Heating thermostat question

WYTAWillis

I recently upgrated my entire heating system and have a condensing oil boiler with three zones. The zone thermostat for the living area (Horstmann HRT3 mechanical) cycles a lot and therefore the boiler is also short cycling. I don't think it is a siting issue and my plumber agrees. The specs say the switching differential is only 0.5 degrees, this seems very small to me and I think a wider band would be more efficient. I am considering a digital therostat with a selectable band, something like the Heatmiser DSL1 which can be set to 1,2 or 3 degrees. Appreciate any advice about this.

Horstmann also have a digital stat with TPI software (HRT4-B) - am I correct in assuming that these are only suitable for a modulating boiler? If not, would they solve my problem?

thanks in advance.

Spear

Moved from N&F.

WYTAWillis

Hello, is there anybody.... out there??
Can't see over the wall.

heinbloed

Get a heating engineer in.

Your thermostat might be the wrong choice, but your distribution(plumbing) might be as well.
If you experience cycling then try to reduce the (flow and return) temperature at the boiler but keep the spread wide (DeltaK).
This would be a temporary meassure. Contact a competent plumber/heating engineer for a check.
It makes indeed sense to install a modulating condensing boiler, better a modulating combi-condensing boiler if DHW has to be prepared as well.

The smallest output of the boiler (kW) should not be bigger than the smallest demand. Only this would do away with cycling.Well, mostly.

WYTAWillis

My plumber has checked the system and flow/return temps look fine. The issue seems to be the stat, it is just too sensitive I believe. I have seen it switch on and call for heat, and in less than a minute it switches off again. The radiators have barely begun to heat when it switches off and if no other zones are on the boiler gets switched off also.

heinbloed

Your plumber seems to be incompetent to solve the problem. Get a heating engineer in.

JohnnieK

WYTAWillis wrote: »

My plumber has checked the system and flow/return temps look fine. The issue seems to be the stat, it is just too sensitive I believe. I have seen it switch on and call for heat, and in less than a minute it switches off again. The radiators have barely begun to heat when it switches off and if no other zones are on the boiler gets switched off also.

To be honest it sounds like the stat. If you have the same one up stairs put that to the same setting and monitor it. If that behaves then consider replacing the stat down stairs for a new like for like one. If it is faulty your plumber should replace it under warranty if it has not expired. Also is it possible that it is installed in a draughty area?

WYTAWillis

JonnieK - I forgot to say that the stat was already replaced and the new one behaves the same. It is in a hallway so draughts are a possibility, but the rapid switching also happens with doors closed and nobody moving around so only air movement should be from the rad. Do you think a wider switching band is worth trying?

heinbloed - wild guess, are you a heating engineer? Don't be so quick to jump to conclusions about competency without knowing the facts.

slavetothegrind

it's usually 1 degree differential in the switching, digital stats are more accurate than their mechanical counterparts and have the benefit of showing you exactly what's happening.

is it possible the convection current from the rad is somehow making directly for the stat?

WYTAWillis

Plumber and I both think the stat is far enough away from the rad and not directly in a convection current. Not easy to move it at this point to test. However the bedroom stat is located further away from the rad and is the same type but does not seem to switch as frequently. It is set to a lower temp but that should not make much difference. I think I will try a digital stat and see if it helps. Any opinions on the TPI energy saving types? They can be set to limit number of cycles to 3/hour but would this compromise comfort?

Iderown

The Horstmann HRT3 room thermostat contains an "accelerating" resistor. This has the effect of switching the system rather more frequently as the set temperature is approached. (Another room thermostat which feature a similar resistor is the Danfoss RMT230)
In the Danfoss thermostat the resistor can be disconnected by disconnecting the NEUTRAL supply at the thermostat. This has the effect of leaving only the live in and switched live out.

The Horstman data sheet at
http://www.horstmann.co.uk/downloads/ElectronicDocuments/Central-Heating/HRT3UserandInstallationGuide.pdf
advises against disconnection the neutral connection. I have no experience of the Horstmann thermostat.
Additional note: Isolate the electrical supply to the heating system before altering any of the electrical connections.

Bruthal

You could try swapping the stats and see if it makes a difference. Certainly sounds like a stat or stat location problem. 0.5 degrees seems very narrow as well.

DoneDL

what is the boiler temperature set to and is there a bypass fitted?

Bruthal

You could try turning the room stat in question up to its highest setting and see does this cause the rads to now heat up to a higher level. At least then you will know if its the stat causing the cycling.

yoshytoshy

If you don't mind spending the few euro on a digital stat ,it would certainly make a difference. At least you'd have more control over it aswell.

What I use at home is digital thermostats to get reading around various rooms ,if you had one you could get an idea of the actual temperature.


The digital room stats that I use have a feature called chronogragh and it can be set, the stat also has a temperature reading display.

WYTAWillis

The boiler stat is set to min and the flow temp is approx 54 degrees. I have measured the room temps with a digital thermometer and actual is very close to the stat set point. Changing the set point does change the actual.

yoshytoshy

Thats a low temperature for radiators ,are you're rads sized for that or are you running underfloor heating?

WYTAWillis

No underfloor heating, just rads. Boiler & rads are new and are matched - as far as I know. The boiler has spare capacity (for future extension) and the burner nozzle was changed to derate it to match current load. The boiler stat was originally set to mid range but turned to min to see if it reduced cycling - did not change anything though. I will put it back to the original setting again.