Get heat pump ready

Here's what we suggest to help make you an informed heat pump purchaser, getting an efficient, effective installation at the best price!

1. Understand the basics

2. Heat pump sizing

3. Hot water approach

4. Understand your heating needs and costs

5. Get quotes

6. Choose an installer

7. During and after installation

 

1. Heat pump basics

Runnng "long and low"

The fundamental difference from a boiler is that the heat pump works best - ie most efficiently - if it works "long and low" - ie it delivers soft heat for a long time,  rather than the short, fierce high temperature blasts you get from a boiler. This means your radiators may sit at only 35C (skin temperature) all day - but amazingly, they work! As temperature changes are much slower, you should reduce zoning, because you can't heat up a cold room in a hurry (well, it is possible with lots of messing about and loss of efficiency, but best to avoid). And you don't let the house go completely cold overnight - instead you use a "setback" temperature (maybe 16-17C) which saves some energy but makes it easy to warm further in the morning. Of course, many people manage boilers in this way already, so there may be little change.

If an installation is poorly done, or if the homeowner runs the pump in the wrong way, then the heat pump may be inefficient and cost its owner too much in electricity. This means that efficiency is the key target of a heat pump installation, and more care is needed with the installation than with a boiler, which is something of a blunt instrument (and typically wastes 15% of the oil or gas energy you paid for).

 

COPs and SCOPs

The critical parameter for measuring heat pump efficiency is called the Coefficient of Performance, or COP. A typical COP of 4 means your heat pump is 400% efficient and  delivers 4x as much heat into your home as it consumes in electricity - yes, really!

This vital COP number is quoted by heat pump manufacturers just like mpg for cars, and as with cars it varies with conditions. It drops for lower outside temperatures and higher radiator temperatures, and vice versa. And just like a car if you abuse it, the efficiency will drop. Because the COP will vary throughout the year, from below 3 in sub-zero up to 6 or more in mild weather, an overall number called the seasonal COP or sCOP is used to describe the overall performance of a heat pump across all seasons, much like the average mpg for a car. This number tells you whether or not your heat pump is cheaper to run than gas. Because standard tariff electricity is currently 4x more expensive than gas, but 10-15% of gas is wasted by the boiler, a heat pump needs a SCOP of about 3.4 to be cheaper than a gas boiler to run. This is eminently achievable. When heat pump owners talk about their systems they may well be bragging about their sCOP. (Ours is running at over 5 as I write this...)

 

The table below shows how sCOP varies with flow temperature for different sizes of Vaillant heat pump. So if you ran at a constant flow temperature (aka radiator temperature) of 40C across the heading season, you would average sCOP of 4.13 for a 5kW pump. Within this seasonal COP you would get a lower COP in cold weather and higher in mild weather. In practice you're unlikely to use a constant flow temperature with a well set up system, as this would use weather compensation, which varies the flow temp with outside temperature. But the table demonstrates the efficiency value of lower flow temps.
Note also that you do need high flow temps for heating hot water - normally 55C, but this is a small portion of your overall heating demand, maybe 20%.

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Your design options: Boiler-lite or heat pump purist?

You don't need to design the installation yourself of course, the installer will do that. But it helps to understand the main design choices that need to be made. The three big choices are Design flow temperature, heat pump size (see next section), and hot water approach (see 3 below). Flow temperature may seem like a technical detail, but it is critical to the design and cost, and thinking about it helps you understand how to get the best from a heat pump.
 

Flow temperature

A heat pump installation is normally a trade-off between two alternative approaches:

1. "Boiler-lite": run the heat pump at higher temperatures (say 50-55C), more like a boiler, in which case installation is simpler and cheaper and operation more familiar, but the efficiency will be lower and it will cost more to run. Or...

2. "Heat pump purist": run the heat pump "long and low", at efficient temperatures (radiators maybe 30-40C) . It should then be 4x more efficient than a boiler and cheaper than gas, but the lower flow temperature means that you may need to increase the size of some radiators to make sure they deliver enough heat in cold conditions. This will often be worthwhile, as radiators are quite cheap. And "increasing size" often just means moving from single to double panel radiators, which take up the same space on the wall. And if you have underfloor heating this works especially well with a heat pump, as it acts like an enormous, low temperature radiator.

