Over flowing into header tank

[Malc]

Hi,

Suggestions as to how I might cure this.

The layout is that the vent pipe is on the inlet side of the pump as in the header feed. The feed comes in looping under then up and the vent comes straight out and up. There is a auto bypass valve around the pump. So all is as it should be.

When the pump turns off there is a whoosh of water into the header tank. I can reduce that by lowering the pump speed. I've raised the overflow pipe as high as I can (low roof). If I only crack a valve in the feed in from the header tank then I think this helps. What it looks like is that the pump stops dead when it turns off and the inertia of the water pushes the pressure up causing a cup of water to go into the header tank.

I was thinking of a check valve so that the water could continue to flow round the pump but the pressure increase may not be enough to over come the spring. The overflow is about 2ft over the header tank water.

I was wondering if a mini-arrestor for water hammer might do the trick?

 

[twgas]

You shouldn't be turning a valve off if its in the cold feed, it shouldn't even be there.
Your problem is most likely a restriction in or around the cold feed/open vent area, if it is piped correctly, as you suggest.

 

[CBF]

as above there shouldn't be any kind of valve in the heating cold feed & DO NOT PUT ANYTHING AT ALL IN THE VENT PIPE

 

[45yearsagasman]

Find the restriction in your pipework and clear it, it will fix your problem.

 

[dcawkwell]

open vented has no place in a modern heating
system go to a sealed system if possible.

 

[Malc]

You shouldn't be turning a valve off if its in the cold feed, it shouldn't even be there.
Your problem is most likely a restriction in or around the cold feed/open vent area, if it is piped correctly, as you suggest.

The valve is the cold feed is for draining down, saves tying up the ballcock or turn off all the cold water. I've actually got 2 with a drain cock in between which allows me to flush the header tank to waste without throwing away all the rad water and inhibitor. Or to add chemicals into rad system directly without needing to wash them down from the header tank (I still add a bit to header though).

I should add I have a problem with fur growing in the header tank. I reckon that this is in part due to warm water being put into it although I think that the overflow has stopped I think it's marginal I reckon so might still do it say when some TRVs have shut.

Also I'm going to change the chemicals to fernox which does have a biological inhibitor. While I've drained the system it makes sense to look again at this overflow issue.

The closing the cold feed tap a bit is not ideal I'd agree. It still wants to be open but with my 2 tap setup I can see that there is still flow via my drain cock.

My theory behind the cold feed restriction is a pressure pulse firing up the vent and then sucking water from the cold feed, hence restricting the cold feed would damp the pulse. I guess I could check that by having the boiler off then run/stop the pump to give the pressure pulse. Tap shut should give no surge at all if it needs the water from the cold feed. The more the tap is open the higher the surge should be. Tricky to tell though. You can here the water slosh up the vent pipe but unless it overflows you don't have a measure to how much that is.

But coming back to the posts:

1. There is no restriction in vent pipe. I've just raised the height of the loop over.

2. I don't understand why a restriction is the problem. The issue is that the surge happens when the pump turns off so it's akin to water hammer.

3. The pipework is routed like this. Boiler on wall in kitchen. Up to ceiling level then a ~2m horizontal run to airing cupboard. Output side of boiler goes to pump mounted vertically so it pumps up then flows back down to floor level to feed radiators (vent in loop for bleeding). In horizontal run, say 1ft from pump is cold feed join this (boiler outlet) pipe from below. 2in before that is vent pipe which goes up to loft. So both feed and vent are near enough at same point and come off outlet.

Also, which might be relevant, the system used to be a back boiler in the lounge with a pumped side and gravity feed side. It was converted to a kitchen boiler setup by using the gravity feed pipes so the pipe work is kitchen to airing cupboard (22m x ~2m), airing cupboard to lounge (28mm x ~10m), lounge to rads (22+15mm). The pump used to be in the lounge next to the fire (back boiler). Hence there are some very long pipe runs.

 

[Malc]

You shouldn't be turning a valve off if its in the cold feed, it shouldn't even be there.
Your problem is most likely a restriction in or around the cold feed/open vent area, if it is piped correctly, as you suggest.

