Around two years ago, my partner and I moved into a new-build ground-floor flat in Edinburgh. This flat was claimed to be energy efficient and cheap to run, with a heat pump supplying both the underfloor heating and hot water. It was not until after we'd agreed to the purchase, and I started doing some research that I started to get worried. However, it wasn't until we moved in and I was able to get a proper handle on it that the flaws gradually revealed themselves.

The Old System

The system that came with the flat was composed of underfloor heating throughout, with an Earth Save Products 300L Ecocent Exhaust Air heat pump which draws air from the kitchen and bathrooms, extracts the heat and moisture from it in its small heat pump unit, then vents it outside. The heating was supplied through a reverse use of a secondary solar heating coil within the Ecocent:

This had numerous issues. First, the Ecocent itself is fairly loud while running the compressor, and it is in a utility cupboard in the middle of the flat. In winter it would run constantly, and while we got used to the drone, it was far from ideal. Then there was the way drawing heat out through the solar heating coil impacted our hot water. If the heating was running for any significant length of time, the tank temperature in most of the volume would drop substantially, but the top of the tank would remain hot. This generally gave us about 50-75L of hot water, and then a sudden drop to water in the region of 30C, unless the electric boiler was running. However, as the electric boiler was on the hot side of the manifold, it typically wasn't able to actually heat the water in the tank if that temperature dropped. Finally, with the Ecocent set to store at 50-55C, this meant the underfloor heating flow temperature could be as high as 55C. The manifold did have a thermostatic mixing valve on it, but that introduced its own slew of issues. Set fully open, as it was when we moved in, it would permit 55C water into the underfloor manifold, potentially damaging the floor coverings. But set to a more reasonable flow temperature of 40C, it would then start passing 32-35C water back to the Ecocent after going through the floor, and it would become impossible for the boiler to heat the hot water at all.

That's to say nothing of the fact that the Ecocent was only able to achieve a delta T between its air inlet and outlet of about 15C, which meant that our overall energy balance would become negative when the outdoor temperature was below 7C:

21C Air is drawn in, cooled to 6C, then vented outside. Outside air would be drawn in to replace the lost air, if it was freezing outside, that meant the heating would now need to heat the same volume of air from 0C to 21C. All that shortfall could only come from the boiler.

During the colder months, I had to constantly watch and manage our heating and hot water in order to get reasonable comfort at a minimum (but still very high) cost. It was exhausting.

Looking for Something Better

After a full year of attempting to work with the system we had, and after more complaints to the builder than I can could, we finally had to accept that it wasn't going to get much better. While I was able to work around the hot water issue by turning the heating off for a few hours overnight, it still left the flat cold and uncomfortable. My attempts to get the existing system working had led me to Heat Geek videos and Urban Plumbers, and I'd already done a heat loss calculation both by hand and in Heat Punk, so I had some ideas for where to start. Depending on air change assumptions, we were probably looking at a heat loss between 2.5 and 3kW at a design temperature of -4C.

The big question was funding availability. As expensive as our system was, my spreadsheets suggested we only stood to save between £400 and £600 per year by switching to a heat pump, and we don't expect to remain in this flat for the 30 years necessary to pay off an 'average' heat pump install. The good news was that we shouldn't be an average install; we already had a plant room, a hot water tank, and underfloor heating throughout. Really all we needed to do was find somewhere to put the pump, and connect it to the underfloor manifold.

The less good news was that our existing EPC certificate already claimed that our heating was by 'heat pump' so it was possible that we'd be turned down point blank when it came time to apply for the Home Energy Scotland grant. It took a few weeks of back-and-forth, and an in person assessment from a HES advisor, but eventually they were able to give us a detailed report which recommended a heat pump, and the EST accepted it.

At the same time, I was also trying to negotiate the mess of permitted development and planning permission.

As a flat, we don't seem to need to comply with the 1m rule, which in theory meant we could get the unit installed under permitted development. However, the best location for the unit was in our front garden, which then removed the possibility again. In addition, we needed to seek permission from the factors for our block of flats, and through them all of our neighbours, too.

However, while all this took weeks or back and forth, it was eventually decided that this was something we could do, provided we met the necessary noise requirements.