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To think about the trade-off consider this example: Say you pay £1000 extra to upgrade radiators, and this brings your max flow temperature needed down from say 50C to 40C. From the table, this might improve your SCOP from 3.4 to 4.1. For a house needing 10,000kWh of heat per year, this would reduce electricity needed from 2941kWh to 2439kWh, ie saving 500kWh, which would have cost about £145 per year on a standard tariff. Your £1000 radiator investment therefore has a probable payback of 6.9 years or ROI of 14.5%, which most people would consider a good investment. You would be moving more towards the purist approach!

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But if those radiators had cost £2000 you might not think it so worthwhile.

Equally, if the electricity price falls relative to gas, or if you avail yourself of smart tariffs which can considerably reduce your heat pump electricity costs, then the cost penalty of a lower efficiency heat pump reduces. In the above example, if the electricity price was halved, the payback of the £1k radiator cost would double, so it's worth looking at smart tariffs at the same time as heat pump specs, to see what you might pay. Your sums might show that, with a cheap tariff, you can accept a lower efficiency in order to reduce your installation cost. You would run the risk of electricity prices rising or tariffs being withdrawn, but if this happened you could still upgrade your radiators at a later date, it is a simple plumbing task.

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From a carbon emissions perspective, the heat pump purist, low flow temperature solution is best, as it minimises grid electricity consumption, and maximises any solar export. Radiators have embedded carbon of course, but there is a good carbon payback and your old radiators will be recycled. (we'll be adding more on this at a later date).

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Meeting MCS requirements

Another factor in choosing your trade-off above is the requirements of the Microgeneration Certification Scheme (MCS), the regulatory body for heat pump and solar installations. You have to meet their requirements to get the BUS grant, and one of them is that the system must be able to deliver an internal room temperature of 21C at its design flow temperature and defined external sub-zero temperatures (normally -2 to -3C, depending on location). Now, this is fine if you like to keep your house at 21C. But for some people this represents overheating, it will cost more and they may be quite happy with 18, 19 or 20C. In this case there is an argument for accepting a higher design flow temperature. It will reduce any radiator upgrades required, saving you money, but in practice you will be able to run at a lower flow temperature, because you're happy with a lower room temperature than the MCS standard, and so your efficiency may not suffer. Your installer should be able to model what this would mean for your house, or you can do so yourself if you indulge in your own heat loss calculation. 

Conversely, if you like it hotter indoors, this adds to the case for higher flow temp and/or radiator upgrades. But cooler indoor temperatures will save you money and carbon, and make it easier to get to a net zero home (you may well literally sleep easier too!).

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Many installers are now designing for higher flow temperatures, to keep installation costs down and get round the overspecified MCS requirements. This is a low risk option, as many people will find themelves quite warm enough without running at the design temperature, and if not it's easy to upgrade radiators.

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2 Heat pump sizing (aka output rating in kW)

You need to get the right size heat pump for your home. Not in metres, but in kW. If it's too big/high powered, it will be unable to turn down ("modulate") low enough when it's mild, and it will "cycle" - start turning on and off to regulate the temperature. Your boiler does this too, and it's inefficient in both cases, but you especially want to minimise it with a heat pump. This has proved a challenge for a heating industry that is still suffering from an epidemic of oversizing. Our house just needs 6kW of heat when it's really cold, but was fitted with a 30kW boiler - 5x bigger than needed.

This is why heat pump installers will carry out a heat loss calculation, which is required by MCS rules. It tells them and you how much heat your house may lose in the coldest weather, and therefore how big your heat pump needs to be to match the loss. But beware, they may well "round up" too much, at the expense of your efficiency. This is a particular problem if they only install one range of heat pumps, reducing the size options available. Eg if an installer uses a brand whose range covers 3, 5 and 9kW heat pumps, and your heat loss is 5.5kW, you will be very oversized at 9kW, but may worry about being cold at 5kW. This alone should shift you to a different installer who can put in eg a 6kW machine from another brand.

It is routine for heat pump customers to have to negotiate down the size proposals from installers, and to do this it helps to know your true heating needs, which see section 4.

