central heating: bad water gushing noise after pump change

[Martin]

hi,

i've just changed my central heating pump. When i put the pump on speeds 2 or 3, there is a very loud "water gushing" noise within the vertical pipes that feed the radiators. On speed 1 there is no noise.

The pump sits vertically above the boiler, and the previous pump (fitted by previous house owners) had the direction of flow DOWN into the boiler. The installation leaflet with my new pump suggested that I have the flow UPWARDS away from the boiler (unless some air vent it fitted somewhere) - this i have done.

I'm wondering if a) My changing of the pump has introduced air into the system which i will need to bleed out - and this is the cause of the noise. b) The changing of the direction of flow has done something to introduce the gushing noise. c) something else

thoughts anyone???

[Dave Plowman]

It's bound to have introduced air, unless you fitted it in a vacuum.

[BigWallop]

The original pump sounds as though it was fitted on the return side of the heating system, drawing the cooler water back into the boiler, so you have actually reversed the system flow. Not a really good idea.

[Dave Liquorice]

If you can change a pump without letting some air in it's a neat trick. This could be source of noise but wouldn't last long before the air got trapped at some high point or vented.

This is the likely cause, you could be now drawing air in down the vent pipe. The fact that the harder you "suck" the more noise you get points to this.

You need to work out how the system is plumbed. Is the pump in the flow or return from the boiler? Flow comes from the top of the heat exchanger, return to the bottom. I'll assue a normal open vented system rather than a sealed one, so you also need to investigate how the system is connected to the feed and expansion pipes. Feed to the return, expansion on the flow both close to the boiler. Or feed and expansion to the same pipe within 150mm of each other, this ought to be close to the top (flow) connection of the boiler.

[Set Square]

I assume you've got a vented system rather than a sealed pressurised one?

Depending on how your feed and expansion pipes are connected, you could be getting pump-over now that you have reversed the pump direction. When you get this gushing noise, have a look at the small header tank in the attic and see whether there is a constant flow of water out of the up-an-over expansion pipe. If so, this is very BAD and must not be allowed to continue.

I guess the simplest advice is "if it ain't broke, don't fix it"! It it worked perfectly happily before with the pump in the return side, why not turn the new pump round and restore the status quo? Incidentally, your boiler may have clearly marked flow and return connections. If your pump is running the wrong way, these will be reversed. I don't know how significant that is, but the connections are presumably marked for a reason.

[Set Square]

Yes, but not very much if there are operational isolation valves either side of the pump.

[Andrew Mawson]

significant

From an engineering standpoint it always puzzled me why nowadays the received advice is to place the pump on the flow side of the boiler. On the return side the water is at least 10 deg C cooler so the pump will last longer there.

Andrew Mawson

[Dave Plowman]

Heh heh. You don't need much to make a noise.

[John Armstrong]

Also, some radiator valves need the flow going in the right direction.

[John Stumbles]

heh, heh - the pump manufacturers obviously cottoned onto that ;-)

-- John Stumbles

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[Martin]

all,

thanks for all the adivce.

24hours on the gushing has dissapeared - i guess it was caused by an air pocket introduced by the pump change, which has found its own way to a resting place.

I've checked in the attic, and i can't hear any flow of water out of any up-and-over pipes. I've also checked the boiler installation instructions (BAXI SOLO) and it states that things can be set up with either a sealed (pump on flow side) or vented (pump on return).

I'm not actually sure which type i've got...but in the attic all the pipes are connected to two black containers - a small one and a rather large one. Both with lid's that can be opened - neither of which look particularly air tight, so i guess i have a vented system??

..so now i have a working system (vented ?) with the pump on the flow. All seems quiet, everything running as it should. So shall I leave things as they are, or swap the pump to the return side???

(Knowing what i know now, i would have put the pump on the return side in the first place if it wasn't for the manufacturers instructions being pursuasive as to put it on the flow. What is said was "you should not put it on the return unless you have a vented system", maybe it would have been better to say "you should not put it on the flow unless you have a sealed system" ).

martin

[Set Square]

If you've got a small header tank for the primary circuit in addition to a large one for the hot water system, it's definitely a vented system. I think your instructions could probably be interpreted to mean "if it's vented, the pump can be in the flow *or* the return, if it's sealed it can only be in the flow"

Presumably you haven't moved the pump to a physically different pipe - you've just swapped flow and return pipes by installing the pump with its flow arrow pointing the opposite way to that of its predecessor? So the water is now going the opposite way round all the pipework?

As I said earlier, the boiler (if it's like my earlier Mk 1 Solo) has one connection marked "Flow" and the other "Return". I wouldn't swap these without consulting Baxi - just in case it matters. [For example, the boiler thermostat is likely to be positioned near the outlet pipe. If you make this this inlet - so that the stat is measuring the cooler return flow - this could result in serious overheating]. As others have noted, some thermostatic radiator valves (TRVs) are fussy about flow direction - and won't work if you get it wrong (some - but not all - are designed to work either way round). You need to bottom out all of these issues before coming to a final conclusion.

[Christian McArdle]

Well, normally, you must ensure that the sequence is directly (using copper pipe and a direct route)

Boiler flow -> vent -> feed -> rest of system (valves, pumps, rads).

This ensures safety in the event of thermostat failure. You should really ensure that it happens in this sequence. However, as the boiler is apparently intended for sealed system operation, the consequences of not abiding by this sequence aren't as bad as they might have been (possible explosion) as the boiler should cut out before it becomes an issue.

You really should ensure that the water goes through the boiler in the right direction. Sometimes, the design of the heat exchanger demands it for efficient (or any) operation, particularly on more modern boilers.

Christian.

[Martin]

So to the conslusion! Having noticed the bathroom radiator was filled with air, I bled the radiator. It filled with air again 2 hours later..and i bled it again...and it filled with air again. So i concluded that i was drawing in air from the vent into the system...I swithced the pump around so that it sits on the return side, and all seems ok. Hopefully there ends the story of my first central heating pump change. :)

[BigWallop]

And now that the DIY bug is in your system, you won't be satisfied with just changing the pump. :-))

Glad to hear all is well. Have a great, and warm, Christmas.