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No hot water but rads of hot water system are hot?

Hi, Just moved into a house built in the late 70's. It has an old oil fired boiler which is original and im guessing not much of the system has changed. Boiler fires fine and CH is working well. There is an issue with the Hot Water system, it doesnt heat the hot water cylinder but the the rads off the same system in the bathrooms get hot. The valve from the boiler in the silver box switches on ok, and ive tried leaving it in manual but doesnt make any difference. Pipe on the CH from the boiler up into the floor above is red hot. Pipes into the boiler thought 1 at bottom and 1 midway are cold.

Thermostat on the boiler looks like is was replaced, but the HW switch and controls look like the switch on ok any way and the rads are hot so im guessing it not that.

Only thin I can think of is a valve somewhere I cant find or an air lock? Any suggestions appreciated.


4 Answers from MyBuilder Plumbers

Best Answer

Hi, It could be that you have either 1x 3 way divertor valve or 2 x 2 way zone valves and these have actuator motors on the top. These may have got stuck. They are usually very near to the pump, trace the flow and returns from boiler and see which type you have, whatever type it may have a stuck or have a faulty motor, get a heating /plumber to take a look. They are fairly straightforward to change.


Answered 17th Dec 2013

It may be that the three port valve is faulty but it could be as simple as the flow pipe going to the cylinder is air locked, most heating systems have a air bleed valve beside the flow pipe connected to the hot water cylinder either an automatic valve which is faulty or a manual one that you either use a flat screwdriver or a small spanner to open and release the trapped air. You can check this by touching the top pipe connection on the side of the cylinder it has to be hot to show the system has not got trapped air which has risen to this point.


Answered 7th Feb 2014

It sounds like you have a pumped CH system and a gravity fed Cylinder, or a semi-pumped system. If this is the case then an air lock in the cylinder pipework is most likely. Try bleeding the system off, and also checking for debris in the pipework. At a push, a hose connected to the vent hanging over the F&E tank should produce water flow into the tank through the tank's outlet. If it doesn't, then debris is blocking the pipework.

Posting a job might be your best solution, as it's one of those issues that can call upon many years of experience to properly diagnose.

Best of Luck though!

Dave - DrainMedic


Answered 1st Feb 2015

If the flow is really hot but returns are cold then you have no circulation either pump not operating or valves closed or bad airlock. Have you vented the pipework at the cylinder.
Rads on that circuit getting hot purly by gravity(hot water naturally rising to the high points)


Answered 26th Dec 2012

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