Hot water pouring into Feed & Expansion tank.

[renard80]

We have just had a new condensing boiler installed (Worcester Bosch 24Ri), CH system flushed and TRVs fitted.

The engineer recommended we set the boiler temperature at about three-quarters of the dial.

Our radiators now seem much hotter than previously, as does the hot tap water despite reducing the cylinder thermostat to 50.

We are experiencing a problem. From time to time, there is a gurgling water sound from the boiler and a rumbling from the loft. Up in the loft, I find hot water pouring from the inverted U pipe into the Feed & Expansion tank. This continues for 5 - 15 minutes before there is a 'click' from either the boiler or the programmer (can't tell which) and the gurgling / rumbling stops instantly.

I have lowered the boiler temperature dial to just below a half, but still we get this problem. It doesn't happen every day, just now and then.

Any advice will be appreciated.

 

[tripty]

Hi,

Sounds like the system is pumping over, water is being pumped up the vent rather than around the rads, what speed is the pump on? Try turning it down if possible.

Hope that helps

 

[renard80]

Hi,

Sounds like the system is pumping over, water is being pumped up the vent rather than around the rads, what speed is the pump on? Try turning it down if possible.

Many thanks, tripty. They had set the pump on setting 3 (max), so following your advice I turned it down to 2 (medium) last night.

However - and this I don't understand - late this morning it suddenly did it again for about five minutes, even though the central heating was off and only the hot water was on. Anyway, I have now turned the pump down again, to 1 (lowest setting).

Also, if the pump is too high, I wonder why it doesn't pump over constantly, instead of only now and then?

 

[tripty]

Ermmmm, have you got any trvs, if you have it may be as they close off the pumping over starts but then why did it pump over when only hot water on.....

Take a look at the water level in the f&e tank, measure the distance between the water level and the highest part of the vent pipe; it should be >450mm. Also, check there isn't too much water in the f&e tank; you only want the water level to be ~30mm higher than the feed/outlet pipe.

 

[TicklyT]

The positions of the boiler, pump, cold feed and the open vent connections may be relevant.

Recommendations have changed over time as boiler and heating systems have evolved.

Older systems often featured hot water heating by gravity circulation. They were often laid out:-
Boiler - Vent connection - Hot cylinder - Cold feed connection - Boiler
with a seperate circuit from Boiler - Rads - Pump - Boiler.

Modern boilers with pumped hot water heating are usually connected:-
Boiler - pump - cold feed - vent - motorised valve(s) - hot water & heating circuits - boiler.

Older boilers had generously sized waterways for gravity circulation, minimising the pressure difference between feed and vent.

Modern boilers, designed for pumped circulation, have more restrictive waterways, so the feed and vent are usually connected close together, usually within 150mm of each other on the same pipe to minimise the pressure difference between them.

If the layout of the system is incorrect, the pressure difference between the vent and feed can lead to water being pumped out of the open vent.

 

[tamz]

The pump needs to be wired back to the boiler which controls the overrun.
Check when you turn off the CH and HW if the pump keeps running for a few minutes. It should if it is wired correctly.

 

[rojaca]

...Boiler, open vent, cold feed, pump.
Open
Crotch
Panties ...is a useful mnemonic I was told

 

[TicklyT]

Sorry, My bad.......
The open vent and cold feed should still be adjacent to each other.

 

[renard80]

Many thanks to everyone for their helpful replies.

I had the heating engineer in today and he thinks he has identified the cause.

On our old system (25 years old . . . ), the pump stopped instantly when both motorised valves (CH and hot water) closed through lack of demand or at the end of a programme.

However, with our new condensing boiler, the pump continues to run for several minutes after the valves close - as tamz points out above. The pumped water can't get past the closed valves but it has to go somewhere - so it gets pumped up the vent pipe and into the expansion tank, causing my problem.
Apparently, the installers should have allowed for this by fitting a diverter pipe (??) to bypass the motorised valves for the period when the pump continues running while they are closed.
My bloke has arranged to return and adapt our system accordingly.

I am not a plumber, so I hope I have explained this properly. I also hope it will help anyone else faced with the same problem.

Thanks again to all you super guys for your suggestions.

 

[tamz]

At least your heating guy knew his stuff
He will just fit an auto bypass which the ones who fitted the boiler should have done.
Glad you got it fixed out

 

[dextrous]

I thought most new boilers came with a bypass fitted as standard nowadays Obviously not

 

[tamz]

I thought most new boilers came with a bypass fitted as standard nowadays Obviously not

Combi's and some systems. Not the regulars

 

[renard80]

Just one more thing:

My bloke is suggesting that when fitting the bypass he also removes the header tank and replaces it with some sort of new-fangled expansion thingy (??) with a pressure indicator. He says I would have to inspect it at least once a year and top it up if necessary.

He has baffled me with science as to why I should have this, but naturally I don't want to shell out megabucks if not needed.

Any thoughts from you professional guys, please? Sorry if I haven't described the thingy adequately . . .

 

[tamz]

He is possibly suggesting a (sealed) system boiler or converting your open system to a sealed system by fitting an expansion vessel. This will do away with your header tank, open vent and feed pipes. the system is then pressurised and is manually filled so you have to monitor this and top up from time to time. It is not necessarily a bad thing and has many advantages over an open system and should not cost much more. Your installer will have (hopefully) based his decision on what he saw.

 

[renard80]

Thanks for that useful advice, tamz..

It's an expansion vessel he is thinking of installing, but as both She and Me are well stricken in years, I don't fancy the idea of either of us having to clamber up into the loft every so often to keep an eye on it.

He says another option is to move the pump into the downstairs airing cupboard, which sounds better from our point of view.

I am going to discuss it further with him when he comes to do the bypass.

Warm regards and more thanks to you.