3 port valve on central heating - how does it work?
Discussion
I rent a house out via an agency, and the tenant recently complained that the heating was on constantly even though they have turned it off. I had doubts this was a genuine fault, because a month ago they complained that the heating didn't come on, and - despite me leaving clear instructions in the house when they moved in - the heating engineer found that it was because they didn't know they should use the Nest thermostat to turn the heating on..........
Anyway - heating engineer was sent out and says the 3 port valve is faulty.
I thought that the 3 port valve simply routed the hot water to the heating and / or hot water tank - hence even if it was stuck (which I know is a common fault) it couldn't be routing hot water to the radiators unless the Nest had switched on hot water or heating.
But the heating engineer has said that "because the 3 port valve is in heating mode, it is constantly sending a switched live to the boiler".
Is this the case? So the 3 port valve is more than just a valve that opens / closes the flow of water? It also turns the boiler on and off? I thought that was purely done by the other part of the Nest thermostat - i.e. the white box that connects to the boiler.
Anyway - heating engineer was sent out and says the 3 port valve is faulty.
I thought that the 3 port valve simply routed the hot water to the heating and / or hot water tank - hence even if it was stuck (which I know is a common fault) it couldn't be routing hot water to the radiators unless the Nest had switched on hot water or heating.
But the heating engineer has said that "because the 3 port valve is in heating mode, it is constantly sending a switched live to the boiler".
Is this the case? So the 3 port valve is more than just a valve that opens / closes the flow of water? It also turns the boiler on and off? I thought that was purely done by the other part of the Nest thermostat - i.e. the white box that connects to the boiler.
My understanding is, the thermostat moves the valve (motor), which mechanically closes a microswitch. The microswitch causes the boiler to fire.
A sort of failsafe, as the boiler won't fire until the valve has opened.
Usual fault with these valves is in the motor/switch "head", rather than the water valve mechanical part. Simple DIY swap.
A sort of failsafe, as the boiler won't fire until the valve has opened.
Usual fault with these valves is in the motor/switch "head", rather than the water valve mechanical part. Simple DIY swap.
Tailender Investor said:
Yes the valves have a switch that sends the signal for the boiler to fire.
If the valve was stuck closed then the pump would be pumping water around the system against the stuck valve which wouldn’t be very good.
3 port valves don't close as such - even when stuckIf the valve was stuck closed then the pump would be pumping water around the system against the stuck valve which wouldn’t be very good.
3 positions are
Open to HW - this is to the coil in the HW tank - no chance of a dead head there
Open to both HW and CH ie mid position (if all rads are off - the pump can still push thro HW tank coil)
Open to CH only (the only time the pump could be dead heading is all the rad valves were shut and no by pass was fitted)
I can understand that the valve may have a signal to the boiler that allows it to fire - but I didn't expect it to have a signal which causes it to fire. But I guess this is why my day job isn't fixing central heating!
I've authorised the work anyway - I guess their gas bill will be a tad high this quarter.
I've authorised the work anyway - I guess their gas bill will be a tad high this quarter.
B'stard Child said:
Tailender Investor said:
Yes the valves have a switch that sends the signal for the boiler to fire.
If the valve was stuck closed then the pump would be pumping water around the system against the stuck valve which wouldn’t be very good.
3 port valves don't close as such - even when stuckIf the valve was stuck closed then the pump would be pumping water around the system against the stuck valve which wouldn’t be very good.
3 positions are
Open to HW - this is to the coil in the HW tank - no chance of a dead head there
Open to both HW and CH ie mid position (if all rads are off - the pump can still push thro HW tank coil)
Open to CH only (the only time the pump could be dead heading is all the rad valves were shut and no by pass was fitted)
If its wired correctly there's no permanent live to the valve, it can only turn the boiler on if the heating controller is "heating on" and the room stat is on.
Hot water should trigger the boiler directly via the controller "water on" and the tank stat, the valve defaults to hot water mode.
https://wiki.diyfaq.org.uk/index.php/Three_port_mi...
It's easy to wire things up incorrectly though!
Hot water should trigger the boiler directly via the controller "water on" and the tank stat, the valve defaults to hot water mode.
https://wiki.diyfaq.org.uk/index.php/Three_port_mi...
It's easy to wire things up incorrectly though!
Can I jump on this with a question? So I have a very old boiler (system I think). Yesterday the heating didn't come on. We checked the wiring on the kitchen controller, batteries in the thermostat & no issues there. Eventually we did get heat by having the water on. I was worrying because I thought the water would be crazy hot, but it was only warm which I thought was odd. Anyhow I turned the water off when I went to bed & of course the heating went off too (even though the thermostat was set to a temp higher than the actual temp in the hall). Throughout this we had kept an eye on the 3 way valve as I previously had this fail, but on going to bed it was in the H position even though the boiler was no longer 'in' to heat the house. It all seems really confusing so wondered if anyone had any thoughts?
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