Friday 22 September 2017

Why I haven't installed a heat pump (yet)

If you read this blog you will know I am keen to promote heat pumps as a low carbon heating technology, so I feel a bit of a hypocrite not having one at home. Since we have insulated our house as much as we can it is a logical next step to a low carbon house. So I made enquiries with two heat pump installers - but no joy. This blog post is a personal story. Every house is different and doubtless you won't meet exactly the same issues, perhaps even none of them.

Our house is not ideally suited, though we have insulated it as much as we can.
Is our house suitable? Heat pumps are not recommended for inefficient, leaky homes. Also they are best with underfloor heating because this runs at a lower temperature and heat pumps are more efficient at low supply temperatures. Our home is a large detached Victorian single brick house and when we moved in, in 1997, it was freezing. Since then we have insulated walls, roof and floors and double glazed all windows and doors. We have good air tightness (except in a few difficult places) and we have mechanical ventilation with heat recovery so we don't have trickle vents on the windows. We are comfortably warm (by our standards) and halved our gas consumption (see A retrofit experience: the savings). We rarely need to use the wood stove that we installed early on. If we had known then what we do now we would have installed underfloor heating but we did not. Consequently we still have radiators (of a range of types and ages). So, probably not ideal but hopefully good enough.

The high temperature option ...
Both heat pump sales companies wanted to sell me a high temperature heat pump. This can supply hot water to your radiators at up to 80C, effectively duplicating the functionality of your existing gas boiler. (In fact you really shouldn't be running a condensing gas boiler that hot as it won't be very efficient.) So there is no need to worry about if the radiators will be compatible. The particular model recommended (Altherma HT) is a split system which means it has an inside unit (the actual heat pump bit) and an outside unit (the bit with the fans). A high temperature heat pump is a relatively easy no hassle solution in many cases. But not for us.

... didn't fit in the cupboard.
We had a space for the outside unit, the problem was where to put the indoor unit. Ideally it should go where the boiler is because that is where the plumbing is. Our current condensing boiler is in a cupboard in the kitchen. This cupboard is mostly empty space as it was built around the old boiler which was a lot larger. But the cupboard was still not big enough for the heat pump - not quite wide enough and a lot too shallow. (The Altherma HT HxWxD is 705x600x695 mm).

High temperature systems are less efficient.
Also, high temperatures are not quite as efficient as conventional ones. The online specs are impossible to compare as they quote COP (Coefficient of Performance) figures at different inlet and outlet temperatures. I went to the MCS database of SCOP (Seasonal COP) values which told me that the high temperature heat pump SCOP was around 2.6 while a low temperature system also from Altherma would give me 3.1 even at an outlet of 55C. I would be using 15% less electricity with the low temperature system. Wth the high temperature heat pump I might end up with slightly higher energy bills than now, but there would still be carbon savings. (For homes not on gas there would doubtless be lower bills as well as bigger carbon savings).

The low temperature option involved replacement radiators.
I also asked for a quote for a low temperature system. It turned out this would fit in the existing boiler cupboard but I would have to replace most of my radiators with more efficient ones. Also I potentially needed a new hot water tank as well, with a larger heat exchanger - either that or I would be using the immersion heater regularly to top the tank up to 60C as required to kill legionella bacteria.

They would not show me the radiator size calculations.
A heat pump system that qualifies for the Renewable Heat Incentive has to supply all the heat required 99% of the time - this means down to a specific external temperature depending on your geographic location. For where I am this is about -3.4C. The installers have to measure your home and, for each room, estimate the heat loss at the design temperature and hence the heat that the radiators must deliver. If they aren't big enough to do this at the design circulation temperature they will need to be replaced. (This is where underfloor heating wins because the large area means a large amount of heat can be supplied at a low temperature.) Apparently mine were not big enough. I asked to see the radiator calculations but they were not forthcoming. They probably don't get asked for them often and have no process for this!

