couple of heating questions...

25 Dec 2008
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United Kingdom
my house has a 12 year old system (i moved here just under 3 years ago). it consists of:

potterton profile 80e boiler
no roomstat (trvs on every radiator)
fully pumped
2 motorised zone valves
open vented
storage tank with coil inside for hot water

it basically works, but a few things have been bothering me:

1 the pump overrun can last for as long as 30 minutes. i measured the length of the overrun circuit and it is 2.9m. the boiler installation instructions say it should be a minimum of 4m. i've removed all lagging from the overrun circuit and set the differential valve at 0.15bar. grateful to know whether this sounds right, or if there is any other way of reducing the duration of pump overrun.

2 the boiler overheat stat will trip when the main stat is set at 4.5 or higher. do you think is it worth replacing the overheat stat?

3 lastly, the lack of room stat has always niggled me. at the moment the pump runs for as long as the heating is on, which causes the boiler to cycle roughly 1 minute on/4 minutes off throughout the evening. would fitting a room stat override this? i know you're not meant to fit a room stat and trv in the same room but, if the trv is set to maximum (or even slightly loosened) i don't think it would interfere with the room stat.

i'd be delighted to get an expert's view on any of the above!
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You are getting close.

On 2 port systems the manufacturer will state a minimum bypass length, however it is very difficult to do in practice, so a bathroom rad is used, not always though.

So take the TRV off in there.

Also as you say all rads have TRVs this will cause overheating, if they close slowly or shut. So a room stat is essential.

I would say 30 minutes is excessive, but due to your setup probably within it's limit.
As you have an auto bypass valve then that fine and allows TRVs on all rads. Its whats required now.

However you should also have a room stat to prevent boiler cycling when all the TRVs are turned off.

Easy to fit as a retrofit!

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thanks for the advice, in particular regarding the room stat. i decided to get one and opened the junction box (LWC1) just to check the existing wiring first. i found that it wasn't quite in accordance with the instructions, nor was the pump wire in the boiler connected as its instructions said either! so now i'm a bit dubious about connecting a room stat.

one other thing...i found that the pump overrun can be stopped by manually actuating one of the motorised valves - a bit of a pain to do every night, but is this normal?

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