Over the last year or two I’ve been pondering the replacement of my gas boiler and possibly quite a lot of the heating system (regular readers may remember that my heating pipes all drop down from the loft).

I’m on mains gas so have only considered gas condensing boilers, not more exotic technologies such as ground source heat pumps or wood pellet burners.

One of the key features of condensing boilers is the need to keep the return temperature down as low as possible – certainly below the dew point (55C), but some manufacturers also quote outputs at 30C (flow 40C) where the boilers appear to operate even more efficiently (possibly due to a lower flue gas temperature).

Therefore I’ve been toying with the idea of running the return from the rads back through a lower coil in the hot water cylinder (and boosting the DHW temp at the end of the heating period), though I’m not convinced that even an oversized cylinder would have enough thermal capacity to be useful.

However I went to the Homes For Good exhibition last week and got chatting to an underfloor heating (UFH) supplier. With UFH you have one (or more) manifolds that take hot flow as required from the boiler and blend it into, essentially separate, primary circuits which loop through the floor. I assume that the return temperature is comparable to that of the water in the floor, so maybe 25-35C.

It’s quite hard to work out the suitability of UFH for retrofit though. Here are my initial considerations:

  • Power output: you get much lower power output than a conventional rad – from what I can tell at lower water temperatures on joisted floors you might only get 40W/m2, so for a 15m2 room that’s 600W. They do talk about up to 100W/m2 but that seems to be at higher water temperatures and probably only for screeded floors.
  • Installation: major disruption (all floorboards up), more joist notching and quite a complicated distribution arrangement as each room gets its own supply from the manifold. I’m not clear whether you’d put the original boards back down or replace them with thinner chipboard panels that would have lower thermal resistance.
  • Controls: so far it looks like you have 240V actuators on each UFH zone, with either 240V or 12V room thermostats. You can get programmable room thermostats so that you can have different temperatures at different times of the day, though if you had them in most rooms setting them all might be a bit tedious.
  • Integration with radiators & DHW: where I have some solid flag floors I’d need to keep traditional radiators so this would have to be designed for higher flow temperatures without compromising system efficiency.

Looking at the output another way: 40W/m2 for a 200m2 house would mean a total floor output of 8kW which is modest even by modern insulation levels, and possibly half what you’d need for an older, more poorly insulated building. In other words if I went down the UFH I route would definitely need to do internal wall insulation for as many of my external walls as feasible. This might be a good motivation, and clearly would have a payback in lower fuel consumption, but would be very disruptive to do all at once.

The controls for UFH are interesting though as I am looking for better control of room heating: e.g. bedrooms and kitchen in the morning & kitchen, lounge then bedrooms in the evening.

I had contacted Myson about an electrically-switchable TRV (a sort of 24V zone valve – see Myson Inc) they sell in the US but without much luck. So now I’m wondering is whether it would be feasible or sensible to use an UFH manifold to control radiators (perhaps oversized and running at a lower temperature, say 50C). This would however need a dedicated pipe run for each main rad from the manifold (increasing complexity and heat losses) so maybe a neater solution would be to put UFH actuators in the pipe run next by the radiator (i.e. like the Myson valve mentioned above).

I suspect this is all getting a bit specialised though and I haven’t found much independent advice. The Energy Savings Trust have several publications about central heating in their “Energy Efficiency Best Practice in Housing” series – the most relevant for me being CE30 “Domestic Heating by Gas: boiler systems.” However really they only mention UFH in passing and are focussed on the importance of the condensing boiler, TRVs and basic controls.

Anyway, that’s about as far as I’ve got so far – if anyone has any thoughts or useful sources of information please add a comment.

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