Controling oil boiler with pipe stats

[Scotch]

Hi,
I have 1500 litre accumulator tank heated by 25kw log boiler.
I have now plumbed directly into the tank an oil boiler as back up, which I wish to control by two pipe stats.
The stats are located on the flow (top of the tank) and return (bottom of the tank) pipes as close to the tank as possible.
When fully charged by the log boiler the tank is 80 degress top and bottom, so the way i want it to work is, the top stat tells the boiler (and pump) to switch on when the temp in the pipe has fallen to about 60 degrees and the bottom stat switches the boiler when the temp in the pipe reaches 80 degrees and repeats.
So my question is how to wire the stats to make this work?


Thanks
Scotch

[dhaslam]

I don't think that would work.   The top stat  would cut out the boiler when the temperature reached a few degrees above 60C regardless of what the bottom one was reading.   You would need to use  a thermostat that has a  settable  hysterisis  and then wire the two thermostats in series.       

[daftlad]

I don't think that would work.   The top stat  would cut out the boiler when the temperature reached a few degrees above 60C regardless of what the bottom one was reading.   You would need to use  a thermostat that has a  settable  hysterisis  and then wire the two thermostats in series.       

Couldn't the top stat be linked into a relay that would latch and would only un latch when the bottom stat was up to 80 degrees?
ta ta

[Scotch]

Hi Daftlad,
Could you show me a wiring diagram of how this would be done?

Many thanks

[daftlad]

Hi Daftlad,
Could you show me a wiring diagram of how this would be done?

Many thanks

Yes I can, may take a while and I have no scanner working at the moment.
But lets wait to see what the plumbers think, they may come up with something better/ different.
ta ta

[johnrae]

If you want your oil  boiler to act as a back-up and its purpose is to heat water in the tank, wouldn't it make more sense to measure tank temerature and use that to control the boiler and pump.  Measuring flow and return temeratures will lead to short cycling of the boiler and ultimately be inefficient.  By way of example, when the boiler/pump are off both flow and return pipes will cool down, irrespective of how hot the tank contents are, and the boiler will restart.   If your counter argument is that the stats will be near where the pipes enter/leave the tank then all they will be measuring are tank temperatures, hence my suggestion to simply measure the tank in the first place and using that as control.

Using measurement of a variable to control a system has to consider what it is you are endeavouring to achieve.  If you want to control the deltaT across a heating system then you measure both input and output of the heater.  These could either vary pump speed to control heat transfer rate or burner modulation to do likewise.  However if you want to use the temperature of a tank contents to control a heating source then you measure the contents ---- simples.  You can of course use PID control so that pump speed and / or burner modulation is controlled dependant upon the error between desired temperature and actual temperature. 
jack

[Scotch]

Hi johnrae,
Thank for your feedback.
The reason I want to use pipe stats is i don't have any spare fittings to install a stat to measure the contents temp of the tank and also to avoid draining the whole tank.
Short cycling should be avoided as the top stat would only switch the boiler on and bottom stat only switch it off, making the boiler run for some considerable time.
I understand that the pipe temps will be lower that the actual tank temps but I was hoping the stats could be set to take this into account e.g. setting the bottom stat at 70 degrees to switch off the boiler maybe enough to achieve approx 80 degrees at the bottom of the tank. I also have the pipes very well insulated.

Thanks

[daftlad]

Do you really want to heat the whole 1500 litres with the oil boiler? I would have thought you would have been better using the oil boiler as a boost to just heat the top.
ta ta

[Alan]

RELAY 230 V.A.C.

http://www.rapidonline.com/sku/Cables-Connectors/Control-Gear-Din-Terminal/Contactors-Relays/IMO-Industrial-relay-RS-series/180819/28-4181

BASE

http://www.rapidonline.com/sku/Electronic-Components/Relays-Solenoids/Plug-In-Power/3PDT-10A-Power-relay/71156/60-1683




Regards

Alan

[dhaslam]

Wouldn't the top thermostat  deactivate the relay once it reaches  it's cut off temperature or am I missing something? 

[daftlad]

That saved me a job. 
dhalsam No, the relay coil has 2 feeds, one from the 60 degree stat and one from one of the relay contacts (via the 80 degree stat)
ta ta

[Alan]

Relay is energised by current flowing through the top stat.  ( Temp < 60 Deg C )

Normally open relay contacts keep the relay coil energised until the bottom stat opens at > 80 Deg or what ever setting you decide.

Regards

Alan

[daftlad]

Relay is energised by current flowing through the top stat.  ( Temp < 60 Deg C )

Normally open relay contacts keep the relay coil energised until the bottom stat opens at > 80 Deg or what ever setting you decide.

Regards

Alan

What is even more spookey is Alan and myself both have the same number of applauds and smites. 
ta ta

[desperate]

It's the "wierdo sparky smite club"

Desperate

[Scotch]

Hi Alan,
Thanks for the feedback.
Got the wiring diagram printed, but unfortunately the relay is on back order    just the usual!!!

Anyway thanks for now

Robert