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Concrete slab desireable for radiant flooring? - Page 2

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Posted by Solar Flare on November 6, 2005, 2:36 pm
A skeptic system! What a concept!...LOL

Posted by Q on November 6, 2005, 5:09 pm

Your heating input needs are going to be driven by house heat loss, which
is dependent on insulation and outside temperature.  You are going to
need the same amount of heat make-up whether you get it from solar or
conventional.  The solar can supply a finite amount each day, depending
on how much panel you installed.  When it gets cold enough to need more
than the solar you have no choice but to supplement with gas, electric,
wood, or yak dung - depending on how close to nature you are.  

Putting the heat into the house via the water heater and fan coil is the
same as using a furnace, heat pump, or stove.  It just takes less
equipment and cost.  You also can't utilize solar or greywater preheated
water in a furnace, heat pump, or stove.  That is how you shoot for a
high solar fraction.

You are missing some assumptions about how the equipment would operate.

1)  The fan coil should see a constant high water temp (130-140 deg f) so
that it can perform at its rated output and minimize fan run time.  Using
a modulated mixing valve to control the water going to the coil would
just result in the fan running all the time blowing slightly warmed air.  
Not energy efficient from an electricity standpoint.  The on-off
thermostatic cycling lets the water heater catch up if necessary.

2)  Yes, the water heater has to burn more gas as the solar tank gets
colder, but you are only supplying what the solar and greywater didn't
provide.  You are going to have to buy that energy somewhere to keep the
house at your desired temperature.

If you put enough solar collection and storage in place you will only
need the gas heat supplement late in the night of the coldest months
after the solar heat has been used.  The more solar heat you can store,
the more gas heat you can avid using.  I have done hourly simulations on
small houses (Habitat) in mild climates that show 75% of the annual
heating+ HW energy needs can be supplied by solar in this manner.  The
payback periods for a combined system were in the 5-6 year range.

You still need the mixing valve to feed the house 110 deg F domestic hot
water from the 130-140 deg F conventional tank.

And just consider the hot tub as a mass storage device.  You supply the
agitation to prevent thermal layering.

Gulf Coast Solar, Inc.

Posted by nicksanspam on November 6, 2005, 5:57 pm

Or a lot more for the same money, with solar siding or sunspaces :-)

What's the cost of shipping yak dung to California?


Posted by Q on November 6, 2005, 11:47 pm
 nicksanspam@ece.villanova.edu wrote in

The rest of the country thinks California has the dung market cornered

Posted by Gary on November 5, 2005, 1:30 am
 Iain McClatchie wrote:

Hi Iain,

We also have been thinking about going to radiant floor heating.

The alternative that we are probably going to use is:
Subfloor is plywood.  On TOP of the plywood, a 2nd layer of plywood
that is the same thickness as the PEX is laid down.  The 2nd layer has
grooves to accept the PEX tubing, which is installed with aluminum
spreader plates. The finish floor is then installed over this (being
careful where you put the nails).  There are several outfits that make
precut panels with the PEX grooves in them, or you can make them on
the site.

I like this better than method B because there is less thermal
resistance between the PEX and the heated area (just the finished
floor) -- this means the water does not need to be as hot (more
compatible with solar heating), and less heat goes to the floor below.

It seems to me that the high thermal mass that you get from concrete
is a 2 edged sword.  Its not so much fun (I think) to have to wait
many hours for the house to heat up after you get back from a
vacation, which I am told is a characteristic of PEX in concrete.  It
also seems to eliminate the possibility of night time setback of your
thermostat.  And, I don't see why the non-concrete solution cannot
provide an even temperature.

To me, having a high thermal resistance between the PEX and the heated
area (as in scheme B) is not acceptable, because it means higher water
temps, and this makes it harder to have some of your heat come from
solar thermal collectors.  I think that one of the nice features of
radiant floor heating is that it is compatible with using solar to
supplement your heating.

Here is a link you might like:

If you go to the "Tech Library" section (you have to register), they
have a download under radiant floors called something like "The Manual
of Modern Hydronic Heating" -- its pretty good -- it provides enough
detail to design a system right down to loop temperatures and flow
rates, as well a providing quite a bit of info on the hydronic heating
system components.

I'd like to hear what you finally decide (and why).



"Build It Yourself" Solar Projects

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