Failed motorized valves

[Richard Chambers]

I have now had both my Danfoss HPA02 valves fail (hot water and central heating on an S plan). It is easy enough to replace the heads, but the failure mode in both cases has been the micro switch failing to open once the call for heat was removed.
With the heating, we were away and came back to a house at 35C as the room thermostat was no long in control. The hot water similarly had the boiler running continuously, but the valve had turned off so the boiler was trending to boil with the small volume of water - I'm glad I was around to manually turn it off.

The question is: how can I make these valves fail safe. in both cases the micro switch contacts were stuck closed, which meant the boiler/pump was powered continuously irrespective of the thermostat setting? It needs an extra relay or something on the live feed (grey wire) to the valve.
Thanks
Richard

 

[SimonG]

And what happens when your extra relay sticks? Another relay? You can go on for ever.

Things fail, replace with a good brand. Unfortunately nothing is made to last, no money in it.

 

[SJB060685]

In your situation above the only thing governing the water temperature was the boiler stat, that along with the high limit stat should prevent any localised boiling. Is there a by pass on the system?
As Simon says above these things happen. You could go down another electrical component route, you could go as far redoing the system with app enabled control whilst you're away, however i would first consider decent two port valves, honeywell were our first choice.

 

[SimonG]

In your situation above the only thing governing the water temperature was the boiler stat, that along with the high limit stat should prevent any localised boiling. Is there a by pass on the system?
As Simon says above these things happen. You could go down another electrical component route, you could go as far redoing the system with app enabled control whilst you're away, however i would first consider decent two port valves, honeywell were our first choice.

I would only fit Honeywell once upon a time, but even finding them to have deteriorated quality wise.

 

[Richard Chambers]

Is there a by pass on the system?
As Simon says above these things happen. You could go down another electrical component route, you could go as far redoing the system with app enabled control whilst you're away, however i would first consider decent two port valves, honeywell were our first choice.

I do have a TADO controller for the system so I can access whilst away, but if the microswitch fails it don't turn off. I could see the house temperature but unable to control it. There is a bypass in the system but the heat capacity of the boiler (Grant Vortex 26 OIl) means it can boil. I shall replace the 6a microswitches with 10a ones as that is easy. Maybe a add a relay as well. Changing the valves now is a long job as the system is a real pain to bleed.
Thanks for the replies.

 

[snowhead]

A simple way might be to use a stand alone room thermostat set higher than normal operating temperature and connect it at the boiler to break the enable signal.
You'd need to pick a room that doesn't suffer from solar gain otherwise you'll loose hot water when the sun comes out.

 

[SJB060685]

Have you actually encountered localised boiling? Your boiler will have a boiler stat and a latching overheat limit stat should the boiler stat encounter problems. The system pump should not only supply the flow rates and pump head required for your loads but should also supply the minimum flow rates through the boiler. I can see your concern about the small water volume in the loop between the boiler and the bypass and the potential for overheating but the fact your boiler was stilling running after you returned tells me the water temperature never came close to boiling, if it did then the overheat limit stat would of tripped and the boiler would cease to fire and the overheat stats are manual reset only.

 

[Richard Chambers]

Yes I have had actual localised boiling but it doesn't trip the overheat thermostat which is 110C. When the heating valve failed it must have left the valve open, but with the hot water recently all the valves were closed. It's a very old (500 years) house with the boiler on an upstairs landing and it's header tank in the loft above it along with the pump. It was quite difficult to get the required head for the pump and the pipework does go up and down a bit so is a real pain to bleed. The hot water main pressure tank is on about the same level as the boiler with various self bleeding valves on the flow/return. The bypass is only about 4m from the boiler so is a pretty short run.
Snowhead: I had thought about a second thermostat as an option, but if that came into play, would it become the primary thermostat and would control the temp.
I am tempted to put a Sonoff Mini remote switch on the supply line so I came override everything remotely if necessary.

 

[SJB060685]

I can picture your system a little better now.
Firstly I personally would want to check the function of the main boiler stat, it could be playing up, it could just be slightly out of place, as in not in its pocket enough. The overheat limit stats on these boilers are usually around 110°c +/- 2 or 3 °c. On some oil boilers you are able to adjust the overheat stat trip setting, however i don't believe you can on the Grant boilers, I might be wrong though.
With both valves closed and still one or both end switches contacting then I can see why the water temperature could sky rocket, of course as you say the tiny loop between the boiler and bypass doesn't help as I imagine its limiting the flow through the boiler.
I suppose one way to give you peace of mind would be to wire in a pipe stat on the flow exiting the boiler, if the set temperature is reached it will cut the power to the boiler, while the pump does its best to drop the temperature, you could even consider a heat sink radiator tee'd in after the pump but before the valves, giving you extra volume and additional heat dispersal.
If you were to choose the pipe stat approach then be sure its capable of withstanding the power the boiler will pull.

 

[Chalked]

you say you are replacing the head. But the problem will lie in the valve itself. The mechanism will be very stiff and this causes the motor to struggle to reach the end switch. Eventually burning the motor out.
you need to replace the entire valve, I would recommend a Honeywell as they work on a ball and lever, instead of a rotating Blade and ar3 the most reliable.

 

[Richard Chambers]

Interestingly, the cost of a Honeywell cpte valve is less than the Danfoss head, so that looks the best plan.
Thanks all