I live in Scotland, in an ordinary 25-year-old detached house with reasonable insulation. I have been getting quotes for heat pump installation to take advantage of the funding that is currently available while reducing both bills and dependency on gas. This would be replacing an elderly gas system with hot water cylinder. Current piping is "microbore" in places.

There seem to be two approaches offered:

  • hybrid: a 5kW Vaillant heat pump for normal heating, with a gas boiler for hot water and occasionally topping up the heating in extremely cold times (I believe it hit -8C at one point)

  • full: a 10kW Aerona heat pump for heating and hot water, with an immersion heater to weekly push the hot water over the legionella temperature, and replacing all the radiators.

The quotes and cost projections I have been shown say the full system is both more expensive and less efficient, but I'm also reluctant to pay for a gas boiler and a decade of gas standing charge when it might not be used all that often.

How common are hybrid systems? How often do they need to use gas, in practice? What is the consensus around the hot water issue?

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    Before you commit to either system, consider getting a quote on a system that does not need auxiliary heat at all. Units that work efficiently down to -25C are available, and are commonplace in Norway and Finland as well as where I live in New Hampshire, USA. You'll never get that cold in Scotland unless you live on top of a mountain. And perhaps treat hot water as a separate issue, replacing your tank with an efficient condensing gas water heater if you are keeping gas service anyway for cooking.
    – MTA
    Jan 27, 2023 at 17:54
  • What are you planning to do for air conditioning? Heat pumps normally come with A/C totally free (it's included) but that's hard to make work with hydronics. I agree with MTA, if you think -8 Centigrade is "extremely cold" for heat pumps, that is very, very obsolete knowledge. "obsolete knowledge" is a running theme here, hydronics are an obsolete approach IMO. Getting heat and hot water from the same system once had merit but combining the systems is not efficient. Simple water tanks are so efficient now that you can use them for energy storage. Jan 27, 2023 at 21:03
  • @Harper-ReinstateMonica I'm not planning on doing anything for air conditioning - that would involve retrofitting a lot of ducting which I don't have. The suggestion of the need for backup heating came from the vendor; but if I were having a condensing gas boiler for HW it would be a no-brainer to connect it to CH as well.
    – pjc50
    Jan 28, 2023 at 11:41
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    AIUI what kills heat pump efficiency is frosting up when the temperature is just below freezing and the air is humid, as then having to heat up the coils to defrost them. That means being efficient at -25 is not an indicator of efficiency in the Scottish winter where it's not that cold. (I used to write software for modelling building efficiency so picked up a bit of domain knowledge but am not any way an expert) So I can't offer an answer, but beware of any answers assuming what works well in continental cold like Chicago will also work well for you. Jan 28, 2023 at 14:01
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    @user1908704 that was my suspicion that prompted the question.
    – pjc50
    Jan 28, 2023 at 22:20

4 Answers 4


First of all, do a heatloss calculation. That'll tell you what your losses actually are. I'm going to assume 10kW is approximately right for your property - it sounds about right for a large-ish regular UK house with moderate levels of insulation.

That figure is a 'worst case' calculation - for the average coldest winter day at your location. On days that aren't the coldest, the heating load will be less and your heat source will be expected to modulate down. Any appliance only has a certain ability to modulate - eg down to 25% or 50% of the rated capacity. Below that it'll have to cycle - turn itself on and off to maintain the required output. This is less efficient than running constantly at a lower load. Cycling is not good news for heatpumps, and not ideal for boilers either - but many boilers do it because they are oversized.

10kW is a fairly small amount of heat. That's quite a small gas boiler. A quick look on the Worcester Bosch site had the smallest one it offered me being 12kW. So even on the coldest day it'll already be in modulating territory. Of course, having extra power will help with warming the house faster and faster water heating, so it's not all wasted.

So it seems to me that 10kW is an average air2water heatpump install in a pretty average house and, assuming installed correctly (sadly still a big issue), that will provide all your heating and hot water needs. It would seem that a 5kW ASHP plus a minimum-sized 12kW gas boiler would overcomplicate things and cost a lot more to install (once you're installing a heatpump the size of the unit doesn't really affect costs very much).