 

3  Hot Water approach

A heat pump will provide all your hot water, just like a boiler. It needs a hot water tank - it cannot provide instant hot water like a gas combi boiler, this requires enormous instantaneous energy. So if you currently have a combi boiler, you will need to find space for a hot water tank (about 50cm square by 150cm high). You can also get smaller heat stores, which we will have more on soon.

The heat pump uses a higher flow temperature than for central heating to heat up your water tank - normally 55C. This means it does not run heating and hot water at the same time. Hot water has priority and your heating will take a break while the tank is heated. You may wish to schedule hot water runs for when you have cheap electricity (via smart tariffs) or solar.

 

Do I need to replace my hot water tank?

Most installers will tell you that you have to replace your hot water tank (aka cylinder) with a special one when you install a heat pump, because the coil inside is not big enough - in a similar argument to upsizing a radiator. But we would question why that is, given that flow temps of 55C are routine in the world of gas boilers (in fact are now the design temperature in building regulations), and work fine with standard hot water tanks. Almost all our installers quoted for a new hot water tank, at an extra cost of around £1000 (we went with the one who didn't). As our hot water had been working fine with a 55C gas boiler, we challenged this, and 3 installers agreed to stick with the existing tank - so why had they initially pitched for new tank?

We found that the system has worked perfectly running the heat pump through the existing tank and piping, so I would urge you to challenge a tank change in the same way, and please let us know your experiences. When we succeeded with the existing tank, our installer was quite excited, suggesting that this was an important breakthrough in bringing down heat pump costs (not to mention keeping perfectly serviceable tanks out of landfill sites).

So why are tanks so often changed? A cynical argument is that this is just installers inflating the job size, and relying on customer ignorance. It's also true that the hot water run has to be a bit longer with a standard tank, about 100 minutes for a 200 litre tank in our case, and this may reduce efficiency slightly. So if a short tank recharge time is important enough to you to spend £1k, fair enough but given overnight is the cheap time to run it, this duration will often not matter. And we're getting a sCOP of around 4 for our hot water, 3.6 as you get near freezing, which is quite satisfactory and of course still cheaper than gas.
(installers, please let us know your views).

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4  Understand your heating needs and costs

A  If you haven't already, get a smart meter. This will help you with step 2, and help you monitor your heat pump once you have it.
 

B  Work out how much heating you need on the coldest day. This determines how big a heat pump you need (measured in kW heat output, eg, 5 or10kW). Installers don't actually work it out his way, they perform a "heat loss calculation" (see step 4), but you can sense check what they do by knowing what your maximum heat needs really are.
The size of heat pump is very important. If you oversize, which is very common, it will run inefficiently and have high running costs (see above).
Work out your max heat demand as follows:

• Identify the coldest day last winter, This will normally be the day you needed the most heating. If you have a smart meter and app, just look for the day's highest gas usage in kWh. and write it down. Probably in the range 30-200kWh. Try to find a day when you didn't use other heating, eg a wood burner, and avoid unusual days like the day you came home from holiday and used extra gas heating the house up from cold.

• Identify how much gas you use on a typical summer day - this is your hot water and cooking use. Probably in the range 3-20kWh.

• Subtract the hot water & cooking use from your coldest day, to give you your space heating gas consumption. Multiply by 0.9 because your gas boiler wastes 10%. Then divide by 24 and you have the kW you would need for a heat pump running 24h (it doesn't normally need to do this, but you design for it to do so on the coldest day).

• Eg if your highest gas use was 100kWh, and your summer day gas use is 5kW, your max heating needs are (100-5)x0.9/24=3.56kW. Note that your heat pump output derived this way will probably be much smaller than your gas boiler max output.

• If you don't have a smart meter, or if you have oil, you'll need to measure usage yourself. Wait for a cold snap, then read your gas meter 24h apart, at the same time, and work out the difference. If you have oil,  measurement is quite crude, unless there is a very long cold snap you won't be able to measure accurately enough using sight or float gauges, so you'll need to rely on a heat loss calculation.

• Note that heat loss in kW is a different parameter to your year's consumption in kWh, as seen on your bill. Total consumption defines what you pay; heat loss is about the power available right now to keep you warm. A typical heat loss may be in the range 3-10kW, while the average UK house consumes around 10,000kWh of heat per year.

