Hi

I have one pump (Grundfos 240 volt) at the moment it's controlled by a thermostat simple on/off 240v. As I am adding a log burner to our system which is a thermal store I need to add a fail safe thermostat 240v so that when the thermal store reaches 90 degree it triggers the pump to prevent overheating. Please don't worry about power failure because I have also fitted a normally open valve to a radiator heat sync to take away hot water from the burner in that situation.

I simply want to have two 240v supplies/thermostates wired to the same pump that can trigger the pump individually and not conflict with each other if that happen to be on at the same time.

Hope this is clear - any ideas would be welcome

Thanks Keith

 

Welcome to AAC.

One simple way to do this is with three relays. Each thermostat would be connected to a small relay and each of these relays would be connected to the third so that it would give you a logical OR. The third relay is connected to the pump so that if either of the small relays activate it, it turns the pump on.

You just have to be sure you select the third relay so that it has sufficient current handling capacity. But, if it is just connected to wherever the existing thermostat is connected, it shouldn’t require anything special.

 

Hi thanks for the reply - really helpful - can you recommend any relays the pump is 60w 240v

thanks Keith

 

I simply want to have two 240v supplies/thermostates wired to the same pump that can trigger the pump individually and not conflict with each other if that happen to be on at the same time.

Maybe I'm not understanding correctly, especially as you mention "two 240v supplies" but if there is only one 240V supply and the pump can be turned on by either switch then I see no conflict. If either or both thermostats/switches are closed the pump turns on, if both are open the pump turns off.

 

It should suffice to connect the 90° back-up thermostat S2, in parallel with the thermostat S1, in the existing circuit.



Nandu.

 

Maybe I'm not understanding correctly, especially as you mention "two 240v supplies" but if there is only one 240V supply and the pump can be turned on by either switch then I see no conflict. If either or both thermostats/switches are closed the pump turns on, if both are open the pump turns off.

Surely if the two supplies are ON at the same time then there would be an increase in voltage to the pump???? Surely I need something like a dpst relay to isolate the two live supplies even though they are from within the same house???? So when live1 is ON and live2 OFF that's one supply to the pump - when live2 ON and live1 OFF that's still one supply to the pump - BUT when live1 and live2 are ON then isn't that double supply to the pump, don't I need a relay to say choose live1 and isolate live2 leaving just one supply, then if live1 switches OFF the dpst will switch to live2 automatically maintaining only one supply to the pump at all times????

 

It should suffice to connect the 90° back-up thermostat S2, in parallel with the thermostat S1, in the existing circuit.

View attachment 307081

Nandu.

What if s1 and s2 are both closed simultaneously does that not overload the pump????

 

What if s1 and s2 are both closed simultaneously does that not overload the pump????

Not at all!

Nandu.

 

What if s1 and s2 are both closed simultaneously does that not overload the pump????

Am I over thinking this - is this really just like a two way switch for a hall and landing light? Which is what your diagram suggests???

 

Surely if the two supplies are ON at the same time then there would be an increase in voltage to the pump???? Surely I need something like a dpst relay to isolate the two live supplies even though they are from within the same house???? So when live1 is ON and live2 OFF that's one supply to the pump - when live2 ON and live1 OFF that's still one supply to the pump - BUT when live1 and live2 are ON then isn't that double supply to the pump, don't I need a relay to say choose live1 and isolate live2 leaving just one supply, then if live1 switches OFF the dpst will switch to live2 automatically maintaining only one supply to the pump at all times????

No 'two supplies', only one.

Nandu.

 

No 'two supplies', only one.

Nandu.

So this is like a two way lighting switch setup???

 

Am I over thinking this - is this really just like a two way switch for a hall and landing light? Which is what your diagram suggests???

It's simpler than that.

The pump runs when either or both thermostats are on.

Nandu.

 

It's simpler than that.

The pump runs when either or both thermostats are on.

Nandu.

Thank you for your help

 

is this really just like a two way switch for a hall and landing light?

Absolutely not the same! Those switches have a Common, L1 and L2 terminals wired like this:

The logic of this arrangement is that the the light can be turned on and off by either switch. It's a good idea to trace this through to understand how it works.

In your case you simply have two on/off switches wired in parallel. If this is in the UK it's likely you only have one phase connected to your home - in some countries there is more than one phase so make sure you don't have the supply coming from different phases - otherwise you'll have an effective fuse tester!

 

Absolutely not the same! Those switches have a Common, L1 and L2 terminals wired like this:
View attachment 307085
The logic of this arrangement is that the the light can be turned on and off by either switch. It's a good idea to trace this through to understand how it works.

In your case you simply have two on/off switches wired in parallel. If this is in the UK it's likely you only have one phase connected to your home - in some countries there is more than one phase so make sure you don't have the supply coming from different phases - otherwise you'll have an effective fuse tester!

thank you for your help - I am in the uk single phase 240v - so I'm actually only using one supply even thought its going through two thermostats and ending up at one pump - so the thermostats are simply switches and if both are on I still only have one supply because I am on one single ring??

 

so the thermostats are simply switches and if both are on I still only have one supply because I am on one single ring??

Exactly right, thermostats wired in parallel as made clear by Nandu. You are right to check and make certain when dealing with mains power. Hopefully everything is well fused and make sure to be touching nothing when you test it!