External bypass valve with modern pump

[MNW67]

We are replacing our boiler (heat only) in the spring and I am just checking a few things.

I have read that the idea of the ABV is to allow a minimum flow through the boiler. When most of the TRVs start to shut, the pressure in the system increases and that opens the ABV.

But what happens if you are using constant pressure or proportional pressure mode? The pressure in the system would not increase, even when the TRVs have closed. So how does an ABV work in this situation?

 

[Mister Banks]

What about when the CH motorised valve closes? You will need the ABV then unless you have a PDHW setup.

 

[MNW67]

What about when the CH motorised valve closes? You will need the ABV then unless you have a PDHW setup.

Could you explain a bit more? I'm not following your train of thought

 

[Johntheo5]

We are replacing our boiler (heat only) in the spring and I am just checking a few things.

I have read that the idea of the ABV is to allow a minimum flow through the boiler. When most of the TRVs start to shut, the pressure in the system increases and that opens the ABV.

But what happens if you are using constant pressure or proportional pressure mode? The pressure in the system would not increase, even when the TRVs have closed. So how does an ABV work in this situation?

In short, you shouldn't use CP or PP mode with a ABV, some pumps like the UPS3 don't even have true CC curves but most arn't too bad, did you have any particular boiler and pump in mind?
But you could install a NO (normally open) 2 port valve as a bypass, that will close once any zone (2 port) valve opens, it will only open when all zone valves are closed, you then only have to ensure that one rad is partially opened (with no TRV) to allow minimum flow when (if) all the TRVs are throttling down.

 

[ericmark]

I have to admit, one not a plumber, and two can't understand why one radiator is left without a TRV, I know my old house the largest radiator in the house output wise had no TRV, or lock shield valve, output control was with the fan speed. This worked A1 with the old non modulating boiler, but for them to work with a modulating boiler I would have thought pipe work needs to be in series not parallel?

Old school and collage it was in series, below each radiator there was a restriction, but mother's house all in parallel and the by-pass valve would open as the TRVs closed, and the return hot water would turn down the boiler output.

I see no reason why output pressure should not be used to control boiler output, but it all depends on how the boiler is designed, the idea of getting a load of parts, connecting them together, and crossing fingers it will work just seems wrong, engineer in this country means degree standard, so when we get a heating and ventilation engineer to design and set up the system they would, we hope actually design the system, if it was easy they would not need an University education so for the homeowner he just hopes his guy knows his job.

 

[MNW67]

In short, you shouldn't use CP or PP mode with a ABV, some pumps like the UPS3 don't even have true CC curves but most arn't too bad, did you have any particular boiler and pump in mind?

It will probably be a Vaillant 418 Ecotec Plus with a Wilo pump.

 

[Mister Banks]

Could you explain a bit more? I'm not following your train of thought

With an S plan system you will have the pump in a short (3min) overrun when both motorised valves have closed, the ABV then opens to allow water to flow hence saving the pump. With PDHW the CH motorised valve will be NO so no need for an ABV if you have a radiator always open.

 

[MNW67]

With an S plan system you will have the pump in a short (3min) overrun when both motorised valves have closed, the ABV then opens to allow water to flow hence saving the pump

In this situation, if the pump is set to constant pressure mode, won't it throttle right back to maintain that constant pressure. In which case, there still won't be enough pressure to open the ABV. I have no idea whether this happens in practice! But it is the logical outcome.

 

[MNW67]

Deleted as confusing!

 

[dilalio]

It's a common issue with modulating pumps... To have or not to have an ABV?

That's when you follow the manufacturer's instructions.

 

[MNW67]

It's a common issue with modulating pumps... To have or not to have an ABV?

That's when you follow the manufacturer's instructions.

Good idea! I've looked at a few now. WB give some pretty comprehensive advice and it sounds very sensible. My situation will be a heat only boiler with three NC zone valves, using constant pressure mode. So they seem to saying I would need a bypass for overrun. But if you are using constant pressure mode, how do you set an ABV so that it opens only during pump overrun but not at other times? Or might it be better to use some sort of diverter valve for the bypass, as suggested by Johntheo5 above, so that it only opens during overrun?

 

[Johntheo5]

A overrun bypass is pretty essential for gas boilers as they have a very small HEX, 2 or 3 litres, vs ~ 20litres for a oil fired boiler, Also the boiler may be running at high output when the last zone valve closes so the HEX has a lot of residual heat. I got the installers of my daughter's now 9 year old Vokera Vision 20S to install a NO 2 port valve close to the HW cylinder which is a good distance from the boiler so the pipework dissipates the HEX's residual heat quite well during the boiler's 3 minute overrun period.

 

[dilalio]

how do you set an ABV so that it opens only during pump overrun but not at other times?

Well you set it so that it opens when the trvs or MCV all close down and when the pressure rises and the pump is dead-heading. During normal CH operation, there should be enough pressure Loss so as it won't open.

 

[dilalio]

You can always tee a towel rail off before the zone valves. Or a hallway rad, depending on logistics!

 

[MNW67]

Well you set it so that it opens when the trvs or MCV all close down and when the pressure rises and the pump is dead-heading. During normal CH operation, there should be enough pressure Loss so as it won't open.

I was wondering if the pump is running at constant pressure, would it slow right down in that situation. Maybe almost to a stand still. So that the pressure never actually rises above the level it is set at (even when all the TRVs and zone valves have closed and the pump is dead heading). The horizontal pump curve at 3.0m shows it maintaining the same pressure all the way down to zero flow. I don't know whether that literally means zero flow! It is very confusing, because it is still using about 10W at zero flow at that setting.