Underfloor heating & programmable thermostats

[SL]

Question's been puzzling me all summer, now I need to turn the heating on... so if anyone knows...

I have hot water underfloor heating powered by a diesel heating/hot water furnace. The floor takes around 5 hours to warm up as the pipes are under a 6cm slab of concrete with stone floor tiles on top, it also takes around 5 hours to cool down again. I have programmable room thermostats in each room controlling that room's floor.

If, for the 5 hours it takes for the floor to heat up, the thermostat is telling the furnace "more, more, more!" surely at the point the thermostat finally says "OK, 22°C, turn off" the concrete slab is still loading itself up with heat. Is it not the case that the floor will continue to get hotter 23°C...24°C... 25°C... for an hour or so more until starting to cool back down again? Then on it's way down it trips the thermostat again but continues to cool... 20°C...19°C...18°C... until the slab can warm up again - and we're back in the "whoa, too hot! Brrrr, too cold!" cycle?

Or is this not the way hot water undefloor heating works in practice?

-- SL

[cupra]

Sounds like you need PID control rather than on/off.....

[Owain]

No. The floor cannot get hotter if there is no source of heat. (Unless you have discovered some really amazing new physics.)

Depending on the thermal lag, the *room* may continue to get hotter as the floor cools.

Owain

[Michael Chare]

I would have thought that the temperature of the floor would stop rising the moment that you stop inputting heat.

I would expect the temperature of the floor surface to keep falling until the material surrounding the hot water pipes has warmed up.

In short I would expect there to be some lag in the system. The proportional control often included with programmable thermostats may help to overcome this - as someone else has pointed out.

[Doctor Drivel]

You have it. Best to have an outside weather compensator to anticipate the coming cold or warm spells. It raises and lowers the water temp, so doesn't pump heat i to the slab when the weather outside starts to warmer. Some have room temperature influence which combines with the outside temp to get the right water temp.

[Guy King]

The message from Owain contains these words:

Or possibly you have /very/ hot cats. Or a large window with strong sunshine beating on a dark tiled floor.

[Peter Parry]

It stops heating when the source goes off. It continues to emit heat for some time.

No, because the tons of concrete act as a large moderator. As you probably know nothing happens quickly with UF heating so the use of complicated controls isn't really necessary and most controllers can't cope with the hysteresis anyway. You will find some plumbers who think outside air temperature sensors are a good thing but as no one has yet invented one which predicts the weather they have limited value. I do know of one person who made a controller which read teletext weather data and used light level sensors to predict solar gain and combined that information to control a time offset on the heating. It worked very well but was rather overkill (and proved that using a much simpler time offset was almost as effective).

What you do need to do is to put in a considerable offset between heating and output, so if you want warmth in the evening the heating needs to come on about midday, but it can go off again at about 18:00 and the residual heat in the slab keeps you warm well into the late evening. The offset can be done by using a control system with a lot of lead capability (if you can find one) or far more simply and cheaply by doing it manually by simply offsetting the on and off times. Once you have it roughly right it is fairly tolerant of outside weather variations as long as the house is reasonably well insulated and draught proofed.

[Owain]

I would class those as sources of heat

Owain

[Guy King]

The message from Owain contains these words:

A fair point and well made.

[The Natural Philosopher]

Mine generally overshoots by about a degree, and is run off a different timer with about 3 hours advance as compared to the main timer, and in winter, its run 24 hours. The stat is screwed to a huge lump of brick which itself takes a long time to warm up, so that doesn't help.

There are thermostats available that will have feed forward..i.e. they will switch off a degree or two before temp is reached if its rising, and on a degree or two above if its falling.

once you are up to temp, the thing is very stable I find.

[Doctor Drivel]

What make did you use?

[Ian Stirling]

Or if the slab is very very thick, and the bit next to the coils is at

30C when they are turned off.