PEX for cooling sub-floor
 

while our new house is under construction, they're currently working on building our 2nd floor. I have an opportunity to install PEX under the flooring of the 2nd floor, for possible cooling in the future. I already have our radiant floor installed in the slab on the 1st floor, so our equipment for circ water in this system will already be there. And I'm eventually going to try out AC Hacker's GSHP later on. I was thinking, why not go ahead and install PEX for possible cooling later on? I'll have temp sensors hooked up for it. And I bet I can also get some moisture sensors installed along the pex as well, to detect moisture, dew point, to shut the cooling down at a certain level.
I've been using 1/2" pex spaced 12" apart for heating our slab.
And our 1st and 2nd floor is roughly about 900 sq ft.
I was thinking that just having the plain pex circulating cooler water in-between floors like that, would take away some of the heat during the summer. But I don't know much about the numbers for the cooling side of things.
Does anyone have any suggestions for how I need to do the calculations and best way to install this?

You are the first person on this forum to show any substantial interest in radiant cooling.

Since cooling is your primary season, you might want to give some thought to efficiency.

So here's what I know...

What you're trying to do is called "high temperature cooling" and also "low exergy cooling". You might try searching on those terms.

You might have to modify your understanding of what heating and cooling are in pursuing your project, and be mindful of comfort rather that the readings from the thermometer.

Our bodies are covered with sensors that have evolved to keep us alive. We generate some amount of heat and that heat must be gotten rid of or we will die... so we have an evaporative cooling system to assist us. When our bodies are able to get rid of heat easily we feel comfortable. If the heat can not be gotten rid of easily we experience discomfort and become restless and miserable and motivated to seek shade under a tree or want to jump in to a glittering blue swimming pool, or some other way to cool off.

Looking at heating for a moment, you have probably experienced the feeling, when standing next to a window on a very cold winter night, of the warmth being 'sucked out of you'... well, that's exactly what was happening. If you had double or triple glass windows, you'd notice that the expected 'heat suck' experience was not there... in other words, you would feel comfortable.

The reverse is also true... if you are in a climate where heat predominates and on a particularly hot day, you go into a tin-roofed structure, you immediately feel the opressive heat radiating onto your skin, and your body senses that it's heat load cannot be dumped and that lethal conditions could be forthcoming... you experience discomfort.

The Germans are using radiant cooling with great success in office buildings. They are using continuous panels that conduct cooled water, in the ceilings. The panels have sensors that monitor dew point, so that it doesn't start raining on the office workers.

You could cool your slab too, but you'll need to monitor dew point.

If it were me, I'd use fairly close spaced PEX with aluminum spreader plates that physically contacted your ceiling material, probably sheet rock.

Using the high temperature cooling approach, you could gain greater comfort in a room that a thermometer would tell you shouldn't feel so comfortable but you'd be save money on cooling.

Humidity will play a big role in this scheme, in maintaining comfort. But the amount of dehumidification will be lower.

Overall, you should save money.

But in all honesty, the very thing that makes convection heating problematic, also makes convection cooling work well. Stratification of air temperatures with hot on top and cool at the bottom, where people lounge about.

You could also cool interior walls, etc.

But all in all, I think your strategy is a good one. You'll probably still need some amount of A/C, but it would be less, and your overall cooling cost will be lower.

Keep us posted as to how this goes.

Best,

-AC

this sounds very interesting. I'm going to have to think on what you've said some more until it all sinks in. I'm hoping to eventually (crossing my fingers and toes!) to build one of your GSHP, install piping underground, and use that to help augment the cooling requirements of our house during the summer.
This picture should help illustrate what I have in mind. for the pex under our 2nd floor. I'll be installing it throughout in-between the joists of our floor framing. I'm told this is also where our AC/heating ductwork will go.
The red line I inserted is also traveling down the inside-corner of our building, where I'll have everything running all of this. My wife calls this my Central-Command, as she thinks I'm half-crazy for trying to do all of this. haha. :)

The PEX in the concrete works because concrete is a pretty good conductor of heat. Heat transfers through conduction very efficiently.

PEX in concrete is a very efficient configuration.

If you are only running PEX through the spaces between the floors, you'll have what is known as "suspended tubing". This configuration ranks at the very bottom, in terms of efficiency.

If you physically fasten the PEX to your ceiling, you will then be approximating what is known as "Staple-up", which is only one notch up from suspended.

If you use aluminum spreader plates and make sure that they are in good physical contact with your sheet rock, you'll have a pretty high efficiency set-up.

-AC

I've been thinking about doing the same thing with my radiant heating installation. I'm thinking in my case the water shouldn't be less than 60F, to avoid condensation (from what I can tell from various charts and graphs online, 85F air temperature at 40% RH gives a dew point at about 59F). We're usually at 30-35% RH, so I'd consider 60F water to be safe.

One question that arises is how to keep the water cool. I've got a closed loop system, so it stands to reason that the water will warm up after a while, reducing the cooling effect. How quickly the water warms up is TBD - I don't think there's a good way to tell without trying.
I've considered peltier elements attached to the manifold to lower the water temperature - any thoughts?

Yeah, it would be really easy for you, with your wizard skills, to build a dew point calculator that was in direct contact with the coldest part of your cooling loop because that is the exact point where the condensation will begin.

Not to discourage you in this effort, but having radiant floor cooling doesn't have all the same advantages as radiant floor heating.

Because of temperature stratification, your highest delta-T in the room will not be so close to your coolness radiator where it would do the most good.

The Germans are doing quite well with radiant cooling by using radiant ceiling cooling, and the dew point issue still remains.

But no doubt, radiant floor cooling would help considerably, and you already have the infrastructure, so why not use it?

As far as cooling the fluid in your closed loop, you pretty much have heat pumps (cold pumps?), ground source, and evaporators as options.

If you're going to take a serious look at heat pumps for cooling, why not also for heating?

Ditto GSHP.

Now that you have the infrastructure, there are many possibilities.

-AC

I wouldn't exactly call it easy, but doable :P

You're right though, it's more complicated than just guessing. The coldest point would probably be right around the manifold, but humidity could be higher in the crawlspace, where it matters most. Sticking a temp/humidity sensor into the crawlspace or somewhere into the floor sandwich, calculating the condensation point and using that data to turn on/off the pump and cooling wouldn't be too big of a deal; but can we assume that the temp/humidity is the same everywhere in the crawl?

I'll probably hook a sensor up and just try to see what sort of data I get. As for cooling, if we assume the water cools to 65F overnight (say via a ground loop), this should be doable...

[WARNING: lots of speculative math without much actual data to back it up follows]

Specific heat of water is 4.187 kJ/kgK, or 4.84 BTU/lbF. 33gal in my water heater is 264lbs, so if we assume we want to get to 78F room temperature with 65F water, that's roughly 16k BTUs of cooling capacity in the water.

Previous data I measured suggested a heat gain of 400BTU/hF or so, which would mean 2800BTU/h for a 7 degree difference. Let's triple that and round up for a sunny day to 8500 BTU/h. 16000-2400 = 13600 / 8500 = 1.6, so it would take only about an hour and a half for the water to heat up to 78 degrees.
Basically, that means if I could get the water down to 65 degrees overnight, I could run it through the loops for a total of 90 minutes *assuming ideal heat transfer* - so basically, assuming I could actually cool the air to 78 degrees on an 85 degree day with the floors alone. I don't expect the transfer to be nearly as good, so the time may be closer to 4 hours with a less pronounced cooling effect.

I guess the numbers make it look not entirely outside of the realm of possibility. I might be able to keep it down to 80 or so, if I can convince everyone to keep the doors and windows closed :P

I think I have to disagree with the statement that PEX is an efficient anything. While the concrete is a very good heat sink, and ALUMINUM plates are very good conductors, PEX tubing isn't. About all that can be said for it(IMHO) is that it is inexpensive. It only works in a radiant system because of the plates. Hot air blowing over the aluminum plating underfloor would work almost as well. I would think that in a chase where the O.P. is running air and heating ductwork, 1/4" or 3/8" copper tubing would work much more efficiently, and after factoring in the cost of the pex,plates and fittings, wouldn't cost much(if any)more.
Of course this is only my opinion and, being in the plumbing and heating industry for forty years, I realize I have a bias against PEX anyway. Can someone show me the error in my thinking, if there is one(or more)!

With your experience of 4 decades in the heating industry, it should be a trivial matter for you to implement your idea.

If it actually works, you will have some awesome bragging rights.

Nothing ventured, nothing gained.

-AC

I would caution the utilization of surfaces for cooling.

The main reason we feel uncomfortable as humans in warm humid weather as AC Hacker pointed out. Our natural cooling method of sweating is less efficient when humidity is high. Our sweat doesn't evaporate very well hence removing less to no heat from our skin.

Air conditioning like the name implies lowers the humidity making us feel more comfortable as the act of sweating becomes more efficient at removing heat from our skin. You can feel quite comfortable in dry 80Deg F room as opposed to a 80 Deg F. Humid room. With the same dry air at 72 Deg F. we can start to feel cold.

Having first hand experience with radiant floor heating it is extremely poor for air-conditioning or space cooling. As mentioned in an earlier post the condensation point is reached quite quickly with little comfort gained as the air remains humid, in-fact more clammy feeling. The surfaces will begin to sweat. I realize that we are able to control temperatures of the surfaces closely to stay out of the dew point but there will be areas close to the entry point of the cool water that hadn`t received the message yet and will begin to sweat. And don`t forget that dew point will change as the temperature & relative humidity changes and the surfaces temperature can`t change fast enough.

The real dangers with cooling surfaces such as ceilings or walls if no dehumidification is taking place is moisture on the surfaces will cause mold.

We find here at home if the concrete floor cools to less than 70 Deg.F with the cool air-conditioned air falling down on to the floor causes our feet to ache as the muscles become cold. If I wasn`t such a energy miser I would like to add just a wee bit of heat to the floor. I couldn`t imagine the discomfort with a cooled floor of 55-60 Deg F

The best solution is to pump the cold liquid through an air handler and properly remove the humidity and reduce the heat. The amount of cooling provided by a heat pump is amazing and very efficient.

The combination of dehumidification and surface cooling for a commercially huge building may offer some greater energy savings as large expansive concrete walls suck up huge amounts of heat. But for a residential home the double complex system isn't good ROI.

Randen