Efficient insulation

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:

Power usage proving heat store insulation is adequate

12 hour power usage

2 thoughts on “Efficient insulation

  1. Richard, how good is this system at providing showers? Do you have a separate heating element to top it up or is the on demand rising main water hot enough? And which system did you get? G

    • Graham,
      Like any mains-pressure hot water system, you’re at the mercy of your mains pressure. If you have good pressure then it’s simply awesome. If you don’t then you’ll likely be better off with a standard hot water cylinder and pumps to power your showers. In my case, I have around 6 bar of mains water pressure; think of a water tank mounted 60m (25 stories) above my house!

      The other thing to consider is the maximum flow rate into the house; if your rising main is too small in diameter then despite having great pressure, you still may not be able to deliver enough volume of water to the outlets for your needs. In practice this only really becomes an issue when you’re running multiple outlets at the same time. It’s also less likely to be an issue in modern houses which tend to be designed for combination boilers, which also need good flow rates.

      The system doesn’t need any additional electric heating to power the hot water. It comes with a thermostatic mixer valve to keep the temperature of the hot water to sensible levels; even at this time of the year when the rising main is at its coldest, the system has no problem producing extremely hot water. I personally limit the water to 55c, which is perhaps still a little too hot, but still under scald level.

      I got my system from DPS Ltd, who were excellent both when helping me decide exactly what system I wanted, and subsequently with the odd bit of technical assistance (eg, when the air bleed valve broke). I ended up with a 250 litre “Pandora” system with a single 160kw plate heat exchanger. If you need more info, just ask.

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