-=> Quoting Alec Cameron to Bob Knowles <=-
Hi Alec
 
 BK> I have always fancied hot water pipe under floor heating with
 BK> thermosyphon but I have a hot water boiler with a small pump that uses
 BK> so little power that I leave it on all winter without switching it on
 BK> demand.
 AC> I was involved with electric underfloor heating, at some power
 AC> stations [admin offices, locker rooms, shower rooms]. The thermal
 AC> inertia/ long thermal time constant was sometimes a horror, a change in
 AC> ambient temp would be accompanied by very long heat up- cool down
 AC> periods. Staff grumbled. 
 AC> Even more serious, elec shocks to the bathers! Not easy to achieve
 AC> perfect grounding inside a PS that deals in huge load currents.
 AC> My personal [human] thermostat is very dodgy and I want rapid response
 AC> to my needs so our house has flued natural gas at one end and reverse
 AC> cycle split sytem at the other.
It is my understanding of underfloor heating that you do not use it as a
the whole heating system but as "background heating" IE you may aim for
16 c (61 f) with your underfloor heating and then top up the rooms that
you are using with other forms of heating. This tends to stop the
problems that one has with quick changes in the outside temps as we have
here in Oz. Mind you I would be aiming at a higher design myself than
16 c as I hate the cold and wouldn't mind if the indoor temperature went
up a bit during the day.
I would have thought that the reverse cycle unit down there would have
been a bit dodgy because of the low out door ambient you experience, do
you have a electric element booster in it?  Usually around 7c (45f) the
output of reverse cycle on heating is only equivalent to electric heaters
of the same power consumption.
 BK> For anyone who does not understand 2 and 4 pipe systems, 2 pipe is a
 BK> flow and return that uses hot water in winter and chilled water in
 BK> summer. The same coils are used for both applications (heating and
 BK> cooling) The 4 pipe system has 2 flows and 2 returns. A flow and return
 BK> for the hot water and a flow and return for the chilled water. The 4
 BK> pipe system need a heating and a cooling pump. It also needs a  heating
 BK> and a cooling coil  in each fan coil unit.
 AC> And you have independent thermostats in each room to adjust the local
 AC> flow rate, yes?
I have 4 fan coil units, each with it's own thermostat and water valve,
the water valves are on/off not modulating. Modulating valves give
better control, but if your pumping cost is a worry then they tend to
hold the pump in with little  heating being achieved. When I designed
the system the chilled water pump used to pump the hot water around and
it was 4 times the size than was actually needed for heating. I find
that the on/off valves and thermostats work well enough and can hold the
room temperature to within 1 or 2 degrees. As a power saving measure I
make the fans cycle with the thermostat, so they only run when heating
is required, it saves quite a lot of power this way. Maybe the final
temperature in the room maybe not so tightly controlled but I do not
notice it. Three way valves are the go on my system so that the water is
hot when heating is called for. Mind you since I leave the small
pump running all the time that is not a problem, but when I get all
energetic and put a relay on the pump that switches it in on the demand
of the fan coil units (and thermostats) the 3 way valves will come into
their own.
I am working on controlling the whole system with a computer, but it is
a rather long time project I am afraid, but I am getting there. I need a
new house and start from scratch :-)
Three way valves allowing the hot water to bypass the coils whilst they
are not actually heating but the pump is running. This helps to stop
"dead" (cold) water suddenly being dropped into the boiler and lowering
the temperature. As my hot water (heating) system tank has a 150 feet of
3/4 copper tube in it to supply mains pressure domestic hot water I
need to keep a fairly stable water temperature in the hot water tank
so as to maintain a reasonable domestic hot water supply. I do get a
fair amount of thermosyphon in my system if the pump is shut down with
the 3 way valves. The boiler and associated hot water tank are only
pressurized by a header tank on the roof and the system is vented to the
air above that, so the actual boiler is only on about 10 psi pressure
(70 kpa). The hot water from the boiler thermosyphons to the hot water
tank.
The actual water flow rates to each fancoil unit is fixed by a globe
valve, but this is set by chilled water needs in summer rather than hot
water needs, as the chilled water is more temperamental because of lower
water to air temperature than the hot water on heating.
BYW I am glad it was you having the 2.5c (36.5f) the other morning, real
brass monkey weather.... I complain when it goes down to 10c (50f) here
even lowder if it drops to 5 which is rare thank goodness.
Catch you later
    Bob
... It has been 2093746529085 seconds since I had sex, but who is
... countin ?  
--- Blue Wave v2.12
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