DIY Doctor

radiatoirs all work but only for an hour

Postby martin_of_cotton » Mon Feb 02, 2009 9:04 pm

have indirect system with new boiler (4 years) and new pump (3 months). Probelm is that when central heating first comes on, it all works fine for about an hour then all radiators start to cool down. The boiler is still on and trying to heat water and the roomstat is still calling for temp to rise and pump is vibrating, though very hot. If I stop the pump by turning off heating on programmer, the motor burbles as it stops, the cold feed pipe down from roof tank starts to cool (was hot) and the expansion feed pipe starts to warm up. If I then restart the heating, pump again burbles, cold feed pipe heats up and expansion pipe cools down yet still no hot water being piped around radiators. I then have to stop the pum a second time, re-start and hot water starts to flow round radiators. The system was working fine until 2 months ago, when we had to fit a new pump. It's only since then that we are getinmg this "limbo" state where pump sounds and feels like it's pumping, but somehow, the hot water is not going round the radiator system. We've bled and balanced the radiators and i've checked there is cold water in the roof tank. I also notice that hot water is coming out of the expansion pipe into the roof tank, just a small dribble. I've tried seetting pump to each of it's 3 settings and medium and high power seem to get the radiators hotter than at low power. Someone mentioned that we may be sucking in air from somehwere or the vent pipe needs to be made much higher in the roof space before it returns it's end to the roof tank.

I'm pretty practical where plumbing is concerned buut this flumaxes me. My thoughts are that the pump goes into vibrate state after an hour as it gets pretty hot (on hot feed side of boiler), but no way of checking unless we replace pump with another.

any ideas appreciated. Martin
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Simply Build It

Postby htg engineer » Tue Feb 03, 2009 1:07 pm

I would start by renewing the pump, if the impeller isn't turning freely or it is slightly off centre it will cause the pump body to heat up as you're describing, It seems that the pump is getting too hot and the impeller is sticking and this is why the heating goes off.

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Postby damp_patch » Tue Feb 03, 2009 7:10 pm

[quote="htg engineer"]I had similar problem. When I had new combi fitted the plumber removed old HW tank to find what the initial problem was. He cut it open to say it was a Climatic Water Heater which were popular in the 70's but now banned. The problem was that the system uses air pressure to keep the hot water seperate from the cold by a sort of air bubble. If this bubble is weakened then the hot mixes with the cold. Another symptom was that the "pressure saftey" pipe that led to the cold tank in the loft was drawing in air slowly which caused the pump and therefore system to run with air in the pipes and the pump got hot and noisey. This gave a rush of guggling bubbles through the pipes when I shut the central Heating off. The air would leave the system through the pipe to the cold tank and when started later all was fine for about 30mins.
I cannot find anything about this type of tank on the net but if you mention it to an older plumber he may have come across them. This may be of help.
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Postby martin_of_cotton » Wed Feb 04, 2009 10:53 pm

thanks for suggestions. I got the plumbing engineers back in today who had installed system and they found the problem. At the link pipe that had the cold feed in and the hot expansion out connections, both separated in distance by 100mm, as per one of the diagrams in this forum re a low head system design. This link pipe was 70 percent blocked by sludge, which was starving the pump of hot water from the boiler feed, which caused the impeller and motor to get very hot and eventually the pump impeller jammed.

problem was solved by removing both the 22mm link copper pipe and the 2 x 15mm cold feed and expansion pipes at this junction and fitting push fit plastic pipes of 22mm diameter to all, with suitable reducers to 15mm for the feed/expansion pipes. If i could draw you the pipes in question, it would describe this area of potential sludge blockage better. Moraleof the story, don't use reducers from 22mm to 15mm at this junction area as they will easily block, espcially if the pump is also fitted very close to this junction, where it is sucking water through rather than pushing. I'll also be flushing then filling the system with some anti-corrosion/anti sludge additive within the next few days as the plumber said my system needed a good flushing, under cold water mains pressure if possible after blocking off cold feed and hot expansion pipes, to get rid of any sludge. All radiators now nice and hot and all the way to the bottom of them as well. (which I'd never really had before)
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Postby Steve the gas » Thu Feb 05, 2009 5:38 am

Primatic cylinder?
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