Perplexing heating / water problem

[RedAl]

HELP !

The problem is that I have hot water in the expansion tank in the loft. It's steaming and wetting the felt and timber in the roof.

I have an open vented central heating system with pumped hot water.

The boiler is an oil fired Gemini boiler located in a utility room.

The heating and water are pumped by a grundfos UPS pump sited in a cupboard next to the boiler.

The house is predominantly single storey with just three rads on the first floor.

The expansion tank is located in the eaves of the loftspace above the single storey part of the house and is only around 1M above the highest radiator.

After reading back over some of the similar problems posted here i've done some investigation.

Initially when either the hot water or heating were on individually everything was fine. If both were on together water was pumping over the expansion pipe into the tank.

I turned the setting down on the pump from 3 to 2 which has stopped this from happening. The level in the tank looks 'normal' about 1/3 full.

However, if both the water and heating are on together i'm still getting hot water in the tank. Either the water or heating on its own isn't causing the problem.

Any ideas ?

 

[kevplumb]

and the system is

pumped hot water

bear with me i'm thick and not allowed anything sharp

 

[RedAl]

The system is open vented.

As I understand it the hot water is pumped from the boiler to the tank. There are two valves after the pump one feeding the CH and one to the HW.

 

[kevplumb]

s plan private joke
i would start with the feed and vent arragment

 

[RedAl]

Any clues what I need to be looking out for ?

Cheers.

 

[doitall]

the cold feed position inrelation to the open vent and the pump.

should be boiler-expansion-coldfeed-pump-by-pass valve.

drain a small quantity of water and see if header tanks fills.

 

[RedAl]

Thanks for the advice. I'll see if I can trace what goes where.

 

[felix]

So you have two valves after the pump, one feeding the CH and one to the HW. Are these motorized valves? If so then I can see at least one possible explanation for your wierd symptoms. These valves are supposed to be both OPEN when you want CH and HW together. Could it be that some incompetent plumber has rigged them in reverse, ie each one CLOSES to force water through the other branch. In this case they will both CLOSE when you ask for CH and HW at the same time. Result - the water has nowhere to go.

At this point, another bit of incompetent plumbing will have the pump sending hot water up the vent pipe into the tank. The points at which vent and expansion pipes join the circuit must be very close together on a fully pumped system. If you have them on opposite sides of the boiler or, worse still, the pump you will get a pressure differential and a possibility of trouble.

There is one other way in which water can spew up the vent pipe. Each time the pump stops, the water finds itself impeded by the stationary pump impeller. Unfortunately, you have a lot of moving mass in the pipes and it can't be stopped instantaneously. There is a momentary rise in pressure which can send a squirt of water up the vent. This is most likely to happen if the vent pipe isn't very long. It sounds like yours is rather short. This, however, is a secondary problem. Check those valves first.

 

[RedAl]

I've taken a few things apart to trace the pipes.

I don't seem to have the configuration people here have suggested I should.

The expansion is connected into the top of the boiler on the return side, while the feed is connected in after the union of the HW & CH returns on the return side of the boiler.

So from what I can see the order is Boiler - pump - valves - rads - feed - expansion

Is this likely to be the cause of my problems ?[/img]

 

[felix]

Let's clear something up straight away. the feed and expansion pipes are one and the same. It comes from the bottom of the header tank. The other pipe is the VENT. It hangs over the top of the tank.

Now I'll assume that your layout is boiler - pump - valves - rads/cylinder - feed/expansion - vent - boiler. I find that arrangement slightly odd. The vent should definitely be on the FLOW side of the boiler so that steam can escape straight up the pipe. Are you sure about the flow direction?

If everything really is the way you've described it then closing both valves will leave static water in the return pipe, no matter which way the flow goes. That won't throw water up the vent. Here are some other possibilities:

1) The flow is as you say. The problem arises with both valves OPEN because the flow rate is higher. The extra moving mass in the return pipe throws water up the vent when the pump stops. If this is the case, water will come out ONLY when the pump stops.

2) You have your flow direction wrong. The return is really the flow so when the pump runs there is a pressure drop between vent and feed. That's over and above the one caused by height difference. With both valves OPEN, the flow rate is higher and so is the pressure drop. Water will come out of the vent only when the pump is running.

 

[RedAl]

Apologies for the confusion in terminology.

When I said expansion I did indeed mean vent.

The order certainly appears to be Boiler - Pump - valves - rads/cylinder - feed - vent - boiler

I'm absolutely certain i've got the flow direction correct as the boiler is in the utility room with the pump and valves housed in a cupboard right next to it and there is a big direction arrow on the pump.

There is a large (38mm ?) pipe comes out of the side of the boiler, goes straight into the pump, this branches to two slightly smaller pipes which in turn feed into 2 valves. These then dissappear off under the floor to the rads / cylinder.

On the opposite side of the boiler is another cupboard housing the return pipes which joint back together. The feed / expansion pipe joins in before the pipe connects back onto the boiler. The vent connects higher up on the boiler on the return side.

I'll take some pics and post them up. Everything is very accessible as it's all housed in kitchen units in the utility room.

The initial problem was a constant flow of water through the vent pipe when both valves were open. Turning the pump down from 3 to 2 seems to have stopped this, but i'm still getting hot water in the F&E tank when both CH & HW are on together but not if either are on individually.

thanks

 

[RedAl]

Attached are some pictures of the connections to the boiler.

Pic 1 shows the flow at the top into the pump and then branching into two valves for the CH & HW. These then disappear off under the floor to the cylinder and rads.

Pic 2 shows the two return pipes joining before the feed / expansion pipe joins before feeding back into the boiler at the bottom. At the top the vent pipe can be seen attached directly to the boiler.

I've managed to get the manual for the boiler off the previous owner of the house (who claimed there was never a problem when he sold us the house) and the plumbing seems to be in line with the diagrams in the manual.

I have to admit to being completely baffled.

Please help !!

 

[kevplumb]

possibly a stupid question but which way is the arrow on the pump pointing

 

[RedAl]

It's pumping down.

I've also go the book for the pump (a Grundfos UPS) it says that pumping downwards in a vertical pipe is acceptable but not recommended.

 

[felix]

What a corker! I spent half an hour studying those pictures and kept arriving at the same conclusion. Unless the pump was somehow running backwards, the vent pipe had to be at a lower pressure than the feed. I gave up and went for a cuppa - and then it struck me.

The vent connects to the top of the boiler where the water is gently swirling around. Now look at your feed. The return pipes come together into a single pipe and that's where the feed comes in. IT'S A VENTURI!
Water is literally being sucked down the feed pipe; the faster the flow the greater the suction. This is why things get worse when you turn up the pump and/or open both valves.

To cure this, remove the feed from that return pipe and connect it into the side of the vent pipe just above the boiler.