 

C Write down what indoor temperature you prefer. This is because the standard heat loss calculation assumes temperatures of 21C. If your preference is different, it will help you to choose whether to round up or down your chosen heat pump size (see sizing, above).
 

D  Test your existing radiators - how low can you go? This is so you have an idea if your radiators need upgrading.

• Find your boiler instructions and turn down the flow temperature (the temperature of the water that the boiler supplies to the radiators) to 55C. (This is something you should do anyway to maximise boiler efficiency). Then see if the house stays warm enough on a cold (sub-zero) day.Make sure all the radiators are turned on for this. Avoid using a wood burner or other heat source because that will give misleading results. 

• See if your hot water heats ok with the boiler set to 55C. You may have to run the hot water a little longer, but this is still efficient, and if it works ok you may be able to keep the existing cylinder, as heat pumps also use 55C for the hot water run.

• Now to see how low you can go by turning the flow temperature lower:

  • If you have a system boiler with a hot water tank: Test your radiators by turning the flow temperature down to 40C or less early on a cold day, and leave it for the rest of the day to see if you're warm enough. If you are, turn it down further to see how low your flow temp can go.
    NB You'll need to turn it back up again to at least 55C before the next hot water run.

  • If you have a combi boiler, you can turn the radiator flow temperature lower without it affecting hot water. Try 50, 45, 40C.... leave it at least a day each time to see if you stay warm enough. Note that if you have a combi boiler, you will need to find space for a new hot water tank if you get a heat pump.

• Once you find you're cold, you have found the limits of your existing radiator system! A heat pump will need to run at at least this temperature in cold weather, unless you upgrade radiators.

• As a rule of thumb, if you find your minimum flow temp is above 50C, you should probably think about upgrading radiators, as your heat pump will lose efficiency. If you can manage at 40, you may well be fine with your existing radiators.
   

E  Do your own heat loss calculation (optional). This is one for the mathematically inclined. A heat loss calculation uses data about the layout and fabric of your house to work out how much heat your house will lose on a cold day. The heat loss is therefore the maximum heat demand, as the house will demand the same heat as it loses to avoid cooling down. This is an alternative way of working out how powerful your heat pump should be, and it is the way installers will calculate it - they are required by law to use a process defined by MCS - the Microgeneration Certification Scheme. There are software packages and spreadsheets available to help you work this out - or you can leave it to the installer, who will do this as part of their quote (they have to), though they may charge for this part of the quote.

 

5  Getting quotes

1  Who to ask: To be allowed to install heat pumps, companies have to be certified by MCS, which helps give you some protection regarding quality. You can't just set yourself up to install heat pumps the way you can with some other building trades! You should check that any installer you're looking at is certified by MCS here.

You also need a current Energy Performance Certificate (EPC) to qualify for the grant. Every house sale also requires one of these, so you probably have one already. Find yours online here. If you don't have one or it expired, you'll need to get a new one, lots of online providers for about £100. Many heat pump installers will use your EPC to give you an initial free quote, but watch out for this: EPCs are very inaccurate and overestimate energy use, so you may get an excessive quote. Better to use your coldest day estimate (see above) for heat pump sizing.
There are really 4 kinds of heat pump installers: Plumbers; general builders; big energy corporates and heat pump specialist installers. Some pros and cons of these are as follows:
Plumbers: obviously have the key skills to install a heating system, but they may be used to gas boilers and only doing heat pumps as a sideline. They may not have as much experience as HP specialists, and might be more inclined to use gas boiler thinking with a heat pump. They may not therefore know the optimum design and setup. It is also sadly common for gas boiler people to try to dissuade you from having a heat pump, coming up with erroneous reasons why it won't work, just because they can bung in a boiler in their sleep and would rather do that.
General builders will be able to supply all the trades you need - plumbers and electricians - and are an obvious choice if your heat pump is part of a wider building project. But again they, or their plumbers, may be primarily boiler people who've got heat pump certified as a sideline. If they have staff who just do heat pumps that is a plus.
Big energy corporates: We're talking here about the big energy supply companies, in particular British Gas, Octopus Energy and Ovo. The latter two in particular are trying to transform, standardise and scale up the heat pump world, which is an admirable and valuable goal. Octopus have even developed their own heat pump, in an attempt to drive down costs. You get the comfort of big brand names, but standardisation can also mean a one-size-fits-all approach, which may lead to an oversized, inefficient installation. We only got a quote from one of these companies, and were not impressed with their crude approach and surprisingly high price. However, their capabilities are developing, and energy companies may be able to pair their offering with an attractive heat pump electricity tariff, which can make a big difference with running costs.
Heat pump specialists: seem like the obvious choice, they will have more experience with HP setup, and offer a more bespoke approach than big corporates. However the best ones will be in demand and therefore may be expensive, and sometimes they do not do the full job, eg they may not be fully gas and electricity certified, meaning that you have to get somebody else in to take out your old boiler and line up the power supply, though these are small jobs.
Most installers only install one or two brands of heat pump, so if you have a preference for a brand, that will limit your choice (which may be helpful!).
Finally, none of this matters if you have someone you trust and have experience with, who can install a heat pump.
 

