Bypass and MPV

[D_Hailsham]

In another topic,

If you have a 3-port valve, close the bypass as you won’t need one.

I know the theory behind this - the path through the HW cylinder acts as the bypass. But is it always true? No only when the MPV is in the "HW only" position or the mid-position. When it is in the "CH only" position there is still a need for a bypass to allow for the increased head as TRVs close down. This would suggest that, if you have an MPV and TRVs, the bypass should be fitted after the MPV in the CH feed.

 

[corgiman]

there is no need for a bypass on a three port vale as there is NEVER not a port open.

 

[ChrisR]

..unless, as D_H says, you have trv's all round, which happen to be shut!

I reckon the hw cylinder is a good bypass route.

 

[D_Hailsham]

...there is NEVER not a port open.

I agree ; but that is not the point.

The whole purpose of an ABV is to compensate for the changes in pressure in the heating circuit as TRVs open and close and the effect this has on the flow rate. There will be times when only the CH port of the MPV is open; when this occurs an ABV is necessary.

I earlier suggested that the ABV only needs to be in the CH branch but, thinking about it more, I realize this is wrong. The HW cylinder (or the inner coil at least) is no more than another radiator, which needs balancing like the rest of the system. When the MPV closes the HW port this is the equivalent of shutting a radiator and will have the same effect on the pressure in the system. So the ABV needs to be in the flow to the MPV

 

[ChrisR]

The HW cylinder (or the inner coil at least) is no more than another radiator, which needs balancing like the rest of the system.

One could take issue with that! They're a highly variable heating load.

The whole purpose of an ABV is to compensate for the changes in pressure in the heating

...and that. A Major point is to provide a bypass route on overrun.

Did you want the £5 argument or the £10 one?

 

[D_Hailsham]

Did you want the £5 argument or the £10 one?

An argument is a conversation betwen two people, neither of whom know what they are talking about.

I can't remember who said that, but I don't mind a discussion - and I don't charge.

A Major point is to provide a bypass route on overrun.

But that is another matter altogether, which is only relevant if the boiler needs a pump overun.

Let us assume, for sake of discussion, that we are in an over-run scenarioWhen the boiler goes off (HW and CH satisfied) and the pump starts to over-run, the MPV will be held in its last position (230v on grey wire only). This means that it could be either the HW or the CH port which is open. Now "CH satisfied" implies that TRVs have closed so the pressure in the CH side will be just as high as when the boiler was running. So, if the CH port is open, an ABV will still be necessary to relieve this while the pump is in overrun.

If, on the other hand, the HW port is open when the pump is in overrun, the pump is able to cope with relatively low pressure and maintain the flow through the boiler.

My brain is beginning to hurt . I have just managed to argue that my first proposal is after all correct: the ABV should be in the CH branch only.