When only heating is selected on my program timer the radiators all work fine and are hot. But when both heating and hot water are on the water gets hot but all the radiators go cold. When the hot water is up to temperature on the tank thermostat the radiators still don't get hot until I turn the heating off then the hot water off then on and off again. Then turn the heating on again and it works OK. I have a 3 position zone valve and this makes a banging noise when the water is first turned on. Could it be the valve that is faulty because I have had to replace it twice now in the past 3 years. The current one is about 13 months old. Or could it be the pump that's the problem?
It does sound likely that the 3 position zone valve is the problem, or likely more accurately the actuator that controls the valve is the problem.
To check that it is the actuator: remove the actuator from the valve body (there's normally a button to press which releases it), and then turn the heating on/off and the hot water on/off and see how the actuator moves - does it go into three different positions? (NB, the total rotation from all-heating to all-hotwater is a lot less than you'd probably expect; something like 40 degrees if I remember correctly.)
Possible faults with the 3 position zone valve (off the top of my head):
- The actuator isn't wired in correctly and so isn't receiving the correct signals. If you've previously replaced the valve (including, I assume, the actuator), and wired up the new actuator however the old one was wired, then this might be the problem.
- The valve itself is gunked up / not moving freely. A symptom of this would be that when the actuator is removed from the body it rotates as expected, but then when replaced onto the body it stops moving. Perhaps if you've previously replaced the actuator but not the valve body, this could be the problem.
- One of the springs in the actuator has come loose/broken. The actuator relies on springs to return the valve to the default position when unpowered. Normally the default position is hot water though, so this would be more likely to be the problem if it you were getting heating but no hot water. If this is the problem you'll probably need to replace the actuator. Or if this is happening repeatedly, buy a better quality valve/actuator!
Do you have an indirectly-heated hot water tank, with hot water supplied by the same boiler that heats the house?
If so, it's fairly common for the water tank to take priority, on the theory that fewer people scream about losing a degree or two in the house than scream about their shower going cold.
I'm assuming the whole system was working well in the past. You need to determine what has changed or what event occurred such that afterwards, the initial problem was noticed. I have had capacitors and other electrical or electronic components go bad and when I trace back with input from the customer on what event had occurred that afterward the problem was noticed, a lot of times they mentioned a severe thunderstorm. Even if you think the house and all the houses circuitry is well grounded, a strong lightning strike nearby can float the ground potential for a fraction of a second to several seconds and fry circuits that are active at that time. It is always a prudent measure to unplug everything and shut down all electrical circuits and HVAC before a thunderstorm.
If the 3 way valve has been replaced in less than a year and the newer one after inspection has also gone bad, then there is another factor that is causing the 3 way valve to go bad. I believe the 3 way valve actuator is replaced with the 3 way valve as one unit. If the actuator is determined to be bad then the problem probably stems from the electrical and or electronic control circuitry that controls the actuator. When you disconnect the actuator, check the impedance at the connection wires for shorts or open circuit if it is a passive electrical control system, meaning the actuator is not controlled electronically from a micro-controller (micro-computer) based circuit board.
If the actuator is directly connected to a IC (integrated circuit) based circuit board, you need to contact the manufacturer for troubleshooting and replacement.
If the actuator and the controlling circuitry are good, but the 3 way valve itself is bad then the problem could be extremely dirty or caustic or acidic working fluid. When was the last time you flushed and cleaned the entire system? Did you properly put treatment chemicals into the working fluid?