I've been thinking about something & if you could make it work..

It seems that it would make sense to put in radiant tubing in a basement or slab & then install solar thermal to heat the water, providing free heat.  The thing I've been trying to figure out is how you can "divert" that heat in the summer, when you don't want to heat your house.  Is it possible to use a "summer/winter" setup where you direct the solar thermal to a water heater in the summer & to the slab in the winter?

It led me to another question - in a solar thermal setup directed to a water heater, what happens on very hot/sunny days when you're producing more hot water than you need?  Where does the "overflow" of heat go?

You would have a buffer tank that the solar heated water would go to... the solar heated water would then would go to the water heater year round, and also to a valve on a thermostat for the radiant floor heat. In the summer the thermostat for the radiant floor heat would be set to off position, which would keep the valve closed. The solar heated water would still go in the buffer tank and through the water heater when there is a demand.

OK, that makes sense.

Is this something that you've seen done? In theory it makes sense to me, but I haven't really heard of it being done (solar thermal heating a slab)

http://www.builditsolar.com is going to be your new best friend.

You can't possibly buffer a summers worth (even a week's worth) of heat dumping in a tank. Most active solar space-heating systems provide for either a heat-dump zone (fin-tube baseboard in a semi-conditoined shed that you can ventilate well is one approach), or sometimes with flat-panels, buffering the daily uptake in a tank an re-radiating the heat back out the panels by running the circulation pumps overnight to bring the temps low enough. Setting it up for a domestic hot water pre-heat is fairly common too, even if you let it run cooler when in space heating mode. (The lower the temp, the more efficient the collectors & system are due to lower losses out the glazing & distribution plumbing.)

Keeping the collectors at high angle reduces the summertime gain, increases the shoulder-season & winter gain. The old-school rule of thumb is to set them at latitude + 15 degrees, but in places with consistently good reflective snow cover there isn't much of a performance hit from simply going vertical, which makes designing in overhangs & shades for reducing summer gain much easier.

"...in a solar thermal setup directed to a water heater, what happens on very hot/sunny days when you're producing more hot water than you need? Where does the "overflow" of heat go?"

This is the reason that a solar hot-water system should not be oversized in terms of collector area, as it presents a stagnation temperature problem that can damage the collectors. In my system, the controller is set to limit the solar hot water storage tank to 165 F (74 C), to avoid damage to the tank, and the pump turns off once the tank reaches that temperature. The "overflow" heat is not used or dumped, so the panels begin to really cook. On my system, the stagnation temperature limit spec is 430 F (221 C), and the measured temperatures have never approached that condition. The controller also includes a "vacation mode" when hot water is not being used, and the pump runs at night to dump heat from the system to reduce the risk of overheating the collectors.

In terms of your overall idea to use the solar hot water system for domestic hot water in the summer and for radiant floor space heating in the winter, you might find it at least as efficient thermally, and far simpler and less expensive to use it for domestic hot water year around. You still bath in the winter, right? :-)

Posted By Lee Dodge on 06 Oct 2010 04:32 PM
"...in a solar thermal setup directed to a water heater, what happens on very hot/sunny days when you're producing more hot water than you need? Where does the "overflow" of heat go?"

This is the reason that a solar hot-water system should not be oversized in terms of collector area, as it presents a stagnation temperature problem that can damage the collectors. In my system, the controller is set to limit the solar hot water storage tank to 165 F (74 C), to avoid damage to the tank, and the pump turns off once the tank reaches that temperature. The "overflow" heat is not used or dumped, so the panels begin to really cook. On my system, the stagnation temperature limit spec is 430 F (221 C), and the measured temperatures have never approached that condition. The controller also includes a "vacation mode" when hot water is not being used, and the pump runs at night to dump heat from the system to reduce the risk of overheating the collectors.

