Help with Pump Over

[Mazzy]

Hi There

I am hoping some of you may be able to help me with a Central Heating issue that has been on going since moving into our first home 18 months ago.

I am not sure how much detail to go into so here goes...

Basically 2 of our downstairs radiators don't get as hot as upstairs even after balancing ( 3 story house, circulator pump on 3). We have also had problems with the boiler locking out.
A service engineer came out and suggested the system was sludged, so advised draining the system and putting in a cleaner for 2 weeks.
We did this (water slightly grey) and even before the cleaner had a chance to do anything the radiators upstairs were red hot and downstairs was a little better.
When refilling the system a small spurt of water came out of the vent pipe! Strange!?
A month on and we seem to be back to square one.

We have now been recommended to have a magnacleanse flush done (full powerflush might put the system under too much turmoil - parts of it are quite old). Have a magnaclean fitted, a gate valve put on the HW tank to restrict the flow (this hogs all the heat when both CH and HW are on together) and also seal the system.

After doing further investigation myself (due to getting so bogged down in so many different options and not knowing what to do for the best) I have discovered that when either the CH or HW are started the pump sucks air into the vent pipe when it first starts. Then when it turns off at the end of the programmed time it spurts out maybe half an egg cup of water.
The header tank water is slightly rust coloured with loads of weird jelly looking stuff.

I have assessed the pipework and based on other threads I think our pipework is kind of ok - but not perfect.
The flow from the boiler has the vent, then cold feed, then the pump. The vent is directly upward on the flow pipe from the boiler and takes a sharp right hand bend and curves up into the roof eaves (this finishes just over the lip of the header tank - with out a 45cm loop) Just before the vent pipe the flow tees off horizontally and then tees again up to the cold feed and down to the pump. Other than the incorrect vent pipe height over the header tank the only slight issue I can see is that the vent and feed pipes are actually 250mm apart, rather than the recommended 150mm.
Could this be the cause of the suction and pump over?
Would fixing this and the vent pipe over the header tank fix the sludge issue?
Is sealing the CH system too drastic? and would our old pipe work deal with the excess pressure?

Any advice would be most welcome as we are new to all this.
Thanks Maz

 

[Best]

Having the heating system converted to a sealed system is the prefered way because sealed systems have no problems with pump over or air absorbing, air gathering in radiators and bad corrosion. But you need to have your heating installer decide if your boiler can be sealed and if the system is worth taking the chance on sealing. You would have to take the risk.
But if you keep it all open vented, get the faults with the close coupling etc fixed and have the rads that aren't working well removed and flushed out with a garden hose, plus have each of their rad valves opened into a hose to see if that improves it all

 

[Mazzy]

Thanks for your reply Best. I understand the many positives of a sealed system and is definitely something we would like to do, we just don't understand really how they work. We are worried that if we don't fix the pump over and suction issues that they would potentially still cause problems in a sealed unit? Putting excess pressure in the vessel or wearing out the rubber diaphragm quicker etc.
We have such a large list of things that have happened in the last 18months, that we feel like we are going round in circles with all the ifs and buts and many opinions from each engineer that comes to help.
Perhaps we shouldn't try to understand it and just let someone loose on it! Haha
thanks again

 

[Best]

No, your thinking is wrong. If the system can be sealed, you will have done away with the small heating top up and vent tank and the vent and feed pipe completely. Sealed system literally means sealed. It works using an expansion vessel which has an air charge on one side of a diaphragm to allow expanding system water as system heats up. Also system needs a pressure relief valve and a filling loop and pressure gauge.
The problem I see you have with your open system at present is the issue of pumping up or any air absorbing. Pumping up in itself will cause oxygen to mix with system water creating terrible corrosion in all metals in system and inhibitor will not prevent corrosion if system remains pumping up.
At the very least get the pipework sorted on open system, but definitely explore the possibility of having the system sealed, after first having it cleaned and any leaks fixed

 

[Billy Bob Bob]

Converting to a sealed system would greatly help in your scenario,all depends if your boiler is suitable

 

[Sparkgap]

Sounds like the problem is air in the system, not pumping over. Doing a back of *** packet calc the difference in water level between feed and vent connections would be about 10mm if that. I suspect there is a pocket of air somewhere and when the pump starts up it compresses it. When the pump switches off the compressed air pushes back the water through the path of least resistance up the vent.
Jelly stuff, sounds like you could do with some biocide. Suggest cleaning out the tank and dosing with some bleach, then flush out and refill with an inhibitor which includes a biocide. Could be some of the stuff has got into the system and bunged the pipes up a bit.

Is your boiler on ground floor or upstairs? Is the pump integral with the boiler or separate? Is the locking out due to flame failure or overheating? If yours is a new built house (maybe not, they tend to go for sealed systems and combis) it could be that if you have TRVs on the radiators and they shut down you aren't getting enough flow through the boiler, but there should be an automatic bypass valve somewhere to let this happen. Another thought is if the installer is relying on the integral boiler pump, these only have a limited head available and could be it's not man enough for the whole system.

Some thoughts to be going on with.