2  How many to get? As many as you can bear! There is an astonishing variation in price and technical approach. We had 6 quotes and all had slightly different technical approaches (buffer tank? Low loss header? Neither?) and installed different brands. The costs varied from £8,000 (exc grant) right up to £13,000!
Many installers charge £100-200 for the heat loss calculation (HLC), which is understandable as it can take them an hour or two (and it should), but it makes getting lots of quotes costly. If you have your coldest day consumption (see above) you should be able to get a preliminary quote for a heat pump matching this for free, before doing the HLC.
 

3  Questions to ask: firstly, you will be much better off than the average punter if you have your coldest day consumption (see above) to put in front of an installer, this makes you a clued-up buyer, and will discourage any supersizing temptation. Your main discussion points are:

What flow temperature? This tells you if they're leaning towards boiler-lite or heat-pump purist approach. If you've read the above you can discuss the pros and cons.
What design? The simplest approach connects your radiators directly to the heat pump. This tends to be cheaper and more efficient, and is the heat -pump purist approach. But many installers will want to put in a buffer tank/heat store or low-loss header which delivers "hydronic separation". These may be needed if you wish to combine your heat pump with another heat source (eg hybrid with a boiler, solar thermal) or you want zoning, but otherwise you should probe into why the installer thinks they're required. If you already have this type of hardware (more common in bigger houses), then it's understandable if the installer doesn't want to mess with it, but ask if you really need to keep it.
And try to avoid zoning. We were fans of zoning for years, it can save money with a gas boiler and you can heat a zone up quickly if you need to. But this is not the case with a heat pump. We found we could abandon zoning, keep the whole house warm all the time, and still get to net zero bills due to the efficiency of the heat pump, smart tariffs and solar.
Hot water: do they want to replace your tank, and how much does this cost? (probably £1000 at least) If you found a 55C flow temperature worked ok for hot water with your boiler, ask why you need a new tank/cylinder for your heat pump.

What controls? Can seem a difficult topic with heat pumps. In effect you don't need complex controls since you run long and low and don't change things much, the way you might with a boiler. But if you have smart controls with your existing boiler, possibly with zoning, you may feel that you are losing some flexibility. Most heat pumps perform best with their own manufacturer's control, rather than 3rd party controls. The downside is that this normally means just one thermostat location. While this is the norm for boilers too, it can seem primitive compared to smart controls like Nest, Tado or Evohome which you may already have. We recommend you go with the heat pump manufacturers' own controls, and don't be tempted to graft your current boiler controls on to it, as this will probably reduce efficiency. Hopefully in future heat pump controllers will be improved to at least allow multiple thermostat locations (because houses often don't have uniform temperatures throughout). We were persuaded by our installer to relinquish our fancy Evohome zone controller, which was great with the old boiler, but we're glad we did as it would not have worked well with the heat pump. But another installer recommended a completely different ("boiler-lite") setup just to accomodate our controller; this would have been an inferior heat pump and less efficient design. They were trying to please the customer, but it wasn't the best design, so they didn't get the work.
Heat pump controllers can often be made available with wireless or internet control, which is helpful as you can then easily get the house warm for when you return from a trip, and reduces wiring and installation time.

