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
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
c) something else
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.
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.
Dave. pam is missing e-mail
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.
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
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
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
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
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.
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. :)
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.
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