Radiators hot even when only HW is calling - confusing
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Radiators hot even when only HW is calling - confusing

by tpot4 » Thu Feb 17, 2022 4:49 pm

I'm really confused about what is going on here.
The symptom seems simple: hot radiators even when CH is off but HW is calling.
But this one is confusing.

I have a Y-plan system...

Boiler: Potterton Flamingo 50s (pretty old but serviced).

Pump: Katsu circulating pump (model 151711).
Mounted vertically.
On 'flow' side with flow upwards towards the diverter valve.
On lowest setting of 45W

Diverter valve: 22mm 3 port ball valve. A, A/B, B.

Diverter Actuator: Corgi A3MID-C

Controller: Horstmann Centaurplus C27 series 2

Room thermostat: Honeywell, manual, circular wheel

Standard hot water tank.

Just to confirm a few things:

The arrow on the pump is pointing upwards toward the diverter valve.
The room thermostat is working (checked the CH calling terminal with neon screwdriver)
The controller is working (checked the CH calling terminal with neon screwdriver, and the other terminals too)
In an attempt to fix this I have just replaced the diverter valve and actuator.

The problem still persists.
So I wired up the old actuator, attached to the old ball valve, in parallel with the live one.
I did this so that I could see what the new one was doing (Big assumption is that I replaced a working actuator and valve when I needn't have).

So when HW is calling and CH is not, the parallel valve is firmly closing off the CH port.
But my radiator are still hot!

I hope someone can help me.

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Re: Radiators hot even when only HW is calling - confusing

by ericmark » Sun Feb 20, 2022 4:39 pm

I have seen radiators get hot in upstairs rooms due to thermo syphon, not likely with micro bore, but with 22 mm and 15 mm have seen it happen.

In the main it happens when central heating has been on, I had it in one house if central heating boiler switched on from cold to heat DHW then radiators stayed cold, but if system still warms upper radiators heated up.

The fitting of TRV heads on the up stairs radiators stopped it.

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