# Using slab as heatsink?

Discussion in 'Home Power and Microgeneration' started by Dave Hinz, Jun 20, 2005.

1. ### Dave HinzGuest

OK, so I've got hydronic tubing in my basement and kitchen floors, which
works great for heating those areas in the summer. But, I got to
thinking - during the summer, the temperature of the slab is _below_ the
temperature of the air upstairs, and no matter how I pull the forced
air, the basement is always considerably cooler than the rest of the
house.

I was wondering if anyone has run a water to air heat exchanger (OK, a
radiator), similar to an A-coil for A/C, to dump the heat from the air
into the water (and the slab). Does anyone have any starting points for
calculations for something like this?

In other words, given a temperature differential of, say, 10 degrees F,
between the thermal mass and the air, and an airflow of (number), how
much can I change the temperature of the air by blowing it through
said radiator full of cold water?

Dave Hinz

2. ### Dave HinzGuest

No, I'm trying to cool the house by dumping the heat from the upstairs
air, into the downstairs slab.
But, I need a water to air heat exhchanger at some point, because the
water is how I'll get that heat into the slab. I think?

3. ### VaughnGuest

First, have you actually measured the temperature differential? Two
thoughts; (not that I want to discourage you) (1) Whatever the temperature
differential is without the system running, it will be significantly less after
a few hours of operation. (2) Unless you are able to cool your room air below
the dew point, you won't get any dehumidification.

I recently got a lesson in just how important dehumidification is; I bought
one of those single-hose portable AC units. For every cubic foot of hot air
that thing blows out of my house, a humid cubic foot enters. The unit cools the
space, but you still are not as comfortable as if you had a REAL AC unit.
Unless you live in Arizona, don't buy a single-hose portable AC unit.

Vaughn

4. ### Dave HinzGuest

12 degrees F. Seems like enough to bother with, I think?
Well, that is the goal after all.
I'm still running the central A/C, so dehumidification is happening;
it's just that I'd like to help dump heat into the slab. Sure, it'll
make the basement more warm, but that's OK - we're upstairs most of the
time.

And, in this case, the dew point issue actually helps, because the water
will come out at the a-coil of the A/C, while the radiator can transfer
some of that heat to the slab.

6. ### Anthony MatonakGuest

Maybe you should simply vent the heat from the upstairs air to the
outside and bring outside air from ground level into the downstairs
to be cooled by the slab and then migrate upwards into the home.

This might present some condensation issues.
Hmm... How about using a swamp cooler (even home made) as a pre-cooler
and then further cool this cool-damp air using, say, auto radiators
and the water from your basement slab. Use an air-air heat exchanger
to cool your indoor air with this damp-cool air so that you're not
putting humid air inside your house. Vent the now-warm-damp air outside.

Just a thought, I'm sure it can be improved on. Perhaps adding some
thermal mass outside, like those piles of rocks the one fellow was
suggesting, that you blow air through at night to 'store' the cooling.

Anthony

7. ### Guest

http://www.magicaire.com/productd.htm

Their SHW2347 2'x2' \$150(?) duct heat exchanger moves 45K Btu/h from 125 F
water to 68 F air with 1400 cfm of airflow with a 0.1" H2O pressure drop,
ie a 45K/(125-68) = 800 Btu/h thermal transconductance. An auto radiator
might do as well.

Nick

8. ### Derek BroughtonGuest

Not "significantly". A slab floor holds an enormous amount of heat. We run
hot (solar heated) air through ours, and it takes more than a day of
operation to make what I would consider a "significant" different in
temperature to the slab.

9. ### Dave HinzGuest

Yes, the total heat won't change, but right now, the basement is _cold_.
Even it out, and the upstairs will be more pleasant.
If the slab is cooler than the air, how would making the air cooler not
be a benefit?

10. ### Dave HinzGuest

Well, that's exactly what I'm saying. Heat from the warm air goes into
the cold water, so the air gets cooler and the water gets warmer.
Thanks for the link, I"ll check 'em out.
Well, I have a forced air system, and I'm taking the basement air in
(which is cool), but the heat transfer never touches the nice cool slab.
That's the part I'm trying to change.
Sounds like phase 3...

