Thermostat fr underfloor heating with high thermal inertia

[jonnyhifi]

Hi! I am trying to find a thermostat for my underfloor heating (which is all electic--- wires set in a screed), which I put in the house years ago in a refurb. Lots of insulation under it, and I like it, but never found a thermostat that appears suitable. The screed means it takes some hours to come up to temperature, so a conventional "on / off " thermostat would be a disaster-- the system has high thermal inertia.

What I need is one which "learns" the thermal inertia of the system, and perhaps pulses the heat input on and aff, as the temp gets near the target. Anyone know of such a thing ? I can't find one anywhere, but I'm sure they must exist.

Given I have "economy 7" cheap electricity at night, even better if it could be clever and only switch on at night.

 

[holmslaw]

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[jonnyhifi]

You could use a stat with a capillary tube. Drill a hole in the screed and insert the capillary tube you will then be monitoring actual screed/ floor temp, rather than air temp.

yep: i agree measuring the screed temp and air is important, as I don't want the surface getting too hot: but it doesn't overcome the inertial problem, which is a time domain problem, rather than a measurement one, ie I have a control issue...

 

[ban-all-sheds]

This is one of the major disadvantages of UFH. Particularly at times of year when the weather is quite variable - either the floor won't warm up quickly enough if there's a sudden cold snap, or won't cool down quickly enough if the weather is quite a bit warmer than the day before.

I've known people get rid of it for just that reason.

 

[jonnyhifi]

This is one of the major disadvantages of UFH. Particularly at times of year when the weather is quite variable - either the floor won't warm up quickly enough if there's a sudden cold snap, or won't cool down quickly enough if the weather is quite a bit warmer than the day before.

I've known people get rid of it for just that reason.

i must say i hadnt thought about the weather changing outside: but of course, you are absolutley right: it can be a moving target. mmm !


In my case i think that is less of an issue though, as my house is exceptionally well insulated, so it is pretty difficultto tell the weather outside for a day or two, as the temperature varies, even with no heat going into the house, and relativley little input via fan heaters can heat it up in an hour or two.

 

[holmslaw]

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[JohnW2]

...my underfloor heating (which is all electic--- wires set in a screed)

Drill a hole in the screed and insert the capillary tube ....

...provided you either have a very accurate and trustworthy plan/map of the buried wires or else are very good at praying

Kind Regards, John

 

[holmslaw]

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[JohnW2]

...provided you either have a very accurate and trustworthy plan/map of the buried wires or else are very good at praying

The poster is obviously not an idiot, therefore certain things don't need to be said.

I'm not suggesting that the poster is an idiot, but I certainly wouldn't attempt such a drilling myself unless I had personally drawn the plans, and even then I might be too nervous. The main problem is obvioulsy that, if one wants to avoid the very thermal inertia that one is trying to avoid, the capillary would necessarily have to be fairly close to the heating wires, which would mean that a high degree of plan accuracy would be required.

Kind Regards, John.

 

[ericmark]

This was one of my university projects using a PLC. The idea was to use a PLC to control stress relief mats used after welding and it became quite complex. As you say it needs to learn so the reaction time alters the mark/space time.

Although it did work it was considered as too expensive to be worth while and a cheaper alternative using a heater in the thermostat worked well enough for the job in hand.

The whole idea of a heater inside the thermostat has been used for years mainly to reduce the difference between on and off temperature. I modern digital will have around 0.5 degrees between on and off but the old type with heater disconnected was around the 3 degrees mark.

Unfortunately I don't know of a pre-made unit with the heater built in. Or a formula to work out size. It would be trial and error. And to fit a resistor could cause overheating and dangers as a result.

Even with my standard hot water system there is a fair amount of overshoot so I have used a fair simpler system. I used a Horstmann DRT2 Room Thermostat which has a clock built in. At 6am it switches the central heating to 17 degrees. I know it will overshoot in my case max temp about 1.5 hours after switch on, on a cold morning. So at 7:30am temp is re-set to 19 degrees the house likely will be above that with overshoot so it then just holds it at 19 degrees.

Problem is the Horstmann DRT2 Room Thermostat only has two heat settings per day at weekend so this system only works 5 days a week. So to get 7 days a week would need two thermostats.

holmslaw's "standard room stat set at 20°C and internal frost stat (that over rides the programmer) set at 15°C" is similar idea.

But unlike a standard thermostat the DRT2 does not simply switch on and off at one set temp but has during weekdays 4 temperature settings and switches between these at the times set. And not too expensive about £24.

 

[holmslaw]

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[JohnW2]

I'm sure that you are somehow connected with the NHS, am I correct?

I'f you're sure, then I don't understand why you're asking me if you're correct. You must have a different understanding of 'sure' from mine.

Kind Regards, John

 

[holmslaw]

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[JohnW2]

Quite right, I should have said 'suspect', so are you in some way connected with the NHS?

I'm not sure I really understand what 'connected with the NHS' means. I'm certainly not employed by the NHS - so the answer is probably 'no' - but what other sort of 'connection' did you have in mind? (I've been known to visit my GP)

Kind Regards, John.

 

[holmslaw]

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