Four years ago I replaced the hot water system in my house with a heat store, which (at the time) was a fairly revolutionary system with some real advantages:
- mains pressure hot water
- high capacity
- enables more efficient boiler usage
- no cold water tank in the loft
- no hot water cylinder in the airing cupboard
- DIY installable
The heat store is essentially a large tank of very hot water, heated indirectly by the boiler. That hot water is then used to instantly heat cold water from the rising main, to provide the hot water for the house. In some ways it operates on similar principles to the hot water part of a combination boiler, except that the heat store is much more powerful (160Kw), allowing it to cope with multiple hot water outlets with ease. It is also potentially more efficient than a combination boiler, as the boiler can run for long efficient burns while charging up the heat store, rather than short on/off burns of the combination boiler while heating water on demand.
In my case, I installed the system in the loft, as it allowed us to reclaim a lot of cupboard space. The system came factory insulated, and it was claimed that it needed no more insulation, even when installed in outbuildings or loft spaces. But I’ve always had a niggling doubt, wondering if I was losing heat to the loft, especially when the weather is cold. But I’ve never been able to prove it to myself one way or the other.
The failure of our central heating boiler this week has given me the opportunity to prove that the heat store is adequately insulated, because while we’re waiting for the new boiler to be installed, I’ve switched the heat store over to electric operation. And electricity usage is very easy to monitor, especially if like me, you happen to have a Currentcost meter.
The heat store has a 3Kw heating element built in, which for the time being I’ve set to run permanently via a thermostat. Whenever the water in the tank drops below 70c, the heating element will cut in and heat the water. Currently the loft drops to about 2c overnight, so if the insulation is inadequate I’d expect to see the heat store lose heat overnight (when we’re not drawing hot water), and for the heating element to cut in, and a 3kw electric load appear on my Currentcost logs.
But it doesn’t. It looks as though the only time I need to top up the heat in the heat store, is when I draw heat off it to produce hot water for the house. Which is good. Incidentally, I believe that the regular “blips” of power usage that are particularly noticeable through the night are caused by the freezer, when the compressor cuts in.
Click the image for more detail: