Heating questions

[spence]

In the autumn i noticed our boiler (Ideal FF250) was short cycling. It's fine when it first wakes up in the morning or evening.... and runs for 30min or so without a break but then starts to cycle... 75sec ON 30sec OFF until the room stat is satisfied. The problem then pretty much disappears...the room stat is subsequently satisfied before the cycling starts.

I've not worried too much about this as the rads all get equally warm and the hot water gets hot. But i have tried... ...changing pump speeds... but it has little affect on the problem. ...opening up the "flow" on the rads with TRVs (which are fitted on the return), and the lockshields on the non-TRV'd rads...again no real change.

...but last night i seemed to cure the problem by just turning the stat on the boiler up from "4" to "6". The guy that serviced the boiler recommended using "4" which is why i'd not bothered messing around with it until now. Just wanted to check running the boiler flat out was "acceptable".

And assuming it is ok...now that i'll be wanting to balance my system...is balancing a rad with a TRV on the return simply a case of having the TRV on Max (6) and then varying the flow?

Apologies for the long post and thanks in advance,

spence

[Set Square]

Assuming that this isn't a modulating boiler, this behaviour is fairly normal. Why is it a problem? When the house is warm and some of the TRVs are partly or fully closed, the boiler is producing heat at a faster rate that it's being given out by the rads. So the outlet water temperature reaches the boiler stat setting - and the flame is turned off. Water continues to circulate, and when it cools to the point that the boiler stat turns on again, the flame comes back on, and so on ad infinitum.

It should be ok. Do you have any means of measuring the flow temperature? [You'll need one when you come to do the balancing!]. 82 degC is usually regarded as the acceptable max - and will, of course, increase the potential output from your radiators. [Could be useful if the promised cold spell materialises next week]. I'd be surprised if that had a dramatic effect on short-cycling - because the boiler will still cycle on its own stat, albeit at a higher temperature. I suppose that if the rads are giving out more heat (and making the house like a sauna in the process!) you are using more of the boiler's max output and reducing the tendancy to cycle by a bit. But if the TRVs do their thing, you'll be back where you started!

You should do the balancing with the TRVs fully open - and adjust the lockshield valves at the other end of the radiator. There's a FAQ at

which describes the process in detail. [The FAQ refers to clamp-on thermometers - but it's a whole lot quicker if you arm yourself with a non-contact IR thermometer, such as the one sold by CPC for £30+VAT - actual link below]

[Lurch]

I was going to say turn your boiler up, but you found that out anyway! I always recommend turning the boiler up full to prevent cycling and it gets the water and rads hotter, quicker. On gravity fed systems the boiler temp would determine the HW temp so this is usually why it is turned down. But this isn't the case on newer Y and S plans etc.. with individual thermostats for heating and water the boiler just heats up the DHW water to the temp of the cyl stat. It will of course increase the temp of the rads but this isn't always noticably too hot to touch. It could be for that reason the installer said to keep it on 4. Sometimes with the rads getting too hot the room suddenly gets red hot then noticably cools before the room stat kicks in again. Experiment with the setting on the boiler til you find a setting that's comfortable.

SJW A.C.S. Ltd.

[spence]

Thanks for the input, and the steer on the IR thermometer...been looking for the clamp on ones at B&Q with no luck anyway, so this i'll save time and hassle.

spence