Adjusting Autobypass valve

D_Hailsham · 29 Oct 2007

I have been searching the forum for instructions and came across the following:

ChrisR said:
Simple:
1. See boiler instrs to see what flow rate is required.
2. See pump curve to see what pump head will produce that.
3. See boiler instrs to determine head loss through boiler, then work out for pipe, at that flow rate.
4. Set ABV to pump head minus flow head losses.

If ChrisR reads this could he explain what he means by item 3, particularly the "work out for pipe" bit?

doitall · 29 Oct 2007

Turn the ABV off. then wind the heating up to full blast.

Slowly open the ABV until you have a flow through it, then turn it back 1 full turn.

chrishutt · 29 Oct 2007

D_Hailsham said:
... could he explain what he means by item 3, particularly the "work out for pipe" bit?

He means work out the head loss for the pipework. It's a fairly complicated procedure. As I recall you take resistance of the index circuit, which is the one with the greatest resistance, usually a large radiator distant from the boiler. You need tables to get the resistance per metre for each tube size involved for the flow rate in question.

Doitall's suggestion is much simpler, though I can't vouch for it.

doitall · 29 Oct 2007

It works Chris, crude but effective

D_Hailsham · 30 Oct 2007

chrishutt said:
He means work out the head loss for the pipework. It's a fairly complicated procedure. As I recall you take resistance of the index circuit, which is the one with the greatest resistance, usually a large radiator distant from the boiler. You need tables to get the resistance per metre for each tube size involved for the flow rate in question.

I understand how you need the resistance of the index circuit and the flow rate when sizing the pump. But as the resistance increases, say when TRVs shut down, the bypass needs to open to reduce the resistance to normal. So surely you only need to calculate:

Pump head minus (boiler resistance plus pipe work from boiler to ABV) ?

to get the setting for the ABV? The resistance of the rest of the circuit is not relevant.

I am probably wrong but can't see the flaw in my thinking.

My system at the moment has a gate valve in the bypass (hopefully set correctly) but, as nearly all rads have TRVs, I would like to replace it with an ABV . The only problem is that the bypass is plumbed in 15mm and I can't find a 15mm ABV only 22mm. Does anyone make a 15mm ABV or will I have to change the pipework?

chrishutt · 30 Oct 2007

D_Hailsham said:
But as the resistance increases, say when TRVs shut down, the bypass needs to open to reduce the resistance to normal.

The by-pass doesn't need to open until the flow has reduced to the minimum level required by the boiler. A by-pass that opens earlier than that might be slightly less efficient, though perhaps not significantly so.

D_Hailsham said:
So surely you only need to calculate:

Pump head minus (boiler resistance plus pipe work from boiler to ABV) ?

to get the setting for the ABV? The resistance of the rest of the circuit is not relevant.

I think the setting of the ABV is the point at which it begins to open. With the above you would need to have the ABV letting by the minimum flow rate (required by the boiler) at that point, not just opening. The ABV needs to begin opening just before the total resistance (boiler and heating circuits) has increased to the point of matching the minimum flow rate, not after it has exceeded that point.

I can't help you beyond that point right now because I haven't worked through the process myself. But perhaps that will help you work it out, or maybe ChrisR will come to the rescue.

doitall · 30 Oct 2007

You shouldn't have any problem getting a 15mm ABV from any good plumbers merchants.

Danfos AVDO for example.

D_Hailsham · 31 Oct 2007

doitall said:
You shouldn't have any problem getting a 15mm ABV from any good plumbers merchants.

Danfos AVDO for example.

Thanks for the info. I looked them up and got a shock. They are about £80 while Honeywell 22mm are about £25. Unfortunately Honeywell and Drayton etc only seem to make 22mm.

doitall · 31 Oct 2007

D_Hailsham said:
doitall said:

You shouldn't have any problem getting a 15mm ABV from any good plumbers merchants.

Danfos AVDO for example.

Click to expand...

Thanks for the info. I looked them up and got a shock. They are about £80 while Honeywell 22mm are about £25. Unfortunately Honeywell and Drayton etc only seem to make 22mm.

No they both do 15mm

I will look up me DVD later for you.

chrishutt · 31 Oct 2007

Just use a 22mm ABV (they're about £15! - try Toolstation or Screwfix for example) and use reducers to connect to 15mm pipe - simple and cheap.

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Adjusting Autobypass valve

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