Why are some radiators not getting hot?

[redbike]

Hi folks,

I have been in this house for a year and one radiator in one downsairs room has always performed poorly. The hot/flow side of the radiator inbound pipe is really hot but the rad itself never gets more than luke warm.

I have obviously checked for air locks and I have tried adjusting the lockshields on the other rads to see if I can force a flow through the rad without much success. Sometimes it has got warmer but on the whole it is very poor.

The previous boiler was replaced earlier in the year for a Worcester combi and made little difference to this particular problem. Since then, we have recently extended the house and added two more rads attached to the flow and return which feed the subject radiator.

At the same time, I have replaced all the other downstairs rads (5) in the downstairs flow/return loop including thermostatic valves and lockshields. I now have the same symptoms on the two new rads leading on from the subject radiator.

The inbound/flow/hot side of each radiator is hot, far too hot to hold. The outbound pipe from each radiator is barely warm. My feeling is that there has to be a pipe blockage on the return side between the original dodgy radiator and the next working downstairs radiator return.

Although I would expect the return loop to be cooler than the flowside, I'm assuming it will not be as dramatic as I have, i.e. the difference between being too hot to touch on the flow and barely warm on the return.

Would you agree there has to be an internal pipe blockage between the working and non working rads on the return loop?

Appreciate your advice.

David

 

[neiltheplumber]

Can you trace the return pipes? Have you closed rads valves off, removed rads and opened return valves to see what flow is like. If no flow try connecting mains pressure Hose to return valve and open end on flow valve (attach hose this side and run to outside) and blasting with mains pressure?
You will need to drain down once you've established no flow on return side.

 

[JohnD]

you say the rad just gets warm, but the others get hot, therefore there is not a blockage.

Turn off all the other rads. Does this one now get warm? If so, you need to balance all the radiators.

You mention adding radiators and tinkering with lockshield, so doubtless the balancing is all wrong.

 

[redbike]

Thanks Neil/John,

I have definetely ruled out a balancing problem at a radiator level. I get plenty of heat upstairs but the lock valves on the upstairs rads were already barely open. I closed them all off as much as possible, any more would have stopped them working altogether.

I then opened the valves on the furthest radiator downstairs completely and graduated the others according to their position down the downstairs loop.
(This is pretty much as it was already).

The symptoms remain identical in that the input tail to each rad is very hot but the rads themselves are barely warm. I'll experiment a little more and lift some floor boards to check the routing on the return side.

Downstairs, when the flow/return pipes reach a certain point (after the hotter radiator) I think they hit a 'T' junction and go left and right. To the left is the rads that won't warm up much. To the right are 2 more rads in the hall and cloakroom.

The cloakroom one is very small and does get v.hot. The Hall one is a metre away and now only gets warm. They both tee off the flow/return in the hall but even by closing the cloakroom lock shield right down, I couldn't get the hall rad significantly hotter. Again, the input side is very hot but for some reason the hot water refuses to flow through it in the same way as the rads on the other end of the loop downstairs.

I'm still convinced there is a 'restriction' somewhere on the return flow.
I'll try testing the return flow pressure if I get a chance but I suspect this will prove subjective. How would I know there is a good flow?

I do know that when I drained the system using the drain cock downstairs it was still dribbling out 2 hours later! Would that also imply there is a restriction somewhere?

I am assuming my system had one downstairs flow/return loop and a separate one upstairs Tee'd off the boiler 22mm CH outlet. If there was a restriction on the downstairs return, even by closing off the upstairs rads, wouldn't the hot water just circulate around the main flow/return pipes upstairs? That seems to be what I get.

Theoretically, if I were to put a full bore isolation valve in the upstairs loop, I could use that to control the amount of flow upstairs versus downstairs?

David

 

[JohnD]

Turn off all the other rads. Does this one now get warm? If so, you need to balance all the radiators.

 

[redbike]

you say the rad just gets warm, but the others get hot, therefore there is not a blockage.

Turn off all the other rads. Does this one now get warm? If so, you need to balance all the radiators.

You mention adding radiators and tinkering with lockshield, so doubtless the balancing is all wrong.

Well its taken a long time to be able to write the solution to this problem. It has confounded a number of plumbers with the symptoms not to mention some forum contributors.

The problem was a creased pipe on the flowside from the boiler to the downstairs rads. There was enough hot water getting through the crease to heat the small amount of water in the pipe to each radiator but not enough to spread the heat amongst all the downstairs rads.

If all the heat was concentrated to one radiator it would get hot after a period of time. Open the system up to other radiators downstairs and there was not enough heat getting through to go around so the pipe to each TRV would get hot, but the radiators remained luke warm.

Another symptom was that if I shut off the upstairs rads, the boiler couldn't get rid of its heat very well, because of the creased flow pipe, so it would go into pump overrun and then wait to cool down before starting itself up again.

It became a bit like trying to heat a swimming pool from a kettle.

The system didn't need balancing and everything pointed to this being a blocked return pipe to the boiler and it wasn't until after most of the return side pipework had been replaced before I found the creased pipe buried in the bottom of the chimney stack!

Bit of an obscure problem but hopefully it may help someone out there
with a similar problem.

 

[Agile]

Thanks Neil/John,

The cloakroom one is very small and does get v.hot. The Hall one is a metre away and now only gets warm.

David

Well obviously that stealing all the flow.

Close the lockshield and open just a SMALL fraction until a flow can be heard/felt.

Tony

 

[Agile]

Hi folks,

Since then, we have recently extended the house and added two more rads attached to the flow and return which feed the subject radiator.

David

Well a 15 mm is never going to properly feed three rads unless they are smalish and the system is properly balanced!

Read the FAQ.

The whole objective is to increase the flow pressure until its high enough to cause a flow in the furthest rads.

Turn DOWN the hotest rads to a minimum.

You should have about 15 c differential across rads.

Tony

 

[redbike]

Hi folks,

Since then, we have recently extended the house and added two more rads attached to the flow and return which feed the subject radiator.

David

Well a 15 mm is never going to properly feed three rads unless they are smalish and the system is properly balanced!

Read the FAQ.

The whole objective is to increase the flow pressure until its high enough to cause a flow in the furthest rads.

Turn DOWN the hotest rads to a minimum.

You should have about 15 c differential across rads.

Tony

Tony, Did you read my last comment? I'm not looking for more advice on balancing which was nothing to do with the root problem. The problem was caused by a creased pipe on the flowside from the boiler to the downstairs rads. It was restricting heat flow by at least 50 percent. At least 3 heating engineers gave up trying to find the problem after many hours. Replacing the creased pipe, which I only found yesterday, has fixed the problem.