Are heat pumps any good?

Is it worth getting a heat pump?  Is a £1,000 mini-split a heat pump?  Does Liz Truss know?

Heat pumps in theory will save the world. For each unit of electricity they use they move about 3 units into your house. However, most of us live in houses that are already built (best kind to live in if you ask me) and not designed for heat pumps. You can see a discussion about heat pumps and radiators here – Radiators and heat pumps – but basically your house is almost certainly not going to work well with a heat pump unless you already have underfloor heating. Otherwise, you’ll need to almost rip it apart and start again.

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Heat pump power

What are the snags?

Apart from the radiator issue there are other integration snags. If you have a combi boiler there is no hot water cylinder but the heat pump will be too weedy to heat water on the fly so a tank will be needed. If you have a hot water cylinder already don’t be too smug. The coil in the tank is designed for very hot water from a boiler. Heat pump cylinders have much bigger coils so you’ll need the upgrade. In practice these tanks usually need a daily boost from an immersion heater which runs on the most expensive energy source you can buy.

So, it’s not just the cost of the heat pump; the new tank etc is very likely to push the cost to circa £15,000. (£5,000 from the Government so make that £10,000).

Assuming you are still up for it you might want to study this chart of current (winter 2022 Truss revision) energy costs compared.

Here you can see that the cost of running the heat pump compares well with natural gas, oil, lpg and wood so the cost of running it won’t be too bad – or will it? Wholesale electricity prices are literally around double the artificially fixed price on the chart so for how long do you think those prices can be subsidised?

It seems to me that you can buy an awful lot of gas for £10,000 so why not stick to a nice powerful and cheap gas boiler while you can still get one.

Ah but I’ve got solar panels

Good for you. And if your array is big enough then a heat pump is the way to go. If we all had a big solar array and a heat pump the energy crisis would be over. It has to be a big array though because on a short winter day the panels lose their sparkle to a large extent and then quit altogether by tea time. Batteries step in here but they are unlikely to make financial sense unless you get them free in a leased electric car and don’t pay for their degradation – all that is another story for another day.

Moving away from the big money discussion let’s talk about mini-splits. Just like air source heat pumps in this note they are proper heat pumps but they are air to air so they blow hot air into the house. With no connections to water they are quick and easy to install and only about £600 odd to buy. As part of a hybrid solution one of these makes so much sense that the Government should incentivise them along with a bonus if they are paired with solar panels. At least this is something most people actually can do, rather than something that they should do, but can’t.

Here’s one I installed in my house paired with a 4kW PV array.

What’s it like?

The panels run it for free on most days of Spring and Autumn but not in the depths of winter although it is so cheap to run that it regularly dries the laundry by blowing hot air over the drying racks. In Summer the air conditioning is a big bonus as the cold air spills across the whole house and on a hot day electricity is always free.

The whole cost was under £1,000 fully fitted but maybe a slightly bigger one with an output of 3kW or more would have been better. You might consider one with more than one indoor unit or another separate one.

The difference between this and a £15,000 air to water system is so massive you’d think the Government should take note. Feel free to forward this to your MP if you agree.

Condensing boilers don’t condense

When hydrocarbons like oil or gas are burnt they produce a fair bit of water in the form of steam and that steam contains energy that can be reclaimed if it can be cooled enough to condense. If your heating system returns water to the boiler at 55c or below then condensing happens in the boiler and that plume of steam outside disappears. The benefit to you, the boiler owner, is immense. The boiler that was sold to you as 97% efficient actually gets there instead of the mid eighties which is probably where it is now. On a gas bill of £3,000 a reduction of £300 – £450 makes this a topic worthy of some perusal. Plough on, it’s boring but not difficult.

There are different accepted design targets for the temperature drop across heat emitters (like radiators). A Dt of 20c or 11c. Both so widely apart to be pretty unhelpful. Maybe all of that is irrelevant as your system already exists and you can only move on by measuring what Dt you get.

So, say a radiator circuit might be set at 90c in and 70c out (Dt of 20) the desired return flow at 55c is far out of reach. Owners of oil boilers are a bit stuck here as the boilers don’t modulate which means they work on full power and are either on or off. Gas boilers, however, can have their output temperature turned down and this is the place to start but, unless you can tolerate a lot less heat in the house, it isn’t the full solution. The trouble is that the radiators don’t move as much heat at lower temperatures so they won’t achieve a Dt of 20 – maybe only 15 or less – so say you drop the input temperature down to 75c the return only drops to 60c. Getting close, but the heat to the house will be down to 12kW from 16kW for example, and we are still not condensing. Turning on more radiators will drop the return temperature and this where the juggling needs to begin. If you time the hot water heating to coincide with the heating that will help too. Looking at that standardised Dt of 11c and working back from a return of 55c the feed would have to be 66c which is quite low for radiators. The chances are your current set up is a bit above that.

Taping thermometer sensors to the feed and return pipes next to the boiler is essential for seeing what is going on.

DIY fan-coil unit

There is one tweak that could help reach down to those last few degrees and this will be particularly useful where oil boilers are concerned. Check out the DIY fan-coil unit featured here.

This works well at lower temperatures, such as the reduced return flow you are reaching for, so it will work just on the return flow pipe alone (via a diverter loop) anywhere towards the boiler end. It’s all about removing power to widen the Dt and it doesn’t make too much difference where this happens but removing heat from the return will leave the main circuit running hotter and with a higher Dt. So, when you have turned down the supply temperature as much as can be tolerated the return flow pipe can be tapped for energy and maybe you’ll reach the magic 55c.

Hardly anyone has a heat-bank based system but these have a return feed to the boiler drawn from the cooler bottom of the tank and that results in the boiler running in condensing mode most of the time and with much longer runs which is particularly good for oil boilers and their short cycling problems. If you are starting from scratch there are many other reasons why a heat-bank is the way to go. There’s a case study on one here. https://originaltwist.com/2016/01/21/eco-heating-news/