Enter the Bosch

As a flat, we obviously have neighbours on all sides, and the windows closest to all of our possible heat pump sites were living and bedrooms. The furthest we could possibly get away from a neighbour's window was probably about 2m, and the best site was only about 1.5m away. This placed some very tough limitations on the noise level from the unit. Many of the installers we arranged to visit for a quote didn't really seem to understand this, and they kept on giving me the same line about how quiet they are, and how I could come to one of their other installs to hear how quiet it is.

I had to keep repeating that I'm not worried about the actual noise. I was purely worried about making sure we passed the MCS test.

While I'd been playing around in Heat Punk, I'd noticed the strange sound level of the Bosch CS5800i unit, which was reported at just 42dB, a whole 10dB less than the next quietest heat pump. I became intrigued by it, as at 42dB I could effectively place it anywhere without needing to worry about noise. All I needed to worry about was the minimum safety clearances due to its use of R290. The Bosch is available in 4, 5 and 7kW versions. I originally planned to go with the 4kW version, but upon reading the specs it looks like the minimum modulation on the 4 and 5kW is the same, so that's what we got in the end.

Finding someone to actually specify the Bosch, however, was easier said than done. We had multiple quotes for Vaillant, Riello, and Nibe units, but only a single installer was willing to go with the Bosch. Fortunately, it was one of the more reasonable quotes at £10,100 before grants.

The Install

After getting our grant promise, permission from the neighbours and factor, and planning permission, we finally began the install around 9 months after I decided to start the process of pursuing a new heat pump. I spoke to the engineer about the plans, and was very happy to hear that we were in agreement about what was needed. We'd remove the boiler and all the existing room thermostats, moving to an open loop system. We'd run the pipework from the heat pump through the ceiling void to the utility room. We'd connect the wall hung indoor unit to both the heating and the Ecocent to provide a quicker hot water heat-up time, but not in series this time. There would be no buffers, low loss headers, or volumisers on the new system as it was all open-loop underfloor with plenty of thermal mass.

It wasn't all sunshine and roses, however.

The installing engineer had never installed a Bosch heat pump before, although he had a high opinion of Bosch from working with their boilers for years before he'd switched to heat pumps. He did however have a lot of experience with Vaillant units and figured it would mostly be the same. The manuals held a few errors and pitfalls which delayed things by a few days due to incorrect wiring diagrams. The biggest woopsie was probably when I noticed that there was no expansion vessel on the heat pump's circuit. The guy who hung the wall unit assumed the plumber was fitting one, and the plumber assumed it was in the wall unit. The two commissioning Bosch engineers missed it too, as they thought the hot water expansion vessel was on the heating circuit. But that issue was fixed almost as quickly as it was noticed.

The sensors for the hot water tank also presented an issue, as different configurations of sensors require different resistance profiles from those sensors, and we ended up with the wrong sensor 3 different times, which might be a record.

Finally, it took nearly an entire day to bleed air from the system due to the design of the wall-hung unit, and even now it will occasionally complain about pockets of air, but fortunately I'm plenty savvy enough to bleed the few pockets that remain.

The End Result

So far, although we're not yet fully into the heating season, we're happy with the unit and the install. It looks sleek and is whisper-quiet. We'll have to see how it is while running a defrost cycle, but at the moment even when running flat out it is basically inaudible beyond a couple of meters from the unit. In the low-noise operation mode it is even quieter, and I basically need to stuff my head down the back of the unit to hear anything more than air movement.

Currently, we're heating our hot water each morning at a COP of about 3.6 (45C store temperature, heat pump assisting up to 42C, then Ecocent alone up to 45C. The combined COP of the two together is similar or even a little higher but it's hard to judge as we can't measure the heat output of the Ecocent) and what little heating we've done has been at a COP of around 4.5 but the weather is still fairly mild. It's fully open loop and controlled only by weather compensation. The maximum flow temperature is 36C at -6C and the design dT is 4.5K. The actual COP since we installed the unit is only around 2.8, however, as the unit has a standby draw of about 25W on average. That actually adds up when we're doing so little else.

All we still have left to do is to paint the outside insulation to match the brickwork.

Total cost was around £10,600, including the cost of arranging for planning permission and for a new EPC.

We were able to claim both the £7500 HES grant, and the £2000 Barclays Greener Home Reward. Finally, our installer was also able to claim £500 from the Bosch Clean Heat Cashback scheme.