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Boiler fires up when demand for hot water but doesn't when demand for C/H?

I have a Potterton Netaheat Profile 80e boiler, Honeywell V4073A mid-position valve, Honeywell ST699B programmer, Honeywell L641A cylinder thermostat, Honeywell T6160B room thermostat, Honeywell Sundial Plan wiring centre and a Grundfos Selectric UPS 15-50 3 speed pump.

When there is a demand for hot water the pump runs, the boiler lights and everything works as normal.

However when there is a demand for central heating nothing happens - the pump doesn't run and the boiler doesn't light.

The problem occurred earlier this year and the temporary get-around I used while it was still cold was to set the mid-position valve to manual so that I got hot water round the radiators when the boiler was running for the domestic hot water.

During my investigations so far I have tried a new ST699B programmer but the problem was still there.

I have checked continuity of cables (we had some mice in the loft earlier in the year) and everything seems OK.

I have checked the room stat connections and they are all sound; I have also used a multi-meter at the wiring centre and have confirmed that a circuit is made when the room stat is turned up.

The fact that everything works fine for the domestic hot water leads me to believe the boiler end of things is OK but I am assuming that it is the same circuitry to/from the boiler for both domestic how water and central heating.

Any ideas?

Thanks.

John P.

John Preston
August 2007

Potterton Heating, Home Heating Systems Up Hot Doesnt Ch

Finally got round to fitting a replacement valve head and things now back to normal so many thanks.

Have just read the recent postings on here - these things take time guys - no need to get shirty.

Anyway thanks again.

John P.

John P.
August 2007

John P.

I have exactly the same symtoms as you in that the only way to get the radiators to heat up is to open the mid-posn valve manually. It does not move as it should when CH and HW or both are selected on the programmmer. Also, my boiler and pump only run when HW is selected. If I only select CH they do not run but do run when I also select HW. I have checked that the required signals (240v) are being sent to the valve actuator (assuming all the relevant switches are closed i.e room thermostat switch, HW tank thermostat switch and programmer switch / switches) and they are but valve does not move. I am going to change the valve actuator this weekend so I will post the results.

Tharg
August 2007

Listen to what the guy is saying, he's clearly a good plumber and know s his stuff, as what he has told you freely "is true".

The level locked open is only use to help to drain and fill system, and a nice guide to watch it open and close.

Les
August 2007

waster

hi-spec plumbing
August 2007

been proved wrong or cat got ya tongue

del the plumber
August 2007

if your not going to listen to advise why come on here.

paul (flowline)
August 2007

>I can't see the valve being the cause of the problem unless the operation of the valve is somehow linked into the lighting of the boiler.

of cause it is the order is
wall stat calls for heat giving signal to
diverter valve opens sending signal to
pump followed by
boiler.

then again when temp reached for shutting down.

all the programmer does is give it power when it's time is set.
it don't detect heat that's the wall stat.

so if you turn the wall stat up and watch the lever and it doesn't move across, remove the silver lid on diverter valve and check the motor for current and if it's working.
you'll find it's not and the motor is faulty.

>I can't see the valve being the cause of the problem

that's probably cause your not a plumber.

hi-spec plumbing
August 2007

Moving the valve to 'manual' was just a temporary solution - it means that water flows to both the hot water tank and the radiators irrespective of where the demand for hot water comes from.

The problem is that the boiler doesn't light when there is a demand for C/H - it only lights when there is a demand from the domestic hot water.

I can't see the valve being the cause of the problem unless the operation of the valve is somehow linked into the lighting of the boiler.

As to the new programmer - I went that route as I thought it was the cause of the problem - turned out not to be the case!

John Preston
August 2007

you said you cured the problem by moving the valve to manual.

so why have you fitted a new programmer when the fault is the valve which you have proved ?

hi-spec plumbing
August 2007