Fitting thermostat to heating pump

[Katie]

I know there are some existing threads about this but it’s easier to ask my own questions outright 

 

I want to install a pipe thermostat to my stove backboiler to automatically turn on my 12vcirculatory pump to my radiators and califorer on/off as the temp rises and falls. My pump is already installed so I’ll be adding the stat to the system to turn off as my stove cools and automatically turn on as it reaches desired temp. 
 

1) can I use any pipe stat or do I need to find a special 12v one?

 

2) wiring- can someone summarise where the negative and positive will go? I assume positive into the stat then from stat to pump but I’m unsure about the off position and the negative? Do I just keep negative on the pump? 

[George and Dragon]

That's going to depend on the stat. You need a stat that is rated for 12V dc.

 

If your stat is capable of switching the surge current of the pump then you could just wire it in the pump circuit in series like any other switch. Otherwise you need to use it to operate a switch, often a relay, that can handle the pump current.

 

Does this make sense?

[Jen-in-Wellies]

As @George and Dragon dragon says, a relay may be needed. Use a standard car relay, easily available and able to handle the current from switching the pump motor. Not convinced a pipe thermostat will work, even if it is very close to the boiler. If it can react fast enough to stop the non-moving water in the back boiler from actually boiling, I don't know.

I've seen people with back boilers that absolutely need their pump place a sign in the stove door with "Pump!" on it when it is cold to remember to switch it on when the stove is lit.

Jen

[Tony Brooks]

What G&D said, but in addition pumped circulation of a solid fuel/wood burning stove has a real danger of filling the boat with steam and spraying rusty water/antifreeze mixture out of the header tank at the least. An explosion at the worst. At least fit a switch in parallel with the relay or thermostat so in the event of either failing you can manually turn the pump on. That does not guard against pump failure so in that case you need to get the fire out PDQ.

[Katie]

Ok will look into this idea further as sounds like not as simple as I thought. Sorry don’t really get the relay thing so perhaps best to find someone else to do this job.
 

The system did use to have a thermostat doing this but it got taken out when I had the pump replaced.
 

I’ve already got the pump  with switch set up fine and always remember to turn it on as soon as I light the fire but i would really like it to switch itself off when it’s cooled down over night for instance so it doesn’t run when not needed and cool down my water 

 

My header tank is external and open so won’t be exploding and filling the boat with steam. 

[Tony Brooks]

5 minutes ago, Katie said:

My header tank is external and open so won’t be exploding and filling the boat with steam. 

 

Your back boiler might if the stove runs away or the pump stops running with the fire going well.

 

Here is a circuit for a relay

 

 

 

I would advise also fitting a switch between terminals 30 & 87 to override the relay and thermostat if needed

 

 

[Katie]

Thanks Tony really appreciate this makes much more sense with the diagram

 

absolutely would want an override switch yes good idea. 

[BEngo]

I have a 12 V pump which assists the thermo siphon circulation of the central heating (Squirrel stove).  This has been fitted for about 33 years and apart from pumps expiring has worked well.

 

The pump is a magnetic  drive centrifugal type by ⁸Johnson, which draws about 1A and the stat is a standard domestic hot water  cylinder stat, held on the short horizontal pipe from the boiler top outlet and in the normally fused  positive line to the pump.  The negative line is not switched.    There is a manual  switch in parallel with the cylinder stat to force the pump to run, for test and bleeding purposes.  I test the pump on light up and daily for about 20 seconds.

 

The  system circulates happily without pumping at normal operating settings and the pump stat is set to come on mainly for the event where the wind gets up and the fire brightens.  It is still possible to boil the Squirrel if you leave the fire full of coal and the bottom door open, but the vent and header tank in the engine ole  cope with this quite happily though it can get a bit steamy.

 

The weak point is the  pump life.  Most small , low current pumps seem to be rated for very short lives.  The Bolin pump was good and very pricey, but is no more and I am dubious about the Chinese solar water heater ones on the web.  The Johnson is not cheap either and  has a life in excess of 100 000 hours, which is not long if run continuously, but is OK in my usage.  I carry a spare and can change it easily.

 

The cylinder stat needs to be of the changeover type and you use the make on rise set of contacts.  If you use the wrong set the pump will run when cold and switch off when hot.  It might not be rated for DC, but like most domestic AC stuff it copes happily with low current 12 V DC.

 

N

[Katie]

Thanks for this

 

my pump was professionally fitted last year but at the moment I leave it running the entire time my stove is lit to be safe. It feeds 3 radiators and heats my calorifier. Had no problems with it but would also like the thermostat added mostly to switch the pump off when cool enough. 
 

ok so a standard AC rated stat should still work good to know as struggling to find anything specifically rated for 12v dc. Yes I agree will also want the main override switch for testing and safety. 
 

 

[Tony Brooks]

The point about using a DC rated switch is that AC circuits turn themselves off 100 times a second so as the switch contact start to open the current almost immediately turns off for a fraction of a second, so the opening contacts don't spark (arc) as they break. DC circuits do not turn themselves off so as the contacts open the spark gets longer and longer. This tends to burn the contacts so for long life the DC contacts will be larger, made of a different material, or a magnet may be placed to one side to pull the spark away from the moving contact. This is why you have been advised to use a relay to switch the pump because car relays are designed for 12V DC.

 

With the fire cooling when the thermostat turns off circulation is liable to stop or slow down. This will allow the water in the boiler to increase in temperature so the stat closes, circulates cool water into the boiler and top pipework so the pump stops, only for the cycle to repeat itself until the fire temperature falls below the thermostat setting. This cycling is very likely to allow the arcing each time the stat opens to accelerate the damage to the AC stat contacts so for longer term reliability please use a 12V DC relay.

[BEngo]

Without any detail of your set up, and particularly the pipe routes,  I would not want to use a thermostat without first trying the system out with the pump switched off.

 

I would want to know how big a fire the system will stand whilst thermo circulating with no pump and not boiling.    The risk is not just of boiling the water, but of causing damage by overheating the stove.  It is quite possible to have boiling in the stove but nearly cold pipes and radiators not far away.  Not really the time of year to be testing fires  but soon it will be.

 

N

[ditchcrawler]

53 minutes ago, Katie said:

Thanks for this

 

my pump was professionally fitted last year but at the moment I leave it running the entire time my stove is lit to be safe. It feeds 3 radiators and heats my calorifier. Had no problems with it but would also like the thermostat added mostly to switch the pump off when cool enough. 
 

That is the safest way to run it. Fire alight pump running

[PaulJ]

1 hour ago, Katie said:

 

my pump was professionally fitted last year but at the moment I leave it running the entire time my stove is lit to be safe. It feeds 3 radiators and heats my calorifier. Had no problems with it but would also like the thermostat added mostly to switch the pump off when cool enough. 

If you are going to go this route I think this is the safest way to look at it. Its actually a way to switch the pump off.

Ive come across several systems that have worked badly and chucked out the coolant as a result -usually for the new owners that had no experience of their set up. All were either because the switch was either set too high (50 degrees) or the stat was installed too far away from the boiler. 

Pipe stat really must be fitted immediately behind the boiler and set low. I would try 30 degrees. 

The pipe stats are not very fast to react to heat and you have to allow for this- they also vary in reaction time , I think from memory (not reliable) , the Honeywell react quicker than the cheapy from Screwfix.

Type of boiler also makes a big difference, my own boiler is a big cast back boiler,  is relatively slow to heat up and so works well with my stat(s)-whereas something like a squirrel can heat up really fast and will boil quicker than a stat could react if the stat is set high.