Problem changing 1-pipe CH system to 2-pipe system

[Steve]

Hi.

I'm trying to change my central heating pipework from an old (I guess 30-ish years) one-pipe system to a two-pipe system. A new boiler in the kitchen will eventually replace the old back boiler, but for the time being I want to run the new pipework with the old boiler/pump.

The old system had a total of about 4m of 22mm pipe with 8mm pipes T-eed off and some 8mm manifolds. The new pipes consist of 2 x 11m 'spines' of 22mm, plus about 20m of 15mm T-ees. There are new lockshield valves and TRVs except for the hall rad which has 2 lockshield valves. My new flow and return pipes are connected to the pump.

The problem is it doesn't work! I've got all valves fully open - the hall rad heats up a bit but the other rads hardly get any hot water.

My only theory is that the old pump (which is a Grundfos UPS Selectric

15-50, on setting '3') can't cope with the extra resistance of the new pipework. Alternatively, and highly possible, is that I'm doing something very stupid.

Suggestions on how to diagnose/fix this will be gratefully received.

Steve

[YAPH]

This sounds very, very unlike a one-pipe system. Are you saying it was like this (ASCII art )

-------- | | | |===== . . . . . . . | | . (rest of system omitted) | boiler | 22mm . | |=====+=+==========. | | | | -------- | | 8mm | | ------- | rad | -------

rather than this:

-------- | | | |=======+==. . . . . | | | . (rest of system omitted) | boiler | | 22mm . | |=====+=|==========. | | | | -------- | | 8mm | | ------- | rad | -------

??

Need to be clear on exactly what it is to start with.

[Roger Mills]

Please explain what you mean by "My new flow and return pipes are connected to the pump".

It that is literally true, the pump will simply circulate water round the rads without it ever going through the boiler! The sequence should be: Hot water comes out of the boiler, through the pump into the flow pipe, through the various rads - connected in parallel - to the return pipe and thence back to the boiler.

Can you draw a diagram of your system and put it on-line, and post a link here, so that we can get a better idea of what it looks like? [Hand-drawn and scanned will do if you can't easily produce a diagram on your computer].

[Steve]

I described it badly - the old system matches your first diagram, the supply and return to each rad are from adjacent T-ees on the same section of 22m pipe.

My new layout matches your second diagram.

Cheers

Steve

[Steve]

Of course, you are right. My description was wrong and the new layout is as you describe: boiler - pump - supply-pipe - inlets on radiator; outlets on radiator - return pipe - boiler

The diagrams in the previous post from YAPH correctly shows my before and after layouts. However I'll see if I can put more specific diagrams on-line. (I can draw diagrams in Word or Paintbox easily, but I'm shaky about putting them online, so this could take a while!).

Cheers

Steve

[Michael Chare]

Don't you need to partly close the lockshield valves on the radiators that heat so as to force the water through the others?

See

thoughts are blockage and air lock!

[www.GymRatZ.co.uk]

If the pump is as old as the system it could be struggling to do anything. Or you just have a real awkward air-lock somewhere. Even in the pump it's self ? IIRC old pumps didn't have any method of bleeding apart from loosening the fixings. Might be worth a go.

Pete

[Steve]

Thanks Michael. I'll try it - but in the old system, all rads got pretty hot. With my new arrangement the hall radiator just gets warm (not hot) and the others are stone cold. So its doesn't sound like a balancing thing. Cheers Steve

[Steve]

Thanks Pete. The pump appears to be relatively modern (currently available on sale) so the original one was probably replaced a few years ago (I've been in the bungalow about 3 years). Yes - air lock or blockage sounds possible. I'll probably have to drain/refill. Cheers Steve

[Steve]

For those interested, here are diagrams of the old and new systems

?action=view&current=oldsys.jpgCheersSteve

[Roger Mills]

Cheers

The new system *should* work ok. Your diagram doesn't show how it gets filled and vented though. Presumably there is a small fill and expansion tank in the attic? Is there water in the tank - i.e. ball valve not seized up? Are there any taps in the fill pipe between tank and system and, if so, are they open?

Your problem sounds to me like an air lock. You need to make sure that

*both* the flow and return pipe to each rad are full of water. Any trapped air could prevent hot water from flowing through the rad. So you need to do a thorough bleeding - just waiting for water rather than air to come out of the bleed screw may not be enough. At each rad, bleed each pipe in turn - by bleeding with one valve closed and then swapping to the other valve. In each case bleed at least a litre of water out of each side - that should make sure that there's no air in the pipes.

Have you already tried turning all rads except one (each in turn) off, and running the pump at its highest speed? That will *sometimes* shift trapped air, and get things moving.

[YAPH]

Then you did indeed have a 1-pipe system. I've never seen one implemented in 22mm + 8/10mm microbore as you describe. Did it ever work? It sounds like incompetence rather than design :-)

(I note you show the pipes to the rads as 15mm on the diagram you posted but you said they were 8mm in your original post. Also you show a break in the loop of 22mm so the flow and return legs are separate: this would have resulted in the entire system flow going through the middle-right-most rad in your picture: I take it that wasn't how it was.)

Your new system as per the diagram you posted is correct. As others have said, assuming the pump is running and there aren't other blockages in the system (e.g. a Y-line filter?) it could be air-locked. How far is the heat getting along the pipes. And are you sure? - double check! - that the pipework is actually connected up as you think it is.

[Steve]

. .

Many thank Roger. Problem cured! I closed all rads except one, put pump on max and thoroughly bled the rad with alternate valves closed. as suggested. The first rad I tried solved it. I had of course already bled the system (yesterday) but obviously didn't do it thoroughly enough.

Many thanks again

Steve

[Steve]

. . .> The new system *should* work ok. Your diagram doesn't show how it gets

Problem solved! As suggested, with pump on max I closed all rads but one and bled it with one valve at a time closed, Problem fixed after first radiator. Must have been an airlock.

Many many thanks

Steve