System pumping back into header tank

[macnic0]

We are having a lot of problems with our CH system. Over the summer we had TRVs fitted (8mm pipes) and a full system flush. We have an Apollo Fanfare wall mounted gas boiler and a pumped system.

Since we had the TRVs fitted we have had two problems
1. air in the rads, every 1-2 hours we have to bleed the air out for around 1 minute - this only occurs in the first two radiators in the chain

2. Extreme noise in the pipes which we think is caused by the hammer effect when the boiler overheats and feeds water back into the header tank. We have watched the header tank and water flowing back into the tank (very hot and fast) occurs shortly after the noise in the pipes

Please can someone advise how we may fix either of the problems. We have already turned the pump down from 3 to 2 to 1 and the problem still occurs.

The TRVs were fitted by a professional but they haven't been particularly helpful in resolving the problems that have occurred since.

Any help or advice will be much appreciated.

 

[kevplumb]

did you have the probs before the trvs where fitted ?

 

[brumylad]

If you had TRV fitted to every radiator then when the house heats up and ALL the TRV's close, the water can't circulate. Hence the pumping over.

Two options, fit an automatic bypass valve or replace the TVR on one radiator with a normal valve.

 

[macnic0]

We didnt have the problems before the valves were fitted no.

Whan we complained to the fitters they helpfully said "yeah you sometimes get problems with microbore systems like yours".

The living room radiator didnt have one fitted as the the main thermostat is in that room.

thanks.

 

[brumylad]

If the new TRVs are uni-directional then it is conceivable that they have been put on the wrong end of the radiator. Return pipe instead of flow pipe or vise versa.

 

[DP]

The system must be able to dispate heat generated by the boiler. It would seem the installation is overheating with resultant venting into header tank.

System bypass and system balancing plus boiler controls should be looked at

 

[kevplumb]

dont think 8mm feed constiutes a big enough bypass

 

[macnic0]

Thanks guys.

The TRVs are on the radiators the opposite end to the bleed plugs.

I was reading about the balancing and just tonight the radiators furthest away from the tank are not as hot.

I will have to invest in a radiator thermometer or try and get the guys who fitted it to come back and finish the job. I am still waiting on their reply to my letter.

Kev, are you saying that TRV's are unsuitable for 8mm pipes?

 

[kevplumb]

no just the amount of flow restriction if your trvs shut down dont think the one rad without would lose the heat quickly enough
wich seems to be your problem

 

[macnic0]

Thanks that's really useful. Is the only way to solve that to have another TRV removed to leave at least another rad on full flow - or would leaving one or more TRV on full flow (top number) maybe alleviate the problem?

 

[kevplumb]

possibly
try it

 

[macnic0]

We have put one on full blast and will see if it makes a difference.
Will post back in a day or 2 with any results.
thanks again.

 

[ChrisR]

Mac it looks like an Automatic Bypass Valve is what's needed. This is a "short circuit" - it opens when the pump pressure goes high because trv's are shutting. It gives the boiler a chance to control itselft by getting the water to go through it quickly enough. If it doesn't do that you can get localised boiling. There may also be a heat dissipation issue - see the boiler mfrs recommendations for the bypass loop.

You need to look at the pump output graph to get the flow required, and set the pressure accordingly on the ABV. Problem with microbore systems can be that the pressure you need , to get the remote rads warm, is pretty high, so you don't get much flow through the bypass.

About 90% of byasses are just a bit of pipe with a valve in it. These are nowhere near as good as using an automatic bypass valve.