Pump on flow or return?

My central heating pump is on the flow side, but a friend told me his is on the return.

Seems to make more sense, as the pump is pumping cooler water, which must be better for it.

Why are most pumps on the flow side?

Reply to
Lieutenant Scott
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The pressure throughout (most of) the system changes when the pump runs, = higher at the pump discharge, lowest at the pump inlet obviously. The point= of no pressure change (PNPC) is where the expansion vessel is connected (a= ssuming a sealed system; the pressure at the expansion vessel doesn't chang= e and its water content doesn't change.

It's best practice to pump away from the expansion vessel, since the pre= ssure throughout the system then increases ith the pump on. It makes cavita= tion and the release of dissolved air less likely, as well as negative pres= sures air being drawn in through leaks. The temperature doesn't much affect= the pump location.

Reply to
Onetap

higher at the pump discharge, lowest at the pump inlet obviously. The point of no pressure change (PNPC) is where the expansion vessel is connected (assuming a sealed system; the pressure at the expansion vessel doesn't change and its water content doesn't change.

pressure throughout the system then increases ith the pump on. It makes cavitation and the release of dissolved air less likely, as well as negative pressures air being drawn in through leaks. The temperature doesn't much affect the pump location.

Ah well, this is not a closed system. I have an expansion tank which is open at the top.

Reply to
Lieutenant Scott

at the top.

Same thing, with gravity rather than compressed air providing the pressure. The PNPC is the connection point of the F&E tank. The water level in the tank won't change on starting/stopping the pump, only by expansion and contraction. Pump away from PNPC, usually.

Reply to
Onetap

at the top.

won't change on starting/stopping the pump, only by expansion and contraction. Pump away from PNPC, usually.

That makes sense. But how about placing the F&E tank on the return aswell? Or should that be on the flow to absorb bubbling over from the boiler getting too hot?

Reply to
Lieutenant Scott

With negative pressure, if you try to bleed the radiators with the pump running, you hear air hissing but the water never spurts...

ISTR that there is also, on open systems, a tendency to pumping over if the pump is incorrectly sited.

Chris

Reply to
Chris J Dixon

While it is true that pumps can rely on the circulating water for some "cooling", cooling to typical boiler flow temperatures is more than adequate.

In reality it does not really make much difference. Its in effect a closed system. However it often more convenient since its a place you know the full flow is concentrated into one pipe. You may have several return pipes that join at various places, and you don't want a pump in a partial loop, rather than the full system.

(With vented systems, you also need a little care in selecting the position relative to the feed and expansion pipes since you don't want the pump sucking gobs of air into the system down the vent pipe on startup. (the reason a combined feed and vent works better))

Reply to
John Rumm

Most pumps are capable of creating a head of 1 or 2 metres. Given the major resistance of a system is likely to be the radiator system, most of this head will appear across the radiator circuit.

If a closed system then I would prefer it to be in the return, such that the boiler is under pressure to reduce boiler noise. If in the flow, the water in the boiler would be the system pressure less the pump pressure.

If an open system you have to be careful about the siting of the cold feed and vent. I prefer John Rumm's policy of a combined feed and vent, either a single 22mm pipe or using a vessel designed for the purpose. If there is a limited head above the top-most radiator, it may be best to place the pump in the flow such that the radiators are under pump pressure, either way the boiler pressure is largely determined by the height of the header tank.

From a pump point of view, life is very dependent on temperature, so cooler the better, but that's not necessarily compatible with system design.

Reply to
Fredxx

There is only ONE pipe going into and out of the boiler. The pump can go on either of those. The feeds and returns both split in equal numbers.

Not sure what you mean by "vented". Mine has an expansion tank fitted which is full of water (and refills from the main with a ballcock should you lose any water to leaks or over bleed radiators).

Reply to
Lieutenant Scott

That's what vented means...

Reply to
Tim Watts

That's a vented system. The alternative is a sealed system, with no tank, a filling loop and pressure gauge, and an expansion vessel.

Reply to
John Rumm

I asked because I only have ONE pipe top the expansion tank. Not "feed and expansion pipes".

Hence the pump could not suck air down it, it would need to suck the whole tank of water down first.

Reply to
Lieutenant Scott

Best thing you can do (36 and nothing up top) is to ask your mate rodders (or continue typing bollocks in uk.rec.driving).

Reply to
brass monkey

Get lost stalker.

Reply to
Lieutenant Scott

You need stalking, pal. You're a bloody idiot.

Reply to
brass monkey

Someone has to "mother you" and tell you you're an idiot. You're obviously incapable of noticing it yourself.

Reply to
brass monkey

Grow up.

Reply to
Lieutenant Scott

Look in the mirror.

Reply to
brass monkey

You need a minimum of two pipes on a vented system, however exactly how they are configured can vary.

The Feed and Expansion pipe is *one* pipe, (it serves both purposes - allowing water to feed into the system, and accommodating expansion by allowing water to be pushed back up into the F&E tank). It will connect to the base of the F&E tank - and its open end should always be submerged.

The vent pipe should loop over the top of the F&E tank so that its highest point is higher than the water level in the tank, the top is then typically bent around such that it discharges back into the tank. This provides a *low resistance* path for steam to escape should the primary water in the system ever boil for any reason.

Traditionally both these pipes attached to the system at different places - often wither side of the pump. However you can attach the F&E pipe to the system at one place, and then tee the vent pipe off that a little higher, as this reduces the chance of drawing air into the system, and if placed on the suction side of the pump can reduce the chances of pump over (i.e. forcing water up the vent pipe so that it spews back into the F&E tank, mixing in air into the process)

See above.

Reply to
John Rumm

Only 6 year olds say that, along with "I know you are".

Reply to
Lieutenant Scott

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