In terms of your overall idea to use the solar hot water system for domestic hot water in the summer and for radiant floor space heating in the winter, you might find it at least as efficient thermally, and far simpler and less expensive to use it for domestic hot water year around. You still bath in the winter, right? :-)

A radiant slab rarely needs water over ~90F to deliver the heat.  DHW basically needs to always be 120F+. The difference in operating temp in winter between a DHW system and a slab -radiant heating system will be well over 30F, might be over 50F if the slab is basically an assist to the primary heating system.  Being able to run 30-50F cooler makes a HUGE difference in collector efficiency- it would nearly double the space-heating BTUs attainable with the solar over a season in a place like Green Bay.  Running a slab at DHW temps simply isn't (or shouldn't be) done, particularly when the heat source is solar.

Running 100' of PEX through the radiant slab as a pre-heat coil for the DHW can make sense though, both winter & summer, even if you switch the panels over to water-heating during the summer.

I've yet to see an active solar space heating system capable of carrying a signficant fraction of the heat load without active heat-dumping provisions.  It's often pretty easy to do  using flat panels as their own heat dump with a bit of buffering (like the DHW tank),  but with evacuated tubes you need a real radiator or well or ground loop or something to dump heat to.

Dana-

You said, "DHW basically needs to always be 120F+."
In my solar hot water system, as well as many of the newer systems in my area, the solar is used only as a preheater. In my case, the solar hot water storage tank is followed by a natural-gas fired, tankless water heater that adds heat to get it to the 120 F+ required. Through the summer, the tankless water heater does not usually add any heat, but it certainly might in the winter months. However, starting with water from the solar tank at 80 F or 90 F would be a big thermal advantage over starting at the ground water temperature of 45 F or 50 F. The tankless water heater has a proportional control with a wide dynamic range, so it only adds as much energy as required to reach to set point. Thus, the collectors can always be used at a high thermal efficiency, even when the outlet temperatures are significantly less than 120 F.

Since I need domestic hot water year around, it does not make sense in my case to pay for all the plumbing for a domestic hot water system, use it for 7 months of the year, and then add even more costs to use it to make a very minor contribution toward space heating, but pay the full costs for heating domestic hot water in the winter. I think most people use one or two collectors for domestic hot water, and many more, perhaps eight, for space heating.

Changing subjects slightly, although radiant floor heating is popular these days, in sunny Colorado, some people have found that when we get a lot of sun, the passive solar heating kicks in about the same time as the radiant floor heating, and the temperature is hard to control due to the high thermal inertia of the radiant floor heating. Cold nights and mornings, and hot afternoons are the result.

Posted By Lee Dodge on 06 Oct 2010 06:38 PM
Dana-

You said, "DHW basically needs to always be 120F+."
In my solar hot water system, as well as many of the newer systems in my area, the solar is used only as a preheater. In my case, the solar hot water storage tank is followed by a natural-gas fired, tankless water heater that adds heat to get it to the 120 F+ required. Through the summer, the tankless water heater does not usually add any heat, but it certainly might in the winter months. However, starting with water from the solar tank at 80 F or 90 F would be a big thermal advantage over starting at the ground water temperature of 45 F or 50 F. The tankless water heater has a proportional control with a wide dynamic range, so it only adds as much energy as required to reach to set point. Thus, the collectors can always be used at a high thermal efficiency, even when the outlet temperatures are significantly less than 120 F.

Since I need domestic hot water year around, it does not make sense in my case to pay for all the plumbing for a domestic hot water system, use it for 7 months of the year, and then add even more costs to use it to make a very minor contribution toward space heating, but pay the full costs for heating domestic hot water in the winter. I think most people use one or two collectors for domestic hot water, and many more, perhaps eight, for space heating.

Changing subjects slightly, although radiant floor heating is popular these days, in sunny Colorado, some people have found that when we get a lot of sun, the passive solar heating kicks in about the same time as the radiant floor heating, and the temperature is hard to control due to the high thermal inertia of the radiant floor heating. Cold nights and mornings, and hot afternoons are the result.

