Worcester Greenstar 30CDi + FW100 weather compensator

18 Feb 2015
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United Kingdom
Hi all,

We've just moved into a 1900s semi which has been extended - 5 rooms have electric UFH and 11 have central heating radiators hooked to a Worcester 30CDi system boiler. Although it's a system boiler it was retro-fitted to a system with two zone valves, one for central heating and one for hot water. This means water and heating always get the same boiler temperature.

I really want to get the gas/electric bills and efficiency under control, so we're working on double glazing the sash windows, and we're a little restricted on insulation possibilities as they are solid brick walls and most of the attic/loft spaces are converted so I can't put a lot more insulation in than there already is, so I'm looking for every trick I can to shave off a bit of efficiency. Hence my interest in weather compensation - as it's said to save up to 11% gas on the basis we just don't put in a lot of heat into the house unless it's needed, and by setting lower flow temperatures for heating the boiler can turn the heat output down and stay on longer, burning more efficiently with less turning on/off and thermal cycling.

I'm planning to get an Evohome or Heat Genius system so we only heat the rooms we use when we use them - because the room stat is not only broken (wired to always on) but I have no idea where I'd put it that would make sense as we have heat leaking between gas CH and electric UFH areas (each of which has it's own timer/stat measuring floor - not room - temperature). After initially implying I was basically daft, Worcester later sent me a technical note which basically confirmed if you have your own room stat, you can set the FW100 to constant "comfort" and the external call for heat will turn it on and off, so I can fit it along with an Evohome or Heatgenius.

Unfortunately the same trick is not possible with the hot water circuit - the FW100 can't trigger an external zone valve to bring the cylinder into the circuit, and nor can it respect an external trigger - it needs the boiler's own temperature sensor fitted to the cylinder to control hot water - so to use the FW100 an internal diverter valve would need to be retro-fitted - as well as at least a new flow pipe from the boiler to the hot water cylinder. Otherwise my hot water would be at 40C or something! So, a certain amount of plumbing hassle and more than I can do myself - so I'd need the plumber out. He doesn't seem to think weather compensation is worth it.

I understand from reading that some people seem to get annoyed with weather compensation because it decides to turn the heating on at 3am at 1C higher than the room temperature in order to get it up to heat at 7am - but with our own "smart" stat like Evohome / Honeywell we get to choose the pre-warming periods so we have a better control over "when" - it's just the FW100 will look at flow/return/outside temperatures and control the boiler target temperature.

So my questions are:
- what do people think of weather compensation?
- are the 11% savings actually realistic, or are they based on the change from a "totally ineptly configured terrible house with a radiator in the garden" to "amazingly well set up with a weather compensator and TRVs" kind of marketing figures?
- what are people's experience with the FW100 and the worcester internal diverters particularly? are either of them known terrible and I definitely shouldn't put them in if I can avoid it?
- and basically, given I have to do some Actual Plumbing to put in a diverter valve and a (relatively short - just along the attic floor 1-2m and down into the airing cupboard) rather than throw a new controller into the facia of the boiler and put a sensor outside, is it still worth it?

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