Do I need a DBV?

9 Feb 2007
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United Kingdom
I'm adding some extra radiators with TRVs to my central heating. Its a pumped vented two-pipe system. I'll end up with 12 rads, all except one having TRVs.

The instructions with the Drayton TRV say "A differential bypass valve must be fitted to ensure that the pump pressure does not exceed 0.2 bar under all operating conditions".

Do I really need a DBV? Would just leaving a couple of TRVs open do the job?

How can I tell if there's an excess pressure problem?

For all i know there might already be a DBV on the system. Where should I look for it?

In the past I've noticed a loud knocking from the TRVs as they attempt to close. Is this due to excess pressure or just unbalanced radiators?

Many thanks for some advice.
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In the past I've noticed a loud knocking from the TRVs as they attempt to close. Is this due to excess pressure or just unbalanced radiators?


cos they are fitted the wrong way round

ie flow is on the return :idea:
They knock because the differential pressure exceeds the manufacturers guide lines.

In the TRV is a spring which holds the plunger or whatever it's called, as the valves shut the pressure differential increases, to a point where the pressure is greater than the spring, the valve lifts, which lowers the differential pressure so the valve shuts again, and repeats giving you the machine gun effect.
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OP you say 1 of your radiators won't have a TRV so if that has 2 lockshield valves fully open that would do.
but if you have to restrict it at all (to get a balance) then I would recommend fitting the bypass valve at the furthest point you can from the boiler.
Many thanks for the helpful replies.

OK I will get a bypass valve!

There's not much point installing all these TRVs then not being able to use them properly.
AVDO is best, and should be fitted near the pump.

If you put it at the furthest point, you put un-wanted heat into the building.
I see your point doitall, but I do also have a room thermostat controlling the boiler and pump, so the heat *is* wanted (somewhere!) whenever the pump is running. The rad nearest the stat (in the hall) is the one without a TRV, so this makes some sort of sense I hope.

I guess the argument for putting the DBV far from the boiler is to maximise the temp reduction in the bypass circuit, and so reduce boiler cycling?

I'm hoping I can install the DBV far from the boiler, because that's the part of the system I have in bits at the moment (I'm adding radiators in my extension). I was hoping I wouldn't have to mess with the pipes in the boiler cupboard.

I'd like to decide where its going now, because I notice DBVs are available in 22mm and 15mm versions. My pipes at the boiler are 22mm, but at the radiators are 15mm.
22mm by-pass is fine.

The theory is that if the trv's are shutting then the rooms are up to temperature, so you are wasting energy putting unwanted heat into the circuit.

It works just as well either way, provided you have it adjusted, correctly, otherwise the last rad before the by-pass may not work very well.
So just to confirm: Its ok to put the DBV near the last radiator on a pipe run, and as long as its correctly set it will control the pressure of the whole system, including the rads on other pipe branches on other floors of the house?

The only disadvantage is I lose some heat in the bypass circuit?

If this is the case I think I'll go for it. However I've read the instructions for several DBVs and some (e.g. Honeywell DV144) specifically say "install immediately after the pump". Its ok to ignore this??
No, you don't just ignore this, it's where it should be.

What happens if you fit it elsewhere is anyones guess and will be different on different systems, may work, may cut the last rad or several rads off, or it may work fine albeit delicately.
Just to throw a spanner into the works, the instructions for the AVDO show two ways of connecting the valve, as in this diagram:

The left diagram is followed when the boiler mfr specifies a minimum flow rate through the boiler; the right diagram when you are controlling the differential pressure.
Both situations are fine DH, and both will relieve the pressure and provide a circulation through the pump.

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