If doing only solar pre-heat biggest advantage/lowest temp in winter use would still be pumping the solar direct to the slab, with a an embedded DHW PEX coil for the pre-heat rather than stored to a tank, switching over to buffered DHW heat in a tank only when the solar gain exceeds the space heating load. (Which wouldn't likely happen before the spring equinox in Green Bay unless the solar array was quite large or insulation levels north of R40.)

Using the same thermal mass as both passive heat storage and as heating system radiation is fraught with issues such as you describe.  Low mass low-temp radiant systems such at WarmBoard, etc. would tend to work better than slabs in homes with high solar gain & very high diurnal temperature swings (as are common in clear-sky high-altitude places.)  Using slabs for the passive solar storage would work OK in conjunction with low-mass radiant though.

Thanks for all the replies! Just to clarify, this was a hypothetical - we just finished our house & don't have any update/upgrade plans for...I don't know....20 years..

I think the short answer I'm hearing is that it probalby could work, but the logistics & cost of using solar for both DHW & radiant heat may make it less than practical?

I'm not quite sure why you'd want to pump direct from the Solar Panels to a slab. You'd be subjecting the house to various different slab temperatures, and would just add to problems of maintaining a comfortable temperature. Most systems I've seen have used a large storage tank, with the radiant system being linked to this via a heat exchanger and possibly an ondemand heater to boost temps if required. This way you run your floors at your set point which should really not be more than a few degrees above your desired temp, else you run the risk of overheating in swing seasons with a good solar gain house. I guess this assumes you have a relatively low heat loss and don't need to maximize the BTU coming from the floor...

Posted By jerkylips on 07 Oct 2010 12:54 PM
Thanks for all the replies! Just to clarify, this was a hypothetical - we just finished our house & don't have any update/upgrade plans for...I don't know....20 years..

I think the short answer I'm hearing is that it probalby could work, but the logistics & cost of using solar for both DHW & radiant heat may make it less than practical?

It's really not that tough of a plumbing & control problem- no different than any dual-temp hydronic system.  Since your heat load is likely to be much higher than your DHW load, but occurs during part of the year with the least solar gain, it's worth optimizing it for both.

Posted By boardom on 07 Oct 2010 01:11 PM
I'm not quite sure why you'd want to pump direct from the Solar Panels to a slab. You'd be subjecting the house to various different slab temperatures, and would just add to problems of maintaining a comfortable temperature. Most systems I've seen have used a large storage tank, with the radiant system being linked to this via a heat exchanger and possibly an ondemand heater to boost temps if required. This way you run your floors at your set point which should really not be more than a few degrees above your desired temp, else you run the risk of overheating in swing seasons with a good solar gain house. I guess this assumes you have a relatively low heat loss and don't need to maximize the BTU coming from the floor...

well, here's my thought process, although it may be flawed..   I was thinking about our climate, in Green Bay.  We get cold winters (8000+ HDD), but definitely have a need for AC in the summer too.  Since central air requires the duct work for the AC anyway, radiant heating through the entire house is a tough sell for me.  If we were to install a forced hot air furnace & supplemented that with the radiant in the basement in the winter, it could reduce the heating demands of the furnace.  That led me to the "ok, but what about in the summer?" questions, & the water heating.

If you're not doing it this year then its probably too early to worry about the details. Now that so many of us are paying attention, advancements in a huge area of "green building" and solar tecdhnology are coming fast. By the time you're ready to do the project they'll likely have a DIY kit at HD. Not to say you shouldn't be thinking about it; thats how new technologies develop. Good luck in your new home!

There's nothing preventing you from running slightly below set temp water through your slab, just like you'd run slight above temp for heat, to get a small amount of cool. Pex tubing is quite inexpensive, assuming you don't design a ridiculous multizone system with multiple pumps and whatnot, radiant isn't really that pricey. If you're concern is cool, I would look into a small air-air heat pump (bonus heat) to provide your cooling primary needs. HRV/ERV to handle minor air circulation in the house. At least that's what I'm planning for my next place. Simple heat/cool, super insulation, tight airsealed. Simpler the better.

boardom- running it direct to the slab whenever the slab is calling for heat is far more efficient at the collector, being 10s of degrees below what your buffer temp would be (unless you've got 1000 gallons of buffer or something, which some do.) Controlling it such that the slab is the "priority zone" taking the DHW buffer off line for the solar heat source isn't a very complicated thing to do, and substantially increases the solar fraction. Overheating the slab with the solar isn't any more a problem than if you'd run it off a boiler- you'd use the same PID/other algorithm thermostats as the feedback as you would with any other hydronic radiant slab.

If I'm understanding this correctly, the radiant slab isn't intended to be the standalone heat source for the basement with a boiler backup, just an "as available, as needed" auxillary heat source. But putting the PEX in now, you COULD run it with a boiler + solar buffered or unbuffered, if you chose to implement it that way. Even if you NEVER used solar, did it all with a boiler, with a basement slab you'd always be well into the condensing temp operating zone, and could squeak the last percent or so out of a condensing boiler with it.

Not arguing the efficiency bonus of running it direct to slab, in a garage, sure... But in a house where you want to maintain an acceptable temperature I think you'd be out to lunch to do this unless you didn't have any walls.

The likelihood of overheating would be pretty close to 100% I would wager. On top of that, assuming you're getting some nice high temps off your collectors, you run the risk of damaging any flooring materials that may be covering the slab. This is generally why people trash high mass systems and the overheating risk, because they design them for the worst days and require a higher output temperature, rather than designing the house to run off a closer to design temp water temp. Hard to overheat much past 76F if your water temp is only 78F, a bit less hard if you're pushing 100F+ through the slab.
Lots of options that's for sure.

Posted By boardom on 07 Oct 2010 01:57 PM
Not arguing the efficiency bonus of running it direct to slab, in a garage, sure... But in a house where you want to maintain an acceptable temperature I think you'd be out to lunch to do this unless you didn't have any walls.

The likelihood of overheating would be pretty close to 100% I would wager. On top of that, assuming you're getting some nice high temps off your collectors, you run the risk of damaging any flooring materials that may be covering the slab. This is generally why people trash high mass systems and the overheating risk, because they design them for the worst days and require a higher output temperature, rather than designing the house to run off a closer to design temp water temp. Hard to overheat much past 76F if your water temp is only 78F, a bit less hard if you're pushing 100F+ through the slab.
Lots of options that's for sure.

I'll take a million USD on that one! 

If there's a room thermostat with a PID algorithm controlling when the slab got the solar heat or didn't, it'll be identical to any other hydronic heat source for the slab.

And it'd be a pretty poor solar implementation to let the solar run at high temp ultra-high delta-T.  As long as the solar array wasn't so ultra-oversized that it's uptake power was faster than the PEX could get it into the slab you'd be golden.

how about one BEEELION.
I guess I misinterpreted what you were saying.. I thought your intention was to circulate solar heat through the slab when it was available. Even when the rooms coming up to temp, I'd still want to reduce the heat coming from the collectors to the slab...
What are you doing with the solar heat when the room is at temp... It just seems like it would be simpler to jack heat into a big storage unit, and then pull heat as required.

When the thermostat for the slab zone isn't calling for heat, the panel output gets dumped to the DHW buffer- a very simple & common hydronic heating "priority zone" type of control. (Although in most hydronic heating systems the priorty zone is the indirect HW heater. Since the solar buffer is only pre-heat, and the slab temp would usually be cooler when there was a heating load, the priority gets flipped.)

Dumping it all into a single buffer and drawing as-needed results in lower efficiency for a 2-temp system, and isn't a dramatically easier plumbing or control issue. If the temp requirements for the & DHW were similar and you were boosting the temp with a boiler to some min as if it were the only heating source for the room it might be conceptually easier. The thermal mass of the slab will likely be higher than any reasonable sized buffer, so running the slab as a priority zone without the buffer in-between will always yield the lowest temp.

Make that a BEEELION Zimbabwe dollars and you're on! (I have a few trillion in my wallet- can't buy a stick o' gum with it, but it's a fun conversation piece.) :-)