Green Building Forum - Underfloor Heating Thermostats

Underfloor Heating Thermostats

tobiasmalfred — Sat, 16 Mar 2013 19:02:54 +0000

Hi,

My ufh keeps running on for a few minutes and then off. Not long enough to change temperature. Is this normal or should it not come on for a period of time until the stat is satisfied. I have manual stats and don't know how accurate they are.

Tobias

Underfloor Heating Thermostats

tony — Sat, 16 Mar 2013 19:10:51 +0000

where are the stats?

Underfloor Heating Thermostats

tobiasmalfred — Sat, 16 Mar 2013 19:41:19 +0000

Hi Tony,

Stats in centre of internal walls. Not near the doors.

Tobias

Underfloor Heating Thermostats

Sprocket — Sat, 16 Mar 2013 22:35:00 +0000

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.

Underfloor Heating Thermostats

djh — Mon, 18 Mar 2013 13:44:32 +0000

Is this actually the stat opening and closing, or is it the heat source (e.g. boiler) cycling, because it can't modulate down far enough to match the low demand?

Underfloor Heating Thermostats

CWatters — Mon, 18 Mar 2013 19:02:11 +0000

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.

Underfloor Heating Thermostats

tobiasmalfred — Mon, 18 Mar 2013 22:17:24 +0000

Hi,

Thanks for the replys. We have a buffer tank so the stat doesn't control the boiler just the pumps and actuators. It is opening the actuator and turning on the pump for a couple of minutes then goes off. You can see what stat is doing it at the manifold control box. Was looking at the Heatmiser PRT stat.

Tobias

Underfloor Heating Thermostats

djh — Tue, 19 Mar 2013 14:11:10 +0000

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?

Underfloor Heating Thermostats

RobinB — Tue, 19 Mar 2013 15:29:53 +0000

I had a similar problem early on, reprogramming the stats allowed us to increase the minimum interval between calling for heat and off again. It was written in incredibly small print and took a few goes to get right!

Underfloor Heating Thermostats

mjansen — Tue, 19 Mar 2013 15:53:53 +0000

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.

Underfloor Heating Thermostats

alec — Wed, 20 Mar 2013 07:04:28 +0000

Whoops

Underfloor Heating Thermostats

tobiasmalfred — Sat, 06 Apr 2013 18:44:36 +0100

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

Underfloor Heating Thermostats

Phil.Chaddah-Duke — Mon, 08 Apr 2013 03:18:19 +0100

I need to decide on the controls for our soon coming updated heating system but feel a bit bemused by which way to go. It seemed to make sense to have a UFH contoller that learns how long a particular zone takes to heat to avoid overshooting etc. Lots of bespoke companies who I get the feeling will be very expensive promise such systems but when I look at actual products like Heatmiser they don't state they offer such features, am I correct or is such functionality common?

Underfloor Heating Thermostats

Sprocket — Mon, 08 Apr 2013 07:38:07 +0100

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.

Underfloor Heating Thermostats

tobiasmalfred — Tue, 09 Apr 2013 12:29:55 +0100

Posted By: Phil.Chaddah-DukeI need to decide on the controls for our soon coming updated heating system but feel a bit bemused by which way to go. It seemed to make sense to have a UFH contoller that learns how long a particular zone takes to heat to avoid overshooting etc. Lots of bespoke companies who I get the feeling will be very expensive promise such systems but when I look at actual products like Heatmiser they don't state they offer such features, am I correct or is such functionality common?

Hi Phil,

The Heatmiser PRT stat can learn how long a zone takes to heat up. It's called optium start. But with our ever changing weather I do wonder how accurate it can be. But the technology is there which is good. It's one of the reasons I'm thinking of these stats.

Tobias

Underfloor Heating Thermostats

mjansen — Tue, 09 Apr 2013 13:10:08 +0100

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

Underfloor Heating Thermostats

CWatters — Wed, 10 Apr 2013 18:43:01 +0100

I believe if you want to use all the features of Heatmiser stats (such as their network controls) I believe the stats need to be wired using data cable not mains wire. Check.