Programmable c/h room thermostat, with adjustable hysteresis / dead zone?

[Adrian Brentnall]

HI Folks As per the subject - does such a thing exist? (Tried Googling - massive amount of hits .....)

Situation is - ground-source heat-pump to conventional radiators, which all works well. Programmable wall-stat in sitting-room - calls for heat, starts heat-pump, temperature then rises slightly, turn off stat.

The heat-pump is just a big fridge-in-reverse - and takes a serious amount of surge current to get the compressor spinning. It's said that short-cycling (stop/start) isn't good for these units.... - and common-sense suggests that, once started, it'd be wise to let the thing run for half an hour or so each time..?

In an attempt to minimise this short-cycling, the engineers installed a din-mount time-on-delay relay (8 mins or so), between the 'stat and the heat-pump - idea being to sort of 'debounce' the call-for-heat from the stat. The pump itself is installed in the workshop. When I'm working out there, I frequently hear the circulation pump start up, then, after the short delay, the lighting dips & the compressor starts - only to cut out again after no more then 30 seconds... This can't be a Good Thing?

I did call the engineers back, who said 'oh no Sir, it can't possibly be doing that' - and went away, promising to return.... that was some months ago.

So - wondering whether fitting a programmer that's more capable would make sense, or if the 'delay-on' timer should be changed to a 'run-on' configuration....?

Any ideas, please? Thanks, Adrian

Danfoss 40704

[dennis]

How often? It may be that it only needs 30 seconds to reheat the water if only one radiator is actually on and the rest are off due to thermostatic valves.

Compressors normally have a protection mechanism to prevent them being restarted too soon. My dehumidifiers, AC, fridge and freezer all have them built in.

My wireless stat has a built in anti cycling system, by default it will only operate six times an hour but I have set it to 12 as I have little water in the system.

Find the manual for the timer and see if its set correctly.

[Adrian Brentnall]

As there's the on-delay timer, can be as often as 6 or 7 times per hour.

Don't think that's happening. I think what's happening is that the room-stat triggers the on-delay timer. The temperature then increases, and the stat drops out, but the timer continues to run. I 'think' that the timer then tones out, and causes a brief 'on' signal to the compressor, even though it's input signal has already dropped.. Need to double-check this with the timer manufacturer though..

What make/model?

The delay-on timer? - yes it is. Thanks Adrian

[Andy Burns]

The heatmiser prog stats allow setting a switching differential from 0.5 to 3 degrees, in 0.5 degree steps.

[dennis]

CMS927 honeywell IIRC.

[Adrian Brentnall]

Ah thanks, Andy. You mean the PRT-B - on the right-hand side of this page?

[Andy Burns]

That, or the similar slimline ones for 230V/12V power instead of battery, also the touchscreen range, in fact I would suspect any of their programmable range have similar configuration options.

[Adrian Brentnall]

Thanks! The current 'stat is battery, so having a replacement that doesn't need

12 or 240v would be a good thing....

[Andy Burns]

They no longer seem to sell 230V models that also have a hot water timer, unless I've not seen the right section of the website.

Don't be tempted by their older wifi based ones they were insecure as hell, therefore I'd also be wary of their newer wifi ones too (though I think they work by making outbound internet connections now, not inbound)

[Adrian Brentnall]

The hot water's on a separate timer (odd way of doing things - but that's what they did, and it seems to work) - so straightforward c/h controller is fine

I'm generally in favour of simpler rather than complicated.. and can't think of any good reason (for us) for a wi-fi-connected thermostat (given that we're usually in the house 95% of the time).

Thanks for the warning, though! Adrian