Bosch CS5800i 5kW - Experience So Far

Steelbadger · 2025-10-02T12:11:05Z

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 ...

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Steelbadger

(@steelbadger)

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Topic starter 14/11/2025 1:49 pm

@ectoplasmosis Buderus is a subsidiary of the Bosch Group, so I assume the different badges are just meant to capitalise on brand recognition in different markets. It does seem to be a proper 'in house' developed unit.

In other news, I've made big strides in improving our heating efficiency. I did this by adding an offset to the heating curve to bring it lower, then adding a minimum flow temperature of 28C. In order to stop the flat overheating at temperatures where the flow should be lower than 28C, I then added a fairly large room influence factor. This should be okay as the flat is well insulated and it takes a long time for temperatures to change significantly. Also, in the event that we do decide to leave all the windows and doors open, there's a maximum flow temperature of 40C configured.

This means that the heating will generally cycle on for a good few hours until the room influence grows enough (at around 1K over the target) that the unit turns off. It seems like if the target flow temperature (before the minimum flow limit is applied) is below 25 it just sets it to 0 and turns off until the room temperature falls enough that the room influence stops blocking the heating.

It remains to be seen how well this works across different parts of the performance curve, but for now we're comfortably around a COP of 4.5 while the outdoor temperature has been around 5.5C.

Sadly, it also seems the flow rate reported by the heat pump via the EMS isn't especially accurate. It looks like it could be off by as much as 25% when doing hot water, so I think any more detailed analysis will require a proper heat- or flow-meter.