central heating not working - motorised valve seems ok

6 Nov 2004
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United Kingdom
I have a Thermecon Option 90/120 boiler.
Hot water is fine, but central heating is not working.
The outlet from the boiler goes through a pump then splits into two pipes, one for hot water, one for central heating. Each pipe has a 2-port motorised valve with a Honeywell head.
This is what I tested so far:
Room thermostat set high
All rad stats set high
All radiators have been bled
Hot water set to off, central heating set to on
When powering up the supply to the boiler, valves, pump etc, the motorised valve for the central heating moves all the way to the 'on' position. Live mains appears on the orange wire from the valve. The boiler fires up, runs for about 2 mins, then shuts down. The first rad gets hot.
So it seems to me that hot water is not circulating through the rads, but I can't figure out why. The pump seems OK because hot water works. Is that a good assumption? What's the best way to check the pump is running when central heating is on?
Is there a possibility of an air lock in the rad loop, even though they've been bled?
What should I check next?
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you can check the pump by undoing the screw on the face of the pump, you should see a small slot where you can put a small screw driver in to it to see if the pump is free ;)
It could be something as simple as the room stat not working. You can check back at the boiler by using a multimeter or linking out the stat.
It could be something as simple as the room stat not working.
This is clearly wrong from what you have said (assuming you tested the CH motorised valve with HW side off). The likely cause is a blockage of debris or air on CH circuit, assuming the pump is working properly. Can you turn the pump up to a higher setting (speed)? Typically they have 3 speed settings.
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This happened to me a while ago.
Even though the pump sounded like it was working. It was faulty.
Once it was replaced, all was ok.
Thanks for all the replies so far.
The pump is already on setting 3
It is a Grundfos UPS 25-80. I tried undoing the screw on the face of the pump, but hot water came out. I closed off the isolator valves either side of the pump, the water slowed to a trickle but continued. With the screw removed, there didn't seem any way to check if the pump is working. No slot that I could see.
I think I still need to determine whether its the pump or a blockage/air.
Are there any tests I can do on the pump in situ, or do I need to remove it to test it?
Where's a good place to get a replacement pump?
Assuming a blockage or air, what's the best way to clear it?
Turning the valves off either side of the pump will not help the pump if its working because its got nowhere to pump to,trust me you will feel the screw driver bounceing off the spindle when you try to locate the screwdriver. you can buy pumps at most d.i.y centres
It is a Grundfos UPS 25-80
This is not the normal domestic size. Those are usually designated 15-50 or 15-60 I think. This is much larger, with consumption on 3 of 245 Watts at 1.1 Amps. These figures are more than double the normal domestic pump.

Do you have an exceptionally large system? Or is it underfloor?
I wasn't expecting that closing the isolating valves would 'help the pump'. I'm just reporting what happened so that you guys who are kindly helping me out get the full information. You suggested undoing the screw on the front. I did that, with the system powered down, and was a bit surprised that hot water started gushing out. I guess I would have expected you to warn me that I could scald myself by undoing that screw, if that was supposed to happen. So I thought it was worth mentioning what happened, in case its significant. Can you or someone else confirm that this is normal, or does it mean that an internal pump seal has failed? Then I mentioned that closing the isolator valves didn't completely stop the flow. This also surprised me - apart from anything else, it has implications if I try to replace the pump - ie I'm going to get wet :LOL: I think there must be particles or scale or something which is preventing the isolator valves from fully closing/sealing. Maybe this is related to the pump problem? Contamination, scale or whatever jamming the pump?

Its not underfloor, just radiators. Not particularly big - three bedroom bungalow. Anyway, I think I should replace like for like if I can find the same one. Any reason not to?

One more piece of info, I tested the system again today, and confirmed that mains is reaching the pump OK when the CH is ON and hot water is OFF (ie when the motorised valve for the CH goes to the ON position, 240V appears at the pump and stays on). So I feel like I've eliminated every possibility except pump failure or a blockage in the radiator circuit.


I guess I would have expected you to warn me that I could scald myself by undoing that screw,
Good point. It is normal for a small amount of water to leak out, and you should have been warned to have a cloth ready to absorb this. In your case it sounds excessive, which may be due to the unusual nature of your pump, or possibly excess wear.

By turning off both pump valves, it should have been possible to check the spindle for rotation - a few seconds of running with shut valves shouldn't do any harm. The valves may shut properly if you open and shut them repeatedly, thus slowly scraping away deposits. If you need to replace the pump, just get a standard domestic one (you don't need extra power or price), but check that the existing distance from union to union is 130mm.
how quickly does the first rad heat up and how far from pump is it? i assume it's quite close to pump but if it goes from cold to hot in 2 mins then i think pump and probably motorised valve must be working, especially if you're getting indirect hot water in normal time. are the upfeeds to 2nd and further radiators cold? does the first rad have a thermostatic valve or is it a sink rad? sometimes thermostatic valve needles stick if not used for a while. otherwise radflush!
I replaced the pump and everything works fine.
Thanks to everyone who posted, I couldn't have sorted this without you guys. Very much appreciated - this is a great forum :D
Just thought I would share this in case anyone had the same problem. My heating was not working but hot water was fine. After bleeding radiators checking header tank and after testing that the thermostat was working. I isolated the problem down to the pump. The power supply to the pump was fine and it became obvious that the pump had packed up. I was just going to check prices online for a new pump when I had a thought. The heating was fine at the end of the last cold season and there was no strange noises from the pump. Perhaps the pump had just got stuck. So I switched the power on and hit the pump a few times (gently) with my lump hammer. Suddenly it started working and within a couple of minutes all of the radiators stared to get hot.

I think I have managed to save myself ££. But perhaps this is a sign of the pump on the way out? and maybe I have just bought a bit of time. if anyone can advise let me know
Why are you posting the same thing on different threads?

This one dates back four years!

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