air in pump

janeoxon · 22 Feb 2006

We have a problem over the last couple of weeks with air gathering in the central heating pump. We bleed it all out and a couple of days later can hear that it\\\'s back again, usually a fair amount! (can\\\'t be good for the pump as it\\\'s getting very hot)
Any clues as to what the cause and cure could be please?

chrishutt · 22 Feb 2006

Any clues as to what system you have - boiler, sealed or open, hot water gravity or pumped, age and history (past repairs, etc.) of system.

janeoxon · 22 Feb 2006

Apologies, I should have given more information.
The boiler is a Potterton Nettaheat and is more than 20 years old. I don't know the difference between sealed and open, how do I tell?
I guess that the hot water is pumped as air is gathering in the pump ?!
We had work done last year to sort out the 3 way valve (?) (silver honeywell) as it wasn't working properly.
Heating system is designed with seperate thermostats upstairs and down so that heating in the two areas isn't necessarily on at the same time or temperature.
Hope this helps.

chrishutt · 22 Feb 2006

So you have a 3 port motorised valve with 2 heating zones and presumably pumped HW! Something wrong there, I think. Are you sure you have only the one motorised valve? Does the boiler also heat the hot water cylinder?

Your system will be an open one, fed by a small tank in the roof space (or similar high level). Can you check this tank to see if there is anything unusual, such as lack of water, water pumping out of the vent pipe (copper pipe inverted over tank), tank water being hot, etc.

Also can you possibly drain a little (half bucket) water from a drain point on the system and check if the tank refills to replace the lost water?

janeoxon · 22 Feb 2006

You are quite correct, we do have two motorised valves, and yes the boiler also heats the hot water cylinder.

Tomorrow will go into the roof and check the tank as you suggest and will also carry out the other check that you have suggested.

Thank you for your extremely prompt and helpful response. I will let you know how we get on with your suggested checks.

chrishutt · 22 Feb 2006

we do have two motorised valves,

If you have three heating zones (HW and 2 x CH) you should have three motorised valves (if 2 ports), or two if one is a 3 port (three pipes) although this would be unusual. Just interested to know which.

janeoxon · 23 Feb 2006

Two motorised valves, one has three pipes as you quite rightly guessed.

We have been in the loft and can see no problems with the tank, we have also drained some water out of the system and it topped back up correctly.

I Hope this gives you some useful clues as to what our problem might be.

ChrisR · 23 Feb 2006

Odd that it's just started after 20 years. Air usually gets out of pumps easily enough with the water, they only need venting the once top get the water into the pump.
Look at the pipes after the pump, what's the next high point? Could be the feed to the HW cylinder, (via a valve), which needs bleeding.

janeoxon · 23 Feb 2006

Mmmm... I'm afraid you've now lost me. There's so much pipework in there it's just spaghetti junction to me. I have taken a picture, perhaps you could take a look and advise which bit I should be looking at next?
picture

janeoxon · 23 Feb 2006

sorry should have said -
boiler is directly above the pump and hot water tank can just be seen in the right hand corner of the picture.

chrishutt · 23 Feb 2006

Thanks for pic. Helps a lot. I think your air problem is related to the position of the vent pipe in relation to the pump. It would help if you could post a pic. of the pipework just above the pump and to the left off the boiler. Also how high is the feed tank above the boiler? And what speed is the pump set at?

Incidentally you don't have independent control of the two heating zones! The upper zone can only come on when the lower zone is already on.

janeoxon · 23 Feb 2006

Here we go with the two pictures as requested, I hope they help

The pump is a Grundfos, type UPS 15-60 130 and is set on the middle speed.
Feedtank is in the roof space, and boiler is on the ground floor, guessing at 12' between?

Thank you for taking the time to try and solve this for me.

chrishutt · 23 Feb 2006

Thanks for that. I'm afraid it doesn't help me much though. It might help you if I explain what some of the pipes are.

If we start with the flow pipe from the boiler to the pump. This has a horizontal branch to the left, which divides into two vertical pipes. One of these is the vent pipe, the other the feed & expansion pipe. Both continue up to the feed tank where the vent pipe inverts over the top and the F&E pipe connects at the bottom of the tank.

Following the flow pipe down below the pump towards the 3 port motorised valve, there is a horizontal branch pipe to the right which connects to the return pipe leading back up to the boiler. This horizontal pipe is the by-pass and has a gate valve to regulate the flow.

At the motorised valve the flow divides. the right pipe goes to the HW cylinder (?) and the left to another divide (tee) and the upper and lower heating zones. the upper one is controlled by a second 2 port motorised valve. The vertical pipe immediately to the right of this MV is the return pipe from the upper zone (which can be isolated by closing the two gate valves).

I was wondering if the pump might be sucking air down the vent pipe, but this now seems unlikely. Could you perhaps post a last pic. of the top of the boiler? I think there might be an AAV (auto air vent) or two which might be the source of the air.

croydoncorgi · 24 Feb 2006

It seems no-one has yet suggested checking the two favourite places for air leaks: the stem seal gland on the pump isolating valve (immediately above the pump) and the large nut which connects the valve to the pump body. These need to be 'tight' (but NOT murdered by strangulation!)

If the pump was disturbed when the 3-port valve was fixed, this might have started an airleak, If the pump was originally fitted using large fibre washers, this is likely. OTOH, since it's a Grundfos, it SHOULD have been fitted with the rubber washers supplied by Grundfos. Check whether the large pump nuts are slack, slight 'spongy' but tight, or rigid. If the first, tighten a bit. If the second, there's a rubber washer that's probably OK, if the third, you can't really tell but assume that it's a fibre washer and plan to replace it with the rubber equivalent. You can do this without draining the system - just thank the gods that you've got decent quality isolatos and close them both (above and below the pump) before trying to slacken the big nuts. If they won't move - take it easy! Only use a cold chisel as a last resort, after you've obtained spare nuts!

ChrisR · 24 Feb 2006

My take on this one is different - yer tiz for what it's worth.

"air is gathering in the pump"

No the pump isn't trapping air, the pipes are. Pumps bleed in a fraction of a second which doesn't tie up with "quite a lot of air". It would simply rise up the pipes, unless they're blocked. It would go up through the boiler, guessing from what we can see. Downward pumping pumps are not a good idea because they can trap a tiny bit of air, but it not enough to stop them shooting it out as soon as they start - must be 20% of the country's pumps are put in like that.
Drain the system and take the pump out, makes sure it/its valves aren't clogged. Find where the feed comes into the system from the header tank and cut the pipes round there to clear them out.

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air in pump

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