It's a radiator bypass valve. The idea is to make sure that there is always somewhere for the water to flow, even with all radiators shut off by TRVs (the pump has to pump the water somewhere!). If you have a radiator or towel rail somewhere that is left at full or almost full flow (and doesn't have a TRV or other automatic shut-off) then you can shut this bypass valve off (or at least close it most of the way).
It probably raises the output temperature.
If the radiators are balanced, and the pump setting is ok, I'd set it to give
10degC between flow and return. In some circumstances I'd expect a 15mm valve in that position to be fully open, but the one shown is a 22mm. This would require a very (impossibly) low resistance radiator circuit if fully open.
According to the manaufacturer's info an external by-pass is not needed on this boiler (probably fitted internally) so just just close it. Yes that could explain your symptoms. Now you can turn the boiler temperature down to the minimum setting which gives you enough heat.
Not impressed with your installer. I imagine he will say it was just a boiler replacement but that's stretching the point. Did you pay for a radiator flush BTW?
Jim A
Thanks for this (and also thanks to mick and me9 for additional info). I noticed in the CDI technical info that is says "no bypass required" but wasn't sure. Also, since I have a radiator without thermostatic valve, it should be shut anyway (?).
Yes I paid for a power flush (£282) (it says also in the Worcester Bosh document that it is required).
I asked for 3 quotes from people recommended to me. The most recommended installer was about £500 more expensive and was offering the manufacter
2 years warranty, while this one has an agreement with Worcester Bosh to offer 3 years. Also, the one I choose had a relatively detailed description of the jobs to do and estimates for potential additional job (like replacing the gas pipe, which wasn't needed after all), while the others were pretty vague.
This valve is probably wrong.
1) The (combi) boiler may well not need this - the installer instructions should say.2) If it /does/ need it then it should be a pressure sensitive type such as screwfix #67854, #34127 or #98894
3) If it's intended as a bypass then it needs to be nearly closed, (but it should be as in (2) anyway).
Now Jim has read the instructions (about not needing any sort of bypass) I'd say the installer is functioning as an illiterate pillock.
IME the 'big companies' and 'con men' are the ones that tend to specify lots of detail in the quotes. Recommendation is to be weighted more highly tan new vans, snappy work clothes, detailed quotes and big promises.
well, as I said, they are a "biggish" company but were also recommended. That just show how difficult it is to choose some decent installer.
Thanks. I haven't paid yet and intend to challenge them on this and see what they say.
Hello again, I bought a IR thermometer and could do proper measurements. It appeared that the balance was really off (all LSV were fully open everywhere, obviously not checked by the installer). The first set of measurements was (in the order along they heat up): flow/return/drop rad1 82/58/24 rad2 86/65/21 rad3 64/48/16 rad4 63/48/15 (this is the living room one, most important) rad5 78/61/17
Following a DIY FAQ, I start closing first the valves for the hottest. I found that I really have to close them almost completely (prob about 1/4 turn open or even slightly less). Anyway, after a few round of adjustments, I got: rad1 58/45/13 (1/4 open) rad2 59/49/10 (1/4 open) rad3 60/47/13 (fully open) rad4 58/46/12 (fully open) rad5 64/50/14 (1/4 open)
Now, I guess in term of balance it's much better, but I don't understand why closing the hottest radiator does not give more heat for the coldest one (living room)? Since my original complaint was that the temperature was not rising quick enough, it seems like in a way it was better before (with at least some radiatiors very hot, rather than having them all balanced but less hot). Anyway, I'm learning! I will contact the installer about this possibly non necessary bypass valve. I have to say I tried to switch it off briefly and somehow, the boiler seems to start making a strange noise (some sort of rattling)so, reopen it. I think I prefer to ask them first to be sure, before switching it off..
Philippe
Update: tried to close this "bypass" valve half way down instead of fully and result! All rads are now very hot ~ 75/55. I'll definitely question them about this! Thanks again for your help
Sloppy installed pissed pipe-work as in the picture above is also a good sign of Pillock at work.
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