Dave Hinz

11. ### Dave HinzGuest

Right, if I didn't have A/C, that might work, but I can't see throwing
away nice dry air.
That's an interesting way to look at it - similar in concept to the
sealed combustion furnace, where you use outside air instead of inducing
a leak to feed the flame. Hmm...
Well, I've got plenty of room for coils of tubing under the back yard,
which is always an option.

Thanks - some good ideas, everyone.

Dave

12. ### Dave HinzGuest

Right. For this particular slab, I can get one degree (F) of
temperature rise per hour, when I heat the water with a 4500 Watt water
heater.

13. ### MeGuest

Just a note but water actually transfers energy significantly better
than air does, so you would expect any Air/Slab transfers to take
considerably longer than Water/Slab transfers of the same energy amount.
What one might consider would be a Fan/Radiator heat exchanger in the
attic, that then feeds a Hydronic tube system in the slab. Pump the
water/glycol fluid around that system and have the fan blow the hot air
thru the radiator. Then let the transfer fluid heat the slab during the
day, and return the heat from the slab via another radiator/Fan system
in the forced air heating system. Use the slab as a energy storage
system.

Me

14. ### Derek BroughtonGuest

Yeah. That didn't hit me until I read Dave's response...

15. ### Dave HinzGuest

I wasn't confused until just now. If I can get a temperature
differential across the radiator by blowing warm air through it, that
means I'm transferring heat to the water, yes? At that point it becomes
an exercise of balancing air flow vs. water flow to find the most heat
transfer possible - which should be "as much flow of each as I can
manage", I think?

17. ### Dave HinzGuest

So, do I want to maximize differential, or (guessing) differential times
flow?

At some point, as flow goes up, will the differential go down so much
that I'm not just diminishing returns, but getting less transfer?

18. ### daestromGuest

Heat exchanger design/usage is a whole science in and of itself. But here
are a couple of points:

1) The metal walls in heat exchangers are *not* the major resistance to heat
transfer. The metal is a good conductor (duh!), but the laminar layer of
air/water right next to the metal is not as good.

2) With that in mind, to get the most heat transfer for a given sized heat
exchanger, you want to minimize the resistance/thickness of the two laminar
films (remember, one on each side of the metal wall). The easiest thing to
do this is increase the local velocity of the air/water. But as you
suspect, there is a point where a further increase in flow doesn't reduce
the laminar layer much more at all.

3) And while the differential pressure needed to move water goes up with
flow-rate squared, the *power* needed goes up with flow-rate cubed. So
doubling the flow through the system requires *eight* times the power.

4) A 'rule-of-thumb' we use in industrial heat-exchangers is to target water
flow for something like 4 to 7 feet/second flow through the actual
heat-exchanger tubes. This reduces the laminar layer pretty far, and
doesn't require too much pumping power. The trick is to figure out just
what the cross-section of the tube is, and the number of tubes that are in
parallel.

5) As one increases the flow through the heat exchanger, the temperature
rise/drop of the water/air will decrease. This is because although the
amount of heat transferred *increases*, the mass flowrate increases faster
(the amount of heat Q=<massflowrate>*<heatcapacity>*(Tin - Tout) ) This
sometimes confuses folks since the outlet temperature approaches the inlet
temperature and some people suppose this means that *less* heat is being
transferred.

6) Counter-flow arrangement is best, but most practical designs are
'cross-flow'. Parallel flow is worst (and easily converted to
counter-flow).

In conclusion, no you will never increase flow the point where heat transfer
does *down*. But extreme velocities can cause erosion problems and pumping
power requirements can be enormous.

daestrom

19. ### daestromGuest

This *should* work rather well. The two things to keep in mind is that the
coil will form condensate that you must provide some drainage for. Don't
let it drip onto the furnace heat exchanger, that's much to expensive to let
rust. The second consideration is that auto-radiators might corrode from
just raw water rather quickly (a year??). In automotive use, the
high-temperature and air-free environment reduces corrosion (and yes, the
anti-freeze also has corrosion inhibitors). The outside of auto-radiators
is not designed for constant moisture, but they often have a coat of paint
to protect them from occasional rain.

I haven't heard of anyone's direct experience though, so it's a bit hard to
tell how well it would hold up to use.

And of course, how much you pay for pumping the water is something to think
about (it may not be a problem, just something to think about). To get 3
A/C tonnes of cooling (36000 BTU/hr), if the water rises from 55F to 70F
through the heat exchanger, you need a flow of 2400 lbm/hour (4.8 gpm).

daestrom