Thermostat low but rads still coming on!

[SooBee]

Probably a rather elementary question but I am puzzled. Our thermostat (Honeywell electronic digital one, freestanding) is set at 15 degrees during the day, to save oil. However, I've found that some room radiators still come on, not boiling hot but warm, and they all have thermostatic valves set to between 1 and frost. When this happens the digital thermostat still shows '15'.

I thought the thermostast controlled the boiler going on or off. Do the rads override the thermostat if the room they're in is cold enough to trigger the rad to come on?

Grateful for advice! (BTW the boiler is a Worcester and only a year and a few months old and has been serviced. The boiler is in the garage.)

 

[JohnD]

you've got a hot water cylinder have you?

I expect that the 3-port valve which sends the heat from the boiler either to the radiators, or the the cylinder, or (possibly) to both is worn and leaking so that heat goes to the rads when you are trying to heat the cylinder.

Look for one of these and feel how hot the two arms of the "T" are.
http://www.screwfix.com/p/drayton-ma1-679-3-3-port-motorised-valve/81844

http://www.screwfix.com/p/honeywell-v4073a-3-port-motorised-valve-28mm/93623

it will probably be a 22mm one.

if the motorised head is faulty, they can usually be replaced without draining.

It will probably be a Honeywell or a Drayton. Other brands are available but perhaps not as good.

If your boiler is in an unheated garage it might be firing up to protect itself from freezing.

 

[SooBee]

Thanks John. I can't find the motorised valve you sent links to. It must be hidden away.

The cylinder (Santon PremierPlus) is next to the boiler in the garage and has its own separate control, again Honeywell digital and mounted on the wall above it. It is set for different times to the boiler and works fine.

The whole system was only installed in November 2011 so surely should not have worn parts already?

The garage is integral (not a separate building) so shouldn't get so cold that it kicks the boiler in, in fact it never gets to freezing, even when snow is on the ground.

I'm just puzzled why the rads are warm when the thermostast says the boiler should be off!

 

[SooBee]

Sorry, me again. I found it it! It's this one (2 port valve)

http://www.screwfix.com/p/honeywell-v4043h-2-port-motorised-valve-28mm/40705

Also forgot to say there are no loft tanks, the system just uses mains water under pressure for everything.

 

[swbjackson]

Can you post a picture of the pipework configuration around the cylinder and boiler?

My money is on incorrect hot water return position causing reverse circulation.

 

[CBF]

mines on the frost stat kicking in

 

[bolshy]

So's mine.

 

[SooBee]

Trying to decipher CBF's very brief reply, I think that might be the answer - the rooms with the stats on lower than 1 are getting cold enough to kick the heating in.

mines on the frost stat kicking in

 

[JohnD]

the frost stat is in the boiler. the temperature in other rooms will not cause the boiler to come on outside its timed periods.

 

[SooBee]

the frost stat is in the boiler. the temperature in other rooms will not cause the boiler to come on outside its timed periods.

yes but that's what happening! Every radiator has its own thermostatic valve (TRV) fitted. Today some of the upstairs ones came on lukewarm (it was very cold) when the TRV was set to halfway between frost and 1. Even tho main thermostat was set at 15.

 

[JayPage]

The trv's on the radiators are in no way linked to the heating coming on and turning off. They are stand alond temperature control valves. The frost protection would be set at 5 degrees? Like one of the above posters mentioned it could be a faulty 2 port not fully closing allowing warm heating water into the radiator side of the system when the boiler fires up to heat up the cylinder?

 

[JohnD]

therefore, the problem is somehere else. Feel the pump, and the pipes going in and out of your motorised valve, see if they are hot.

Did you say you have a combi boiler with no hot water cylinder?

 

[CBF]

your boiler will have a built in frost stat that over rides all the other controls, i'm not sure what it is set at on your boiler but it's normally 5 degrees, if the temp drops below that where it is the boiler will fire to protect itself until it reaches a certain temp then it will turn off, which is why your rads are only getting lukewarm when it does it, if the boiler is in a unheated space i always fit a seperate frost stat along with a low limit pipe stat on the ret

 

[SooBee]

therefore, the problem is somehere else. Feel the pump, and the pipes going in and out of your motorised valve, see if they are hot.

Did you say you have a combi boiler with no hot water cylinder?

No, it's a separate boiler and cylinder.

 

[SooBee]

The trv's on the radiators are in no way linked to the heating coming on and turning off. They are stand alond temperature control valves. The frost protection would be set at 5 degrees? Like one of the above posters mentioned it could be a faulty 2 port not fully closing allowing warm heating water into the radiator side of the system when the boiler fires up to heat up the cylinder?

The cylinder is only on 40 minutes morning and then 40 minutes in the evening so I'd have thought not long enough to affect rads in the middle of the day. It's a huge cylinder and suits us well - keeps water hot for hours and hours.

I did speak to the plumbing co. who installed it and he said something about the return but did not think anything was wrong. (And I wouldn't have thought they'd miss a chance to earn a bit of dosh coming out to check!)