Heating & water will only run for half an hour


Postby anglebox » Mon Oct 29, 2007 10:43 am

Both the central heating & hot water will only run for about half an hour when the pump seems to stop. The boiler works ok as it continues for a bit untill the boiler stat cuts in. When the water temp drops the boiler will then switch on for a minute & then back off again.
I had a underfloor heating circuit added earlier in the year which has an idenpendant pump & this seems to work ok.
If the system is switched off on the controller for a while (couple of hours, not sure exactly) then the system can be started again & the pump will work agin for another 30 minutes.
The pump is mounted above the boiler with only 6 inches between it & the ceiling.
Any help or advice would be greatly appreciated.
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Postby The Heating Doctor » Mon Oct 29, 2007 11:50 pm

Has this problem only started after the underfloor system was installed? Which manufacturer is the underfloor heating?
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Postby anglebox » Tue Oct 30, 2007 10:51 am

The polyplumb underfloor heating was installed in January but apart from comissioning has not been used. The problem with the existing central heating/hot water has only apeared in the last few months.
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Postby htg engineer » Tue Oct 30, 2007 5:30 pm

"Both the central heating & hot water will only run for about half an hour when the pump seems to stop. The boiler works ok as it continues for a bit untill the boiler stat cuts in. When the water temp drops the boiler will then switch on for a minute & then back off again."

If the boiler heats up, then cuts out, then comes on then cuts out. But the radiators and the water stay hot, then the boiler is working correctly. The thermostat is controlling the temperature.

If you turn the boiler off for a couple of hours then back on it will stay on until it reaches the temperature again.

You haven't really explained it properly, do the radiators/water go cold before the boiler comes back on ?

Are the radiators not getting hot enough ?

have you tried turning the thermostat up ?
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Postby The Heating Doctor » Tue Oct 30, 2007 10:42 pm

This in my opinion has got to be something to do with the underfloor and the way it has been wired up. A limit thermostat may have been connected to the main pump and not the underfloor pump causing it to sop when a set temperature has been reached, the boiler is not swiching off until the boilers thermostat kicks in. Did a company install the underfloor or was it a DIY project?
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Postby anglebox » Thu Nov 01, 2007 8:39 am

With the hot water switched on only - boiler stat on max & the tank stat on 70c the water is the only way to get just about hot enough to have a full bath.
With the heating on only - All the radiators initialy get hot but then go cool even with the controller on, room stat set at max & the boiler stat at max.
If the water has been on for half an hour (into the pump failure mode) & you switch on the heating the valve opens but as the pump is not going the rads stay cold. And visa versa, the water will not heat if the heating has been on.
If they are on together they both get warm but then go off after about after half an hour & go cold.
The underfloor heating along with 4 extra rads (which are all currently switched off) were installed by a plumber in a 2 storey extension in January & we have not noticed the problem with the existing heating/hot water up untill a few months ago.
Like a lot of people on this forum we suffer with a continual loss of pressure which we have been trying to resolve for years even with the adding of some "sealer" by the plumber in January.
I have managed to check the wiring box in the airing cupboard & across the terminals of the cable leading to the pump there is still 240v when it goes into the failure mode. ie not pumping.
Not sure whats is stopping the pump are there any checks that can be done bearing in mind the access is very limited due to its position.
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Postby marrtin » Thu Nov 01, 2007 3:08 pm

If you are definitely sure there is 230 volts at the pump terminals when in fault mode (not just lighting up a neon screwdriver), this points to a faulty pump. Many induction motors have internal thermal trip fitted. (Not sure if yours has - check). These can be manual or automatic reset. As the windings overheat, then the trip will operate and isolate the power. When they cool, it re-sets.

WITH THE PUMP OFF, check to see if it is partially seized by removing the large centre screw and inserting a narrow bladed screwdriver, turn the rotor shaft. It should rotate freely. If you feel a resistance, you may have found the problem.
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