Does anyone use hydronic underfloor heating systems to move passive solar heat around the mass?
felixcollins
| Posted in Energy Efficiency and Durability on
I’m in the early stages of planning a new build. I’m considering a heat pump underfloor heating system in concrete slab. The house will be carefully optimised for passive solar gain as well. Has anyone heard of using the underfloor plumbing to move heat around the slab from areas receiving direct solar illumination to other parts of the slab?
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Replies
Felix,
Q. "Has anyone heard of using the underfloor plumbing to move heat around the slab from areas receiving direct solar illumination to other parts of the slab?"
A. Yes, I've heard of it. But as far as I know, the disappointing results don't justify the pumping energy or the necessary controls.
Slabs heat up slowly, and act like flywheels. Moving heat from Slab Area #1 to Slab Area #2 doesn't result in quick changes in air temperature. In other words, this proposed system isn't very responsive.
And doesn't concrete "equalize" pretty quickly anyway?
Thanks for the responses. Do you have any examples documented that I could review or contact?
I'm not worried about the response time (the response time of a concrete slab to insolation is slow in any case).
My thought is that by convecting the heat to parts of the slab that never get sun, I'm effectively increasing the amount of direct solar exposed thermal mass that I can use in my performance calculations. This would mean that there could be less thermal mass installed elsewhere in the house and/or the slab could be made thinner.
If I'm installing hydronic underfloor heating anyway then the fixed marginal cost of using it to distribute within the slab is two changeover valves and a more complex controller (if it is automated). There is of course power consumption of the pump as well, depending on the controller this could be kept low.
In response to Dan Kolbert. In my current house with a well insulated 100mm slab floor I can walk around in bare feet 4 or 5 hours after the sun was directly illuminating it. The parts away from the glass feel cold and the parts close feel warm, to my cold feet anyway :-)
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
Concrete slabs do not behave like pools of water. The solar heat absorbed would