Choosing between hybrid and all-electric heatpump system

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.