Drained + refilled h/w system - heating works OK, but no H/W

Postby Steve Block » Thu Mar 22, 2007 5:20 pm


I recently drained down the h/w system and refilled it with and added a cleaning agent, with the idea to remove sludge build-up etc.

I took time to ensure that all of the radiators were bleed, started at the top and worked down – not sure if that’s the correct way round. I made sure that the system was fully filled before turning the boiler on again.

However, now only the heating works OK. The radiators all got hot, but for some reason there is no hot water. It appears that the when I set the boiler to hot water only, the boiler starts-up OK, but after about 1 or 2 mins, the boiler stops and appears to be very hot. I’m wondering if it is stopping as a result of any over-heat safety feature ?

Could there be an air lock somewhere in the system that could be preventing the water circulating around the boiler, causing the boiler to start over – heating and cut-out. As mentioned before, the central heating appears to be working fine.

Any pointers would be appreciated ?

Best Regards
Steve Block
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Joined: Tue Mar 20, 2007 9:54 am


Simply Build It

Postby dancbr900 » Fri Mar 23, 2007 11:47 pm

Hi Steve,

You need to open the 3-port valve when filling the system. This will allow the water to run into the cylinder coil, this heats your hot water. You might also have a small bleed nipple next to the cylinder. If so it may help to bleed this as well. Hope this helps
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Joined: Fri Mar 23, 2007 11:38 pm

Postby Recardo » Sat Mar 24, 2007 8:38 am


I have a similar problem to yours - did the exact same, drain and fill adding cleaner agent.

I'm getting heat and hot water but finding when the water & heating are 1st switched on there is about 2 mins of banging and clattering before it settles down.
The boiler seems to cutt off after a while and takes about 20 mins before it fires up again, I suspect its over heating.

I have also noticed that the pump gets very hot and stops too, just making a kind of electrical humming noise.

hopefully the fix to your issue will apply to mine ;)
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Joined: Sat Mar 24, 2007 8:32 am

Postby Recardo » Sun Mar 25, 2007 9:17 am

this migh seem like a really daft question, but where is the 3-port valve likeley to be located? near the boiler?

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Joined: Sat Mar 24, 2007 8:32 am

Postby wishiwascorgi » Thu Mar 29, 2007 12:17 am

if the pump is making that noise you need to open the silver screw on the front of the valve. Water should seep out then place your screwdriver into the hole, if you feel it push back the pump is turning, if not your pump could have seized. If it is seized it will need replacing. The diverter valve is inside the boiler just below the main heat exchange. Usually on the opposite side to the pump as it is on the flow. If the diaphram has split that could cause the intermintant problem or the motor that opens the valve could need changing. It depends what boiler you have
Posts: 34
Joined: Thu Mar 29, 2007 12:00 am

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