hot water system plumbing design??

[Thornwhite]

Spent the weekend trying to understand my central heating plumbing system.

Looks like it has undergone several changes in its life.

Once I got through all of the old unused pipes that have just been left in place, it appears to be a fairly standard gravity based system - boiler, HW cylinder, Cold water storage, hot water expansion/feed, a flow pump that powers both HW and CH and return pump that just runs for the CH - but it does have a couple of abnormalities.

There is a device which I have come to believe is an air separator? It sits in the flow line and vents into the HW expansion tank, It also seems to take the system feed from the expansion tank.

These don't seem to be very common in domestic systems. Does anyone have much experience with them - why they might need to be fitted, how useful they actually are...?

The other thing is that I have got a 15mm pipe with a check valve that runs between the flow and the return pipes near the boiler. I am guessing it is some sort of balancing line? Any ideas?

The check valve is currently set for max flow which means that some of the hot water out of the boiler could be returning straight back into it.

Any ideas why the system would have this?
Should I leave it as is or should I switch it to no flow?

Any comments or ideas would be appreciated.

cheers,

 

[Madrab]

That'll be an auto bypass valve that activates when the TRV's on you radiators close or zone valves close and there's boiler pump overrun - It automatically allows a flow through to the boiler to avoid a boiler overheat situation and minimise velocity noise. It needs to be setup to ensure the boilers minimum flow rate is met and limit circulation pressure.
I would suggest that when your next service is due get the engineer to check the autobypass is set correctly as it uses boiler min flow rates and pump setting (head of pressure) to gauge correctly.

 

[Thornwhite]

Appreciate your comments Rob.

The way you explain it, I can understand the need for an auto bypass valve.
I guess what i have is a fairly agricultural version of same. Would you be surprised that this bypass flow is being controlled by a basic £1 isolating valve.

Just for reference, I'm not planning on doing anything much about it, just wanted to understand what it does, why it is there, and whether it should be set to allow flow or not.

if in doubt leave well alone i guess.

 

[kbdiy]

There is a device which I have come to believe is an air separator? It sits in the flow line and vents into the HW expansion tank, It also seems to take the system feed from the expansion tank.

These don't seem to be very common in domestic systems. Does anyone have much experience with them - why they might need to be fitted, how useful they actually are...?

Yes definitely an air separator and well worth having in my opinion. I fitted one some 20+ yrs ago and never had any issues with air in the system. It takes it out before the water circulates through the pump and rest of the system.

 

[Madrab]

I'd be a bit surprised, yes. Building regs require that if a system requires a bypass then it must be an automatic bypass type and an isolation valve doesn't meet this.

Do you need one? It depends Thorn, if you have a radiator that does not have a TRV fitted, i.e. it is always on then, that that allows for pump overrun and keeps flow traveling through the boiler to avoid the overheat so the valve wouldn't be needed unless the boiler manufacturer requires one to be fitted.