RED Heat Pump

We’d heard only recently about heat pumps but became interested when a resident nearby had one installed under the RHI grant scheme. This scheme ended in April 2021 (although other grants are now available). On the face of it, our house would possibly not be an ideal candidate for a heat pump due to its size and the fact that it is a 100-year-old house with less-than-ideal insulation. It has loft and cavity insulation that was done in the 90s. We topped up the meagre 100mm loft insulation to 300mm soon after we moved in (2005).

Why did we think about going for a heat pump?

Our current boiler was frequently breaking down and, although only seven years old, seemed to be a bit of a lemon. The hot water cylinder had also started to leak. A couple of plumbers said that a repair was possible, but couldn’t guarantee it, especially as the cylinder was fifteen years old and out of warranty. So we were already in the position of having to invest in a new system, and the RHI grant would mean a heat pump could probably be installed at the same cost as, or very little more than, a new gas system.

We contacted several installers to survey and quote with very mixed results. Before a heat pump can be installed, the installer needs to make quite accurate measurements and projections in regard to the amount of heat required for any particular house. It became obvious that the more conscientious installers measured up all the rooms and did the calculations, with a consensus that we need close on 16kW. This is when we discovered there may be one or two cowboys out there as one company just had a quick look round and came up with a quote for installing a heatpump that we subsequently discovered would not be adequate. Another installer suggested we needed two heat pumps.

In the end we decided on an installer who recommended a modern “new technology” pump that was super efficient and so would be adequate for a large detached house. We were further impressed by his idea to split the existing pipework into two zones: this allows the upstairs to have a different (lower) thermostat setting to downstairs. Assuming we didn’t need or want the bedrooms quite as warm as the living room, this would save energy and money.

Because heat pump systems usually require larger radiators than gas systems, prior to installation we turned down the temperature of our existing gas boiler to simulate the type of radiator temperature we would be getting with a heat pump. Although the installer will do calculations based on room volume and house efficiency, this is a good way to get a rough idea before biting the bullet and swapping out more radiators than is actually necessary for your needs.

We were all set to go for a February install in order to get the RHI grant but at the last minute, the installer called to say he could not go ahead as his refined heat loss calculations meant that this pump would not quite be adequate for the house. This is crucially important as all installers are obliged according to their MCS certification (required not only for peace of mind but in order to qualify for the RHI grant). The obvious solution was to retrofit underfloor insulation which in our case was not too difficult as we had traditional board suspended floors. After doing this, we were all ready to go ahead and had it installed mid February 2022 – catching the tail of winter to get a decent idea of how it was working.

This all went reasonably smoothly. We did need to add/enlarge some radiators – however, we decided to keep a couple of existing radiators in bedrooms that are not used regularly, and in any case we tend to prefer cooler bedrooms.

What do we think of it so far?

First of all, as made clear by the installer, with current energy pricing, a heat pump is not likely to save you money compared to a gas boiler. Looking at our bills, this year we paid about the same for electric during February and March as we did for electric and gas combined in the previous year.

We are very impressed by the two-zone system. Overall it works very well, but we quickly became aware of a couple of issues caused by fitting a new heat source to an old system of pipes and radiators.

Kitchen radiators

With our old gas system, our kitchen was heated by a kick-space (plinth) convector radiator. This is a small but powerful radiator that sits at floor level inside the cabinet plinth. It uses a fan to be able to convect a lot of heat from a small radiator. The new design to use with the heat pump added a conventional radiator but also relied on retaining the kick space heater. As it turned out, this kick space heater does not work with the low temperature of a heat pump system so we now need to either add another conventional radiator or to find a kick space heater that will deal with lower temperature.

Extension pipework

Our 1920s house has a 1960s extension with a separate shower/toilet. This now serves as a home office, but with the new system it does not quite get warm enough. The installer’s plumbers initially had various theories about this:

  • Too many isolation valves within the pipework.
  • System needs rebalancing via the valves on other downstairs radiators. They did this but no difference.
  • Pipework to extension was the wrong size – ie the plumbers in the 60s may have (literally) cut corners and should have run a separate branch from the main 22mm feed.
  • Blockage in pipework.

The plumbers then discovered that the TRV valve that came with the radiator was faulty so they changed this, however the radiator still was not heating the room adequately.

They then found a lot of gunge in the valve and cleaned it out. The radiator still was not heating the room adequately.

We found that by turning off the extension shower room towel rail, the main room gets warmer, but of course this doesn’t seem an adequate permanent solution.

No power flush?

While all this was going on, we also noticed that the hall radiator was cold at the bottom, so wondered if the system could do with a power flush. Although the installer said a power flush is not required by the heat pump manufacturer, the distributor pointed out that in older properties a power flush was essential. So this is where we are currently: we will get a powerflush and then we can re-evaluate whether the pipework to the extension is adequate. Renewing it directly from the master 22mm feed is not too much of an issue, so one way or another we are confident about getting this all working soon. Touch wood.

What we have learned

Always make sure that the installer does a thorough survey of the system in advance. In our case it was obvious (looking at the instruction manual) that the kick space heater was very unlikely to work with a heat pump. The extension pipework, although adequate for high temperature and flow rate from a conventional system, does appear to have been running too many radiators already, so this should have been flagged up as a potential issue in advance. And finally, insist on a power flush!

[UPDATE Jan 2023]

Some of these issues were resolved:

  • A powerflush was carried out on zone 1
  • After some more plumbing work the kick space heater does work, but depends on the flow temperature setting. This makes sense but what it means is that it will only come on when the the system temperature is high enough. Possibly this is not an issue , as it can be viewed as a “top up” to the main radiator system and may only be necessary when very cold and weather compensation causes the temperature to rise.
  • A section 15mm pipework has been replaced by 22mm to adequately serve the extension radiators.

During extreme cold < -0 degrees the system does not reach design temperature of 21 degrees. This may be due to an issue with defrost cycle. Although we would expect the design temperature to be possible this is actually higher than we would mostly require.

It seems that there may still be a lot to learn, both for the customer and the installers, especially when it comes to retrofitting as any older house can have more unknowns than are expected. There may still be a lot to learn in regard to optimising the system and we are in touch with the manufacturers who advertise data collection, in order to facilitate getting the best performance..

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