For hot water, let's say you have a 250 litre cylinder. To heat it from cold, input temp of 10C to 45C takes 36.75MJ or 10.2kWh. With a 10kW ASHP at optimum output it'll do it in an hour and take 2.5kWh of electrical energy. On colder days it might take a bit longer, if you want it hotter a bit longer and more energy again. Why would you need an additional gas boiler for this? It'll heat it quicker, but unless you have a lot of people bathing in quick succession the ASHP can recharge the cylinder just fine.

Data point: I have a 13kW Aerona3, house heatloss somewhere between 5 and 8kW. Last week it was -5.5C outside when I woke up and it coped with absolutely no problem. There is scope for an electric backup heater when it gets really cold (-20C kind of level) but there is not much need to wire those in the UK. Most of the year the weather is not sub-zero and you reap the rewards in efficiency on those days, which more than compensates for a few days at lower efficiency. 200 litre cylinder gets heated from cold in about an hour, but one tank is enough to have a couple of back to back showers.

The only issue I can see is the microbore: heatpumps need larger radiators to run at lower temperatures for best efficiency, and larger radiators need more water flow to them. If it's impractical to replace the microbore with larger pipes then that could be a problem for a heatpump install. Flexible PEX or polybutylene pipework might allow replacement in awkward spaces.

Finally, there is the question of whether you trust your electricity supply, and what happens if it goes out? Having a backup plan is good. For example, in a rural area where outages are more common, you might have a wood burner. In an urban area you could keep a gas bottle heater. Worth thinking about, but I wouldn't let your one-week-per-decade outage determine your heating choices for the other 519 weeks.

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    Yeah, option #1 re-uses the existing microbore and radiators, option #2 replaces the radiators, and I've not quoted for option #3 "replace all the piping at tremendous expense". Thanks for the calculations, very informative.
    – pjc50
    Jan 28, 2023 at 22:26
  • If it goes out, wouldn't gas heating also break down? Need electricity to pump gas to the boiler and then hot water through the radiators.
    – gerrit
    Oct 11, 2023 at 15:37
  • Often when there are power cuts the gas network stays running - the compressor site may be some distance away and be unaffected and/or have backup power. It's possible to run a gas heating system with a UPS to power the boiler, controls and water pump - it only needs about 200W. Oct 11, 2023 at 16:40

Modern heat pumps can be efficient at producing heat down to very low temperatures. For instance, the performance data for my current air-to-air heat pump is specified all the way down to -17C, and at -8C it has about 65% of the nominal specified heating capacity. So it's entirely possible that the full heat pump system will work just fine. With that in mind, there's two things you need to consider here:

1 - Has proper engineering been done to size the system? If these are based on guesses you're likely to be disappointed in either the efficiency or the performance. With a heat pump system you really do need to have all the numbers properly figured to get good efficiency.

2 - Do you want the ability to perform arbitrage on the price of energy between the two different options? With heat pump systems paired alongside a combustion heat source, it's best to regularly the point at which you switch systems based on energy costs. Since gas is sometimes quite cheap per BTU / kW when compared to electricity, even a quite efficient heat pump can be beaten at low temperatures on a heat per unit cost basis.

I'm a big fan of heat pump systems with resistive backup. If those are both clear for you, it can be a great option.

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    It's fun how "very low temperatures" vary across the world. I've got months where the average low is -15C, and we tend to face at least a week or two of -30-40C. Important to keep in mind for those of us at the low (and high) extremes, what works in many places doesn't always work for us.
    – mbrig
    Jan 28, 2023 at 5:54
  • Of course you can almost always find a place with lower temperatures. Most humans live somewhere where -15C would be considered very cold. See journals.plos.org/climate/article?id=10.1371/… for reference.
    – KMJ
    Jan 29, 2023 at 6:03

Since we're talking "efficiency" here, I would consider unhitching your wagon from the concept of a hydronic system. I certainly see where they were a "seller" 40-50 years ago - sales pitch of "Look! Heating and hot water from the same system!" But they pay for it in efficiency.

A simple hot water tank is very efficient now due to high insulation standards now required. It's so good that it can even be used for energy storage if the set temperature is high enough - and you said you wanted 60C for legionella. So at least, unhitch your hot water supply from the hydronic nonsense.

Once hot water is removed from the system, it now begs the question of why use hydronic at all. Historically, the answer was "it's easier to run pipes through walls than ducts". Well, it's also easy to run refrigerant pipes. Freon not water.

