Radiators: feed hot, return cold - what's going on?

[sharon-d]

Hi everyone, first post. Moved into our new house a few weeks ago but now realising that the central heating system isn't working that well. Hubby's been trying to fix it but he's not making much progress and hates the thought of being ripped off by BG.

Thought I'd try you guys for some advice. Might be something simple or really expensive. I'll explain the setup and the problem ...

The flow and return circuits are 22mm. From the boiler, an equal tee has been used to split the feed for upstairs and downstairs – all 22mm. It’s the same for the return. The boiler is located upstairs.

There are ten radiators, five upstairs and five downstairs. Each radiator has an 8mm microbore feed-branch and return-branch with a distance of not more than 2 metres from the main 22mm branch.

There are no problems upstairs – all radiators are hot, so too is the 8mm feed and return. All upstairs radiators have a TRV. We’ve checked to make sure that the TRV arrow is pointing in the direction of flow.

All’s not so well downstairs. Again, the feed and return is 8mm microbore. All but one has a TRV. Again, we’ve checked to make sure that the arrow on the TRV is pointing in the direction of flow. However, on each radiator, the feed is hot - but the return is cold, or lukewarm at best. Only the top-half of the downstairs radiators gets hot. The bottom is lukewarm to warmish, but never hot. We've tried to balance the system - by opening the the tap-screw "thingie" at at opposite end of the TRV full for those radiators furthest from boiler, and almost closing those closest to the boiler. This helps a little, but not alot.

Thinking that it was a problem with the pump,hubby replaced it with a new Grundfos 15-60 and set the speed to III. We also pumped up the expansion tank to 1bar, which helped a little but didn’t last long. (Expansion tank shows 1 .. 1.5bar when the CH is running). The system water pressure is set to 1Bar, rising to 2Bar when the CH is running. The system is a Vokera Option.

Sorry to ramble; hope someone can offer some advice. A friend suggested a power-flush, but thought I'd seek help here first. Hopefully we don't need a new boiler

Thanks guys

 

[hi-spec plumbing heating]

where is the boiler up or downstairs ?

 

[sharon-d]

Hiya, its upstairs.

 

[hi-spec plumbing heating]

could be a blockage in return, balancing.
try shutting all the rads upstairs and leave rads open down stairs.
just shut all the trvs to 0

 

[sharon-d]

Ok, we'll try that. A friend also suggested that it might be a blockage - he said to try using a magetic to see if its attracted to a section of the copper pipe, and if it was then we'd need to power-flush the system. Thanks for your help.

 

[hi-spec plumbing heating]

try the manifold where it's 22mm to 8mm.
maybe just air in return side or even slight block that putting all the flow downstairs may move.
or add some cleaner to one off the rads and let it circulate for a while.

 

[bengasman]

Judging by the mentioning off pump replacement, it would be an open vent system. Combine that with microbore and you almost surely have a sludged up system.
Start with adding - hyperflush - from kamco if there is still some heat in the rads. Leave in for a week. Drain system, refill and run on max for half hour and drain again. Repeat this until clean. Refill system and add another dose of hyperflush and leave in for 3 weeks. Drain, refill, run for half hour and drain again etc.

 

[hi-spec plumbing heating]

Judging by the mentioning off pump replacement, it would be an open vent system.

it's a vokera option

 

[sharon-d]

Hiya, thanks for the replies.

Tried what "hi-spec plumbing heating" suggested - we’ve turned all the upstair radiators TRVs to 0. These radiators are now cold. We also turned all the downstairs TRVs up to 5. There’s no difference in temperature with the downstairs radiators, still not very warm – and the feed-in to each radiator is still hot; return is cold or lukewarm.

One thing we noticed after doing this is that the boiler burner is cycling on and off more frequently - it heats for about 30 seconds then pumps (without the burner being on) for about two minutes before cycling again. We also noticed that the 22mm return to the boiler is now cool, whereas the feed is very hot. The pump is as hot as the feed.

Another interesting thing we noticed - when we power off the boiler, after a few minutes the return feed to the boiler becomes as hot as the flow feed. No power, no pump, no burner. Weird???

I'll try and get hold of this hyperflush. Is it something that homebase or B&Q stock? Thanks

 

[hi-spec plumbing heating]

sounds like a block on the return.
try shutting all but one rad down stairs and remove the trv thermostatic adjuster on that rad.
see if that one heats any better. if it does try the next.

 

[zimmers]

sure hes put the pump on the right way if so sounds like as usual microbore is blocked on the drops somewere

 

[lagunaglen]

Try taking trv heads off and seeing if the pin moves up and down, but do it carefully, if they dont move use pliers to gently lift up the pin, the trvs may just be stuck.

 

[gigz]

I would take off each downstairs rad and flush them outside with a hosepipe, while rads are off open each valve into a bucket, this should get rid of some of the crud, then add some good cleaner/desludger, leave it running for at least a week, then dump the water out and refill adding a good inhibiter. If that doesnt work a powerflush may be required.