Their estimate of heating demand is higher than mine.
What I did have were some figures for my total peak heating demand at various external temperatures. Comparing this with my actual gas use in different weather condidtions (and applying some fudge factors to allow for mean versus peak temperature) I am 95% certain we actually use about 20% less than their calculations. This may mean that we don't need to replace our radiators really but I can't be sure. The discrepancy could be due to:
  1. Incorrect calculation of the heat loss: perhaps not taking into account our air tightness and MVHR system
  2. Lower room temperatures than standard: we never run the thermostat higher than 19C even in the evening. Our house is cooler than the standard heating configuration used in MCS.
I suspect it is a bit of both really but I still don't know if correcting for these issues would be enough to allow us to use our existing radiators.

We could have insisted on a low temperature system without changing the radiators. This might leave us rather chilly in very cold weather and we would end up using the wood stove more often, like we used to. However, if we did this we would not qualify for the RHI subsidy and so it would cost us a lot more.

The heating system is designed for the house, not the people.
The rationale behind the RHI rules is that the heating system is configured for the house, not the people in it. We might move out and new people come in who like the place conventionally warm. If they decided to top up with conventional electric heaters this would be both expensive for them and increase the load on our electricity generation system. The last thing we need is lots of people demanding more power on cold winter evenings, our worst peak demand time.

On the other hand, this means that eco minded people that have adopted a slightly more spartan lifestyle for the sake of the planet (and/or to save money) get less subsidy.

The monobloc option - leaking refrigerant?
I should mention that we were also offered a Panasonic Aquarea which is a monobloc system. This has a bigger outdoor unit but just a manifold inside where it connects to the plumbing so would fit our existing boiler cupboard. I was a bit concerned about this because I had one salesman tell me they never offered monobloc because they were known for leaking refrigerant while another told me it was fine.

The heating controls proprietary lock-in problem.
The final show stopper was the heating controls issue. It seems a minor point but we currently have a Vaillant boiler with Vaillant heating controls that use a proprietary interface. I know you can use a 3rd party controller with the Vaillant boiler but as I understand it the Vaillant controller will not drive a 3rd party boiler. This controller manages the solar hot water system as well as the boiler and it has weather compensating controls. We really like the way it integrates all the functionality we need in one place. It even reports how much energy we have saved from our solar hot water panels.

Unfortunately this integration requires wired connections to three completely different parts of the house. At the time it was installed we had floors up everywhere and it was no issue running new wires in. Replacing it now would probably mean lifting our beautiful pine floor with umpteen coats of sealant and ripping into plaster all over the place. My beloved put his foot down.

An alternative solution would be to stick with Vaillant all the way. They do sell a heat pump which presumably works with their controls but from the specification I found its performance is considerably inferior.

For us, installing a heat pump is best done with other building work.
The main lesson I learned from this is that, for us, replacing a boiler with a heat pump is a big deal and probably best attempted only when we are doing other building work at the same time. Then the need to reconfigure cupboards or rip into plaster and replace radiators would be less of an issue. In fact we would probably put in underfloor heating at the same time.

On the other hand, if you have a suitably large space for a high temperature heat pump, and if you don't mind the lower efficiency, and if you don't have proprietary heating controls that will not interface to another system, then installing a heat pump might be reasonably straightforward.

You may be interested in these related posts:
How to make sense of your meter readings.
High temperature heat pumps go better with weather compensation.
Do heat pumps deliver?


  1. If the Renewable Heat Incentive doesn't work for eco-spartan types, you wonder who it does help. Surely environmentalists who are committed enough to do all the legwork and even contemplate the trauma of building work, and those who intend to stay in their homes long enough to see the benefit, are precisely the ones we should be helping with the last mile.

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    You could get more information if you have a smart meter; it could record daily gas use(you may need to set this up), then you could determine what your maximum gas use was throughout a year, and pick a heatpump that could match that daily energy. I'd also suggest dropping the gas boiler temperature very low and setting the timer to continuous ON during a cold snap for a week. This will allow you to practically find out what temperature your current radiators must run at to keep your house warm. Most gas boilers are set very hot which is less efficient, and turned on for short periods - better to leave it on low for longer (this is even more important for a heatpump). If the gas flue is too hot for water to condense, it can't run efficiently.


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