The valve is the cold feed is for draining down, saves tying up the ballcock or turn off all the cold water. I've actually got 2 with a drain cock in between which allows me to flush the header tank to waste without throwing away all the rad water and inhibitor. Or to add chemicals into rad system directly without needing to wash them down from the header tank (I still add a bit to header though).

I should add I have a problem with fur growing in the header tank. I reckon that this is in part due to warm water being put into it although I think that the overflow has stopped I think it's marginal I reckon so might still do it say when some TRVs have shut.

Also I'm going to change the chemicals to fernox which does have a biological inhibitor. While I've drained the system it makes sense to look again at this overflow issue.

The closing the cold feed tap a bit is not ideal I'd agree. It still wants to be open but with my 2 tap setup I can see that there is still flow via my drain cock.

My theory behind the cold feed restriction is a pressure pulse firing up the vent and then sucking water from the cold feed, hence restricting the cold feed would damp the pulse. I guess I could check that by having the boiler off then run/stop the pump to give the pressure pulse. Tap shut should give no surge at all if it needs the water from the cold feed. The more the tap is open the higher the surge should be. Tricky to tell though. You can here the water slosh up the vent pipe but unless it overflows you don't have a measure to how much that is.

But coming back to the posts:

1. There is no restriction in vent pipe. I've just raised the height of the loop over.

2. I don't understand why a restriction is the problem. The issue is that the surge happens when the pump turns off so it's akin to water hammer.

3. The pipework is routed like this. Boiler on wall in kitchen. Up to ceiling level then a ~2m horizontal run to airing cupboard. Output side of boiler goes to pump mounted vertically so it pumps up then flows back down to floor level to feed radiators (vent in loop for bleeding). In horizontal run, say 1ft from pump is cold feed join this (boiler outlet) pipe from below. 2in before that is vent pipe which goes up to loft. So both feed and vent are near enough at same point and come off outlet.

Also, which might be relevant, the system used to be a back boiler in the lounge with a pumped side and gravity feed side. It was converted to a kitchen boiler setup by using the gravity feed pipes so the pipe work is kitchen to airing cupboard (22m x ~2m), airing cupboard to lounge (28mm x ~10m, old gravity pipework), lounge to rads (22+15mm, old pipework). The pump used to be in the lounge next to the fire (back boiler). Hence there are some very long pipe runs.

Update: This link //www.diynot.com/forums/viewtopic.php?t=158037 seems relevant to my problem. The keyword is surge which I hadn't searched on before. My overflow is also about 300mm above the top of the F&E tank and is touching the roof.

 

[Agile]

So wrong you post it twice!

 

[Malc]

So wrong you post it twice!

You'll have to explain that as it makes no sense to me.

Oh, OK, now I see that the forum has posted twice when it should have just edited the one post with the update.

 

[CBF]

it's wrong because you shouldn't have any valves in the cold feed & by you turning them off you are introducing a point that it could block (look at any drawing of a open vented system you can find anywhere not one of them will show a valve in the cold feed why do you think that is ? because it shouldn't be there), like others have said you have a blockage or restriction in the pipework

 

[Malc]

The valves in the cold feed are gate valves which are normally fully open so no different from pipe work. It seemed to me that restricting the flow reduced the overflow into the tank. A bit like there was a pulse sending the water up the vent pipe and sucking in fresh from the cold feed. But that may be just my imagination. Even with the cold feed taps fully open there is still a surge.

And in any case the cold feed pipe is not part of the main pumped route. It's a 15mm pipe that comes in underneath and up to join the pumped route. The whole idea is that water doesn't flow in the cold feed on a regular basis. That's why you go down then up, to stop thermal cycling. The cold feed is just there to top up system losses when you bleed radiators. A regular change of water would lead to a warm F&E tank, heat losses and moisture in the loft space.

If there is no water flow in the pipe I don't see what difference a restriction or not makes (and the valves are 3-4 ft away from the 22mm join. And if anything I though restricting the flow helped (something I'll have to check after xmas).

I suppose it depends on exactly what is happening. Clearly there is a surge but for water to flow up the vent pipe new water has to come in somewhere, or radiators/boiler have/has to pull in to reduce the volume of water (which I can see is the case), or there is air pocket which expands (I don't think so).