Radiators hot upstairs but cold downstairs

[Wintom]

We have had British Gas out (£300 for a 30 min visit) and a local heating company (£86 for a 30 min visit). Our boiler is only 4 years old. Our upstairs radiators are red hot but the downstairs ones are Luke warm at best. The boiler fires up then switches off after about 15 seconds. The pump has been checked and is ok.

British Gas man turned all the radiators off then turned them all up to max, he bled the radiators and all seemed good. 3 weeks later the same problem is back.

Local heating chap, reset the boiler then fiddled with the thermostat, radiators and temporarily all good. Yesterday the problem was back, this morning everything was working ok, but this evening the problem is back.

Any thoughts or ideas of what the problem might be?

Looking online this is a common problem, with multiple causes. If the boiler is working, but going off quickly, I would suspect an issue with the thermostat. On the Screwfix board someone mentions being advised by a plumber to change the batteries in the thermostat, which fixed it. If the boiler were staying on, I would suspect air building up due to the way the system is designed. Have you tried bleeding the downstairs radiators yourself?

Otherwise, if it is something more complex, it may be that the two engineers who have visited haven't looked hard enough to get to the bottom of things. They will always try the obvious stuff first.

Definitely sounds like a thermostat issue.

My friend had this issue. The pipes in downstairs were clogged up. The plumber tried to flush through but ended up having to replace the pipes. 🫤

Have you got thermostat valves on the rads? Just a thought, but a couple of my downstairs rads weren’t getting warm- it was the pin getting sticky I believe on the valve interface- common if they haven’t been on for a while. Removed thermostat valve (as per YouTube instructions), tap with a hammer and cycle it a few times to free it up and that solved those cold radiators for me.

might well be a more complex problem but that solution worked in my case.

@Pozz it’s a wired thermostat like this one… no battery

https://www.screwfix.com/p/honeywell-home-1-channel-wired-mechanical-room-thermostat/696FV?tc=DT4&gad_source=1&gclid=CjwKCAiA2pyuBhBKEiwApLaIOxNqSOkhNUkJHXdkNkKHWF89yUO_xDQaPF833f2z8B3stt7fR9sPbxoC_4MQAvD_BwE&gclsrc=aw.ds

The boiler may only be 4 years old, but how old are the rads.

Its a flow issue, either the rads are full of muck or the water is not circulating through the cold ones.

Check the following:
-slightly warm at top, cold at bottom? Get them flushed out
-slightly warm at bottom, cold at top - air in system needs bleeding
-check thermostatic valves work properly
-check circulation pump works properly
-check non return valves are not sticking.

this might be worth watching

Radiators probably 30 years old both upstairs and down.

I have had this- it's easy to fix. My dad (retired plumber) sorted it for me. He calls the hot radiators the 'robbers'- they rob all the heat. It's something to do with the flow of the pipes in the house, but I lost interest in that part of the explanation!

Turn your heating on full for an hour. Then, go to the hot radiators and very slightly turn off the valves (just a tiny bit at first). You need one of the radiator key things to do this. Also make sure the valves are fully open on the Luke-warm ones. Give it time and then check them. Keep adjusting the valves on the 'robbers' slightly until the heat balances out.

Hope that helps!

They may be clogged in which case you would need a flush. That can be about £5-600 and take a good few hours.

I had a similar issue a few months ago and bleeding the radiators worked in the short term, but I ended up needing a power flush to fix it properly. If you're thinking about replacing any old ones downstairs, I found some cheap radiators online that actually worked better than I expected, and installing them was pretty straightforward.

If you turn the radiators upstairs off, do the bottom ones get hot?
where is the thermostat and does the nearest radiator to it get hot or stay cold?