Bosch CS5800i 7kW replacing Greenstar Junior 28i

Air Source Heat Pumps (ASHPs)

Last Post by Slartibartfast 12 hours ago

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Slartibartfast

(@slartibartfast)

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Joined: 13 hours ago

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Topic starter 19/02/2026 3:42 pm

My heat pump journey began a couple of years ago when I had a survey by a small local company and a quote for a Stiebel Eltron system. I wasn't totally convinced, but it went onto the list of things to do later. Eventually we looked at a Bosch 2000 8kW, which seemed ok, and when Bosch offered a further discount on top of the government grant, it looked even better. So good, in fact, that I asked them to quote for the newer 5800i - more expensive, but quieter and with a higher maximum flow temperature. This, too, came in under my budget, so it was time to move forward.

Naturally I approached other suppliers, including Bosch's "official" distributor in the area (30% higher quote) and my energy supplier Octopus. They declined to quote because my central heating is an older, single-pipe system, and anyway they refused to install the heat pump in the most logical place where I wanted it to go. So it was a green light for the local company.

I knew beforehand that I'd need a hot water cylinder (the Greenstar is a combi boiler) and an internal unit (about the same size as the old boiler) but I wasn't expecting the two expansion vessels and the absolute maze of pipework between them all. The internal unit was designed with inputs on the left and outputs on the right - which was a problem, because the external unit was to the right and the cylinder to the left! We could have wished for a choice of handedness there, as it meant some awkward routing of big pipes. (My Bosch dishwasher has a similar problem with its power lead, and I can't help thinking there's some Germanic logic at play here...) Fortunately I had a reasonable amount of space to play with, but you can see the final result here.

Siting the external unit was a key consideration. With the previous system, the gas supply was on the right hand side of the house next to the garage, with the boiler on the back wall of said garage. For me, the logical place for the 5800i was next to the garage wall. There's a 2m separation between my wall and the next house, a sort of shared alleyway, so there was enough room for the unit and its clearance area. However it was pointed out that my neighbours could, if they wished, put up a fence along the nominal property line, and that wouldn't be good for the pump. So we agreed to put it on the wall, something over 2m off the ground. There are no windows to the side of either house, and although it's a big beast, it's hardly visible from front or back. The unit weighs in at 143kg and is well supported by three wall-mounted struts, which the installers tested by doing pull-ups on each one. And they were big guys... A Genie lift was used to raise the unit to the right level, and the pipework was routed through the old boiler vent, which was another convenient factor.

The 5800i is compatible with Bosch's HomeCom app, which has had some poor reviews but seems to be working fine for me. All the day-to-day settings can be done via the app, as well as remote operation, and there's a control touch panel on the internal unit with access to more settings. Unfortunately there's not enough information about either of these, online or anywhere else, so I've spent some time on the phone with technical support. Connecting the system into the home wifi network was a bit of a pain, as its wifi range isn't good - I had to move the hub closer. Also make sure you've switched off any VPN or ad-blocker apps.

The system went live in the late afternoon and the following day was one of the coldest this year, minus 4 outside. I'd usually light the log-burner on such a day, but even without it the heat pump kept the house cosy and comfortably warm. I was definitely impressed. Since then it's performed pretty well, though I discovered a couple of gotchas along the way. One was the water heating, which I set to eco mode to run for a couple of hours in the early morning. This worked fine, but I noticed the water was getting hotter through the day, because of a default "daily heating" setting - now disabled! The other was the auxiliary heater, which was a little too active because it was set for too high a temperature. I'm going to play with it and see how much - if at all - it's needed. I set the "active" heating zone to start at 4am and setback from 8pm, which works very well. Octopus gives me 6 hours of cheap electricity each night, so it's a balancing act to heat during low-rate times but not use too much energy by starting too soon. We will see.

Overall I'm very happy with the system. It's too early to see how it's working out cost-wise, but I'm expecting it to be somewhat more expensive than gas in very cold weather and somewhat less expensive once we get above 7 or 8 degrees. I'll update this from time to time. All I need now is a gas tariff with a low standing charge, as I'm still, as they say, "cooking with gas!"