What's the purpose of a split before 3 port motorised valve

[diystar1980]

Hiya,
My home (1980s build) has gravity fed central heating system. I understand the purpose of pump and the Y plan 3 port motorised valve near hot water tank in the airing cabinet. But what I don't get is why there is a split before the 3 port motorised valve. I am referring to the split shown by red arrow resulting in hot water going along yellow arrow. Is its normal and is a return back to the boiler? Any thoughts??
Thanks

 

[ianmcd]

looks like a by-pass

 

[diystar1980]

looks like a by-pass

Yes it is, I did some search on bypass and came across automatic bypass valve and now I know it's purpose. Thanks

 

[Mottie]

I didn’t think you needed a bypass for a three port valve?

 

[picasso]

Depends if the radiator side has has trvs on all rads , if so then the (automatic) bypass keeps the flow rate through the boiler constant.

 

[diystar1980]

Yes, we have fitted TRVs on all rads. There is one room thermostat. However we keep TRVs on two the radiators in living room at Max, rest are set to various levels between 2-5. One of our rads in living room isn't getting enough hot water hence I was trying to userstand the reason why this bypass exists. If its not needed in this case then I can try shutting the gate valve on bypass completely or reduce it further down as currently it gets as hot as the A output from 3 port valve.

 

[ianmcd]

Yes, we have fitted TRVs on all rads. There is one room thermostat. However we keep TRVs on two the radiators in living room at Max, rest are set to various levels between 2-5. One of our rads in living room isn't getting enough hot water hence I was trying to userstand the reason why this bypass exists. If its not needed in this case then I can try shutting the gate valve on bypass completely or reduce it further down as currently it gets as hot as the A output from 3 port valve.

I wouldnt close it completely, even if the TRVs are set at max they can still shut down, by all means reduce it, you dont have an auto by-passs you have a manual one

 

[diystar1980]

I wouldnt close it completely, even if the TRVs are set at max they can still shut down, by all means reduce it, you dont have an auto by-passs you have a manual one

That's right, half a turn down. Or fitting new auto bypass valve instead of current gate valve could be another option.

 

[Mottie]

Or remove the heads from the TRV's in the room with the thermostat?

 

[Madrab]

On such an obviously old system I would probably be more inclined to look at the bypass as being there to avoid pumping over when in hot water mode, due to the resistance through the coil. Even if TRV's were fitted to the system from the start, It would have been the norm to have a manual stat and a TRV free rad in the same space (usually the hall)as a CH system bypass.

It will still bypass if there is now more resistance through the rad circuit, especially if modern TRV's have been fitted. I would recommend you perform a balancing exercise of the CH first before you start to alter bypasses.

 

[ianmcd]

try closin all the other rads down to closed and if the poor performing rad heats up well, leave it like that for a few hours and see if it works better after that

 

[diystar1980]

try closin all the other rads down to closed and if the poor performing rad heats up well, leave it like that for a few hours and see if it works better after that

I've tried closing TRVs on all rads except the one that was not getting hot enough (living room) and it did result in that radiator getting piping hot correctly. This ruled out the possibility of any clog in the pipes and any air in the rad itself. Then I tried turning lockshield on all upstairs radiators down by half a turn to increase flow to the downstairs radiators. This did improved that one radiator slightly, however not to the extent needed. This living room has another radiator and that does get piping hot under all conditions. So I suspect the one that dosen't get hot enough is last one in the circuit and is being starved off by others or too much water going through bypass valve or the 3 port valve is sending heat to hot water tank all the time. The next thing I am going to try is turning off hot water at programmer switch and run only heating for a day and see if there is hot water at the taps. This is to rule out if the 3 point valve is working as it should by restricting water to the hot water tank and only passing heat through to the CH circuit.

 

[diystar1980]

On such an obviously old system I would probably be more inclined to look at the bypass as being there to avoid pumping over when in hot water mode, due to the resistance through the coil. Even if TRV's were fitted to the system from the start, It would have been the norm to have a manual stat and a TRV free rad in the same space (usually the hall)as a CH system bypass.

It will still bypass if there is now more resistance through the rad circuit, especially if modern TRV's have been fitted. I would recommend you perform a balancing exercise of the CH first before you start to alter bypasses.

There was a manual stat in the hall and all radiators free of TRVs. I changed the stat to a Honeywell DT90e and installed modern Drayton TRVs on all rads. Your recommendation of balancing is good one and I better do that before looking into bypass valve and the 3 port valve.

 

[ianmcd]

If the problem rad works fine with the HW off , then you need a restricting valve on the HW return

 

[ETAF]

Note a DIYer NOT in the trade - so this may not be the best way to balance

Then I tried turning lockshield on all upstairs radiators down by half a turn to increase flow to the downstairs radiators.

When you had them ALL OFF and the rad was hot , i would have turned the bedroom rads up a little allowing a little water through until hot and see if the lounge one is still hot and repeat until its all balanced - There are lots of instructions on how to balance a CH system , but not just turning half a turn down.
I had an issue with a new rad in cloakroom i fitted and playing around with the 5 rads on that part of the circuit - others are microbore on a manifold , I managed to get ALL rads working fine