 

6  Comparing quotes and choosing an installer

As with any quote you will start with price and  your subjective impression of the installer. If possible also check out their ratings on sites like TrustPilot, Checkatrade or Google.

Then use the following checklist:

• What is the intended flow temperature at design outside temperature (~-2C) and MCS spec of 21C indoors, and what is the anticipated SCOP? (expect this to come straight from the manufacturer's spec).

  • < 40C: more Heat pump purist. Expect to have to upgrade some radiators, be clear which and how much this costs. sCOP should be >4, if not, why not?

  • > 50C: more boiler-lite. With this approach you should get a cheaper install, potentially few or no new radiators. You still want sCOP>3.6, if not question this approach. If your real intended indoor temperature is <21C, you will find you can run below this flow temp and the design temp is just for MCS compliance purposes - discuss what your real flow temp and sCOP is likely to be. If you want to minimise install cost, ask what flow temp this would require, and what sCOP that would deliver.

• What is the Heat Loss for the property vs Heat Pump size (kW)? You want these to be as closely matched as possible. If your candidate installer has done the HLC this should also be close to your own coldest day figure. If not you need to review the HLC - they are quite inaccurate compared to your coldest day figure. If suggested heat pump output rating is >120% of your heat loss, then it may be oversized. Look for a closer match from this installer or elsewhere.

• Design: do they want to put in a buffer, heat store or low loss header? if so why, how much extra does it cost, where will it go and what is the efficiency impact? Be sceptical if you have no zoning or other heat source to consider, or if you have no such device already.

• Hot water: it's a cost advantage if you can keep your existing cylinder, and if it worked fine at 55C then this should be the default. The installer might be unsure as they err on the side of caution (and expense), so you can suggest giving it a try - if it doesn't heat well enough then you can always replace it later, you won't lose much.

• Controls? Default is the manufacturers own control, sometimes called "native" control. If this is not suggested, why not? Try not to cling to your existing controller, tempting as it may be, unless there are really no compromises in doing so.

• What heat pump brand and why? Yes this comes last. The system design and installer skills are more important. Output (kW) is the most important feature, but you may have a preference based on on noise figures, efficiency, price or appearance - check out the specs of the installer recommendation and compare it to others.
  Mainstream brands include Daikin, Grant, Hitachi, Kensa, LG, Mitsubishi, NIBE, Panasonic, Samsung, Vaillant, Viessmann.

 

7  During and after the installation

You've done the hard work, now it's time for your chosen installer to take over. Expect an installation to take about a week, though it may be less. It won't feel much different to any standard plumbing job like a new boiler.

Make sure you get a comprehensive handover, so you know how the controls work and what options you can tweak as you get used to your new heating. There will be obvious basic settings like timed heating, temperature set point and set back (for night time), for both heating and hot water, and you can finesse these taking into account your electricity tariff and day to day needs. As noted by us elsewhere, try to avoid big temperature changes eg from setback 17C straight to 21C at 6am, this will make your heat pump max out! We have no set point changes >1C.

One more obscure but worthwhile setting is Weather Compensation, also known as the heat curve. This defines what flow temp your heat pump uses for a given outside temperature, and you want it as low as possible for efficiency, but high enough to keep your house warm. So after the first week or so, and assuming it's heating season, find the setting and turn it down from wherever the installer left it by the smallest increment, leaving it a few days then turn it down again. Keep doing this until you find you're a bit cold, then turn it back up again one notch. You've found your correct Weather Compensation setting (WC). This will be lower, the more energy efficient your house is.

 

The only other task which is important is to make sure your heat pump is performing as designed (or better). For this you need to know what real sCOP it's achieving. Depending on the heat pump and controller, it will probably track this number. It may be presented as a COP on a particular day, or sCOP over a month or since installation. For some reason (probably avoiding any litigation possibilities) Vaillant, which we have, refer to their sCOP tracking as the "working figure". If you can't see how to get this info check with your installer. Remember that sCOP is an average over the whole season (that's what the S stands for), so you won't have a representative figure until you've had a whole heating season. You will get a flattering figure compared to the prediction if you start in October, and a worse one if you start in a very cold snap. If you want to get an early indication, look for 24h where temperatures were around 5C in the UK, which is roughly the winter average, and see how your pump performed that day.

 

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