Modern heat pumps have no trouble with Chicago cold. Alec mentions "5 degrees F" a lot, that is -15°C.

So the new paradigm is very much replacing water radiators with refrigerant radiators. This has a neat side effect: It gives you Air Conditioning.

What's more, most heat pumps (or rather, their radiators) are not "Bang-Bang" systems. They don't "bang on" when it's too cold and "bang off" when it's too hot. They are able to adjust their speed for continuous comfort, and that improves efficiency too.

Typically this is implemented as a "mini-split" system and they are not costly. Look at any of the footage of damaged Ukrainian high-rise apartments - those boxes on half the units, those are mini-split exterior units.

And you can do energy storage with this system too - just this way.

And one thought, heat wise, is that you could install a propane gravity furnace that does not require electricity, and plumb its supply out to a propane bottle which you would simply fill at a store when you need it. That will give you emergency heat during power outages.

  • In Scotland there is rarely a need for air conditioning. Glasgow's average high is 20C, record high 31C (wikipedia). Inverness is roughly the same. Without A/C, hydronic systems are easier to fit (no ducts, no insulated refrigerant pipework requiring an F-gas certificate) and quieter in operation (no fan blowers). Jan 28, 2023 at 11:46
  • Interesting view, I'd not heard of the refrigerant system; would that be a drop in replacement on existing piping or something more complicated?
    – pjc50
    Jan 28, 2023 at 11:51
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    @pjc50 The refrigerant is like the piping on the back of a fridge: evaporator absorbs heat from the inside of the fridge, condenser grille dumps heat outside, compressor forces the phase-change. In an air to air system you put one pipe loop inside (on a wall or ceiling), and the compressor and another loop outside. Both have fans to force air past them to heat exchange. You can't reuse hot water pipework for refrigerant: it needs to be pressure tight and designed for the job. Jan 28, 2023 at 12:38
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    @Harper-ReinstateMonica Also, the vast majority of UK heating systems are hydronic - the same 'boiler' does heating water and bathroom hot water; valves switch between the two. We don't have dedicated tank-mounted gas water heaters: the boiler is usally in a different room to the hot water tank (the tank is often central in the house, the boiler on an outside wall for the flue gases). There's not much to be gained by separating heating and hot water - you only need the one heat source. With an ASHP it's the same. A hydronic ASHP can also do cooling, you just need to add fan coils for that. Jan 28, 2023 at 17:04
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    It does matter a lot for retrofitting an existing system, this is not a new build.
    – pjc50
    Jan 28, 2023 at 22:21

Key question: Are electric power outages a serious concern?

If they are not, then either system is fine.

If they are, then having the gas system as a backup is a huge advantage. Even if that gas system needs some electricity to run (controls, pumps, etc.), it won't need nearly as much as the full heat pump system. Which means that a small generator could provide backup for heating, provided you have enough gas on hand (utility connection or a large propane tank), as well as power other critical loads (lights, refrigerator, internet connection). That wouldn't be enough to power air conditioning in the summer (if it was then it could power heat pump heating in the winter) but that is, for most people, far less of a concern than heating.

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    OP has a hydronic system currently. Hydronic systems don't work without electricity because they need pumps to circulate coolant. I don't know if the UK has gravity furnaces like our Williams/Empire but they wouldn't be hydronic. Jan 27, 2023 at 21:27
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    @Harper-ReinstateMonica my point is that electricity requirements to run the hydronic are, like what I need to run my central natural gas furnace, easily handled by a smallish generator. As efficient as the heat pump may be, it would certainly require more electricity though less total energy Jan 27, 2023 at 21:37
  • No oil system was mentioned? It's a natural gas system.
    – pjc50
    Jan 28, 2023 at 11:42
  • @Harper-ReinstateMonica gravity-fed (convection) systems were once common in the UK but they've mostly been replaced - as well as inefficient, they limited your tank position too much. But today's systems require 100-200W for the boiler/controls/valves/pump, so a generator or house battery can handle it. It is a good idea to wire everything on a separate circuit so that can be hooked into external power if needs be. Jan 28, 2023 at 12:56
  • "Oil" === "fossil fuel", @pjc50
    – FreeMan
    Jan 28, 2023 at 15:13

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