New-ish house.....central heating badly behaved.

[aztek]

Hello,

We have a problem with the CH in our 5 year old Barratt house.....i've tried for weeks to track the problem down but i'm totally stuck now....i'd be grateful if anyone can help me out.
The boiler is a 'Potterton Suprima 40' (with the temperature knob set to about 70%), the programmer is a 'Potterton Mini Minder E', there's a single room thermostat downstairs, and a cylinder mounted thermostat (set to 60 deg). There's a single Myson pump and a Myson 3-way valve and attenuator. I'm assuming the hot water cylinder is gravity fed by a tank in the loft and the water is heated by the pumped loop which gets diverted to the radiators by the room thermostat. I know very little about plumbing and have tried to read up as much as possible in order to solve the problem before the weather starts to get really cold....but to no avail.
The problem is that when the temperature of the cylinder has reached the required level, the CH never comes on i.e. no pump and no diverter movement. As soon as the water in the cylinder needs heating (say, when i run a hot tap for a while), the motor kicks in and the 3-way valve goes to the correct position. I replaced the valve actuator but this didn't fix it. I put a multi-meter on the actuator input wires and the CH 'ON' doesn't seem to go live when the HW 'OFF' is live....but as soon as the HW 'ON' goes live the CH 'ON' comes on (if the room thermostat is turned up).
Is this a problem with the programmer perhaps? It's worse at night when the hot water cylinder has reached its temperature and we want some heat on but it just wont fire up the pump. Any ideas? Any help will be much appreciated.

thanks,
Bill.

 

[sifu]

There should be a HW satisfied/off connection at the cylinder stat and/or the timer (sometimes only the cylinder stat connection is used). This provides power to the CH when the HW is satisfied via the grey wire of the mid pos valve. Make sure both the cylinder stat's satisfied connection and the timer's HW off connection are switching correctly. This could highlight the faulty component.

 

[breezer]

here is a thought. what happens if you put a thermometer on the hot water cyinder, what temperature is it when it switches off.

I am wondering as you havent changed anything is the cylinider stat reading the correct temperature.

you say its set for 60 degrees but is it really switching off at 60 degrees or is it say switching off when it thinks its 60 degrees but really its say 40 degrees

 

[oilman]

you say its set for 60 degrees but is it really switching off at 60 degrees or is it say switching off when it thinks its 60 degrees but really its say 40 degrees

I've seen this where I was almost accused of not fixing the boiler as it wouldnt come on in the morning. The stat was set at 6o but the operating temp was actually 50, so the water was hot enough for the boiler not to start. There won't be much wrong, as it worked for 5 years.

BTW, when did you last replenish the corrosion inhibitor? nothing to do with the problem, but should be done every 3 years.

 

[aztek]

I checked the temperature of the cylinder just after the thermostat switched off and it read 59.6 degrees. I've got the day off today so i'm going to pull off the programmer box and test the signals coming out of it.

 

[aztek]

I think i've found the problem....i pulled out the programmer and tested the CH 'ON' wire coming out and it was still wrong....so i tried pushing both switches (HW and CH) over to 'always on'....it fixed it!! The central heating has been on all day The 'twice' setting must be broken in some way.

I'm assuming it's just a replacement programmer....i might try an electronic one (maybe an EP2002) as there's less moving parts.

Thanks for the help.