Boiler Thermostats

[JohnC51]

No response to my previous posting so perhaps a simple question may provoke an answer

Does the boiler thermostat control the temperature of the Flow or Return water?

Thanks in anticipation that someone out there knows the answer.

 

[ChrisR]

Neither, though nearer the flow. Boilers vary, but certainly not the return for the main boiler stat. Some overheat stats are in unfortunate positions because they were added to the design as an afterthought so the boiler could be used in a sealed system. eg Ideal Mexico.

 

[JohnC51]

Update - Having looked inside my Glow-worm Fuelsaver 45F, the stat is in the return pipe, as is the overheat stat thermocouple.

 

[ChrisR]

Lovely. One notorious result of that was a burst pipe. The pipe was plastic, which only just copes with the "fault" temp and pressure in a sealed heating system. If the water leaving the boiler is too hot, pop. The manufacturer's get-out is that the British Standard on CH systems quotes a minimum flow rate (hence the need for a bypass valve). If the return is within 11C of the flow, then nothing bad happens. In theory.

But in the real world with stats drifting, getting gunged up, pumps slowing etc, it can be a problem. There's a difference between the people who look at something which says it's OK for heating and just use it, and the ones who ask lots of questions and come up with an engineering judgement.

The overheat stat won't be a thermocouple by the way, it's probably a bimetallic strip in a little round black plastic thing. It may well "interrupt" the gas valve's thermocouple, which puts the gas off.

 

[JohnC51]

Hi again Chris,

The oveheat stat does look like a thermocouple - the copper tube is tightly coiled at the end resembling a spring. I didn't remove it from the sheath around the return pipe but it was loose inside and there was evidence of some heat compound similar to that used mounting heatsinks to PC processors. I'll see about getting a replacement control stat for the boiler and see how it goes. Until then, I have to fiddle with the bypass valve.

I noticed your comments in another thread relating to bypass valves. My rads are fitted with trvs except the bathroom which isn't and should act as a bypass but doesn't. I still have to mess with the bypass (valve) in the winter months to get the rads hot enough. I should get my MPV fixed as well or maybe just chuck it all out and start again!

Ho hum ..

 

[ChrisR]

You have got the water going the right way round?? It should go left to right through the h/exch. The high limit or overheat stat is a liquid expansion type, same in principle as the main boiler stat though it's manual reset and not adjustable.

 

[JohnC51]

Erk! On my system (installed by previous owner at least 15 years ago) the flow is on the left and the return on the right. That would explain a few things!
Any idea where I can get hold of a manual for one of these boilers (Glow-worm 45F)? I've tried the GW web site without success.

Thanks again!

 

[JohnC51]

Final one of this episode - having consulted a local Corgi chap, the boiler is fine, the MPV was stuck in the almost water off position, hence the need for bypass if the heating was off as there was insufficient flow through the water heating coil to dissipate the heat from the boiler. The Glow-worm 45F only has a 1pint heat exhanger so heats up pretty damn quick!
New MPV being fitted Thursday.

 

[ChrisR]

I got my instrs from GW, no probs. WHich is why I know which way the water goes.