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

SL wrote:
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?

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

"SL" wrote in message
...
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?

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


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?

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.

--

Michael Chare

"SL" wrote in message
...
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?

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.

The message
from Owain contains these words:

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

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

--
Skipweasel
Pay no attention to that man behind the curtain.

On Fri, 08 Dec 2006 15:35:01 +0100, SL wrote:

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?

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

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?

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.


--
Peter Parry.
http://www.wpp.ltd.uk/

The message
from Owain contains these words:

I would class those as sources of heat

A fair point and well made.

--
Skipweasel
Pay no attention to that man behind the curtain.

SL wrote:
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?

It sort of is.

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.


--
SL

"The Natural Philosopher" wrote in message
...
SL wrote:
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?

It sort of is.

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.

What make did you use?

Owain wrote:
SL wrote:
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?

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

Or if the slab is very very thick, and the bit next to the coils is at
30C when they are turned off.