Rapid Response Pipe Thermostat (All about speed)!!!

[chris202]

Hi,

I've recently installed a pipe thermostat to my UFH circuit to help regulate the fluid temperature within the UFH sysytem.

When the UFH is operating as a single circuit - in other words the CH + HW are off, the boiler sometimes beats the system at dissipating the heat which causes the boiler to shut down on the boiler thermostat as it should. Thsi is because we have a high output boiler pushing all it's heat through a small 15mm UFH circuit (understandable).

However, for various resons, I would prefer to shut the 'call for heat' supply down by simply cutting a pipe stat into the switch live to the boiler.

This I have already done and electronically (and in principle) it works fine. However, I have used a Danfoss frost pipe stat which operates on a bi-metallic principle and the response rate is so slow that it often will not click out before the boiler.

I know it does work, because eventually I see the supply from it cut by the time the bi-metallic strip heats up.

Does anyone know of some sort of 'electronc' pipe thermostat that will have a mush quicker response rate than the bi-metallic strip type.



Many thanks,

Chris.

 

[TicklyT]

A relatively slow response and a wide hysteresis are usually considered an advantage for nearly all conventional cylinder and pipe 'stat applications, so you may need to consider a form of thermostat not normally used for central heating controls.

A fast acting 'stat with a narrow hysteresis curve may lead to excessive rapid on - off cycling of the boiler

 

[oilman]

Set the pipe stat to a lower temperature so it will cut the boiler just before the control stat. there will still be a bit of overshoot, but the boiler stat won't be doing the switching. This will possibly give you reasonable hysterisis too. If it doesn't, you could configure two thermostats to do it, failing that you will have to go to an electronic programmable controller from someone like Eurotherm. Should be able to get something reasonable for around £300, perhaps less.

 

[chris202]

Thaks both for your replies.

The trouble is - the pipework temperature ramp up can sometimes be so sudden (quick), despite turning the pipe stat back - it still has trouble controlling the overshoot due to the very slow response rate.

I have tried winding the thermostat right back - but this only causes another problem with the 'switch in' temp being very low.

If I could find a thermostat with a immediate response as soon as we get to temp that would work. But based on the first response - I may also need to find one with perhaps an adjustable hystersis range so the thing is not kicking in and out like a yo yo.

I would like to know more about how two thermos would work - this may help but dont fully understand how?

As I seem to have two experts on to the case - can you advise if I amn 'creating' over nothing here and should simply let the boiler do the switching - or is that not good practise - I just personally do not like the idea - but dont have a solid argument.

Thank you....

Chris.

 

[Agile]

If you want a rapid and accurate response then you would need to build a simple electronic sensing circuit using a temperature measurement IC such as the LM35 costing about £3.

Ideally the circuit should have an adjustible hysteresis so that the problem of rapid undesired switching does not occur.

Oddly ( cost? ) the boiler makers still stick to NTC resistors for boiler designs even though accurate digital temperature sensors are now available.

Tony