where are the stats?

What type is your stat?

Most modern electronic thermostats are PID (Proportional, Integral, Differential).
It means they have a microchip that provides a measured response proportional to the temperature difference required. They settle at the set-point temperature rather than oscillate about it like traditional stats do.
However they do this by controlling the on/off ratio within a fixed-time window that is usually 10 or 25 minutes so you will usually see on and off at that period.

For electric heating this is fine but it is very bad for GSHP where they cause this short cycling. A traditional thermostat is much better in such cases.

Some electronic thermostats (like some of those from Drayton) have a jumper inside to let you set the control to be either PID or traditional.

What djh said... Check that the cycling is caused by the stats and not a boiler issue. There might be lights on the wiring center you can use to confirm when one starts/stops calling for heat. If the boiler is switching on and off without any of the stats changing state then it might be the boiler.

If it's the boiler... is it an oil boiler? They can't modulate the burner so if the dial has been turned right down that might cause it to cycle on and off to achieve the required average flow temperature. Turning it up might not cure the problem. You would need to measure flow and return temperatures before I could comment further.

If it's the stats.... Some of the latest stats from companies like Heatmiser have adjustable hysteresis and time delays to help prevent short cycling.

Posted By: tobiasmalfredIt is opening the actuator and turning on the pump for a couple of minutes then goes off.

I'm no expert but I think some systems turn the pump on for a short period to move the liquid so as to get an accurate temperature reading. It's not something like that is it?

I've got Heatmiser stats controlling UFH (networked version to wiring centre)
I haven't paid too much attention to any microbehaviour but I'm pretty sure they just call for
heat until the set temperature is reached.
You may have to change the 'Switching Differential' and/or 'Output Delay'
from the default under the Optional Settings.

Has anyone got Heatmiser PRT stat's with the UH3 wiring centre? Look's very good but want to hear from anyone that has it before I buy it.

Tobias

The usual method of adapting to load size and avoiding overshoot is PID - it's an (electronics) industry standard control algorithm that is very widely used and it works well.

eg. http://en.wikipedia.org/wiki/PID_controller

The down-side is that they pretty much all work with fixed intervals that are rather too short for heat pumps, resulting in a lot of on/off switching that is better avoided.

Even if PID were available with eg. 2 hour interval times I would still prefer a single on/off with hysteresis ie. a normal thermostat. That's what I use. Our GSHP comes on once a day. I don't really notice any temperature fluctuation but I suppose it gets just a little cooler in the evening.

It's not just about how long your floor takes to heat up... remember that outside temperature (and air pressure/wind) fluctuates over hours anyway so rate of heat loss from your building also changes.

I've got Heatmiser stats connected to a UH1 wiring centre, so 12V instead of 230V between stats and wiring centre. I suspect the logic embedded in all the heatmiser stats, whether 12V or 230V is pretty much the same.

I've got a mix of newer rooms, insulated to a decent standard, and existing rooms that still need their insulation sorting out. All the newer rooms have an individual heatmiser stat in the room, all run UFH loops, all have 'optimum start' turned on.

I'd say that for the well insulated rooms the heatmiser 'optimum start' copes well with any sudden outside temperature changes, they just don't lose enough heat to get the system into trouble.

For the badly insulated rooms that are still heated by rads my feeling is that the old Honeywell CM67 that
I used to have for years would be preferable to the Heatmiser stats. Mostly that's because it's just got a
bit more functionality, e.g. more on-off and the ability to set the next day to the Sat/Sun program.

Boiler is a Viessman 200 so it can modulate down a fair bit when demand reduces, no outside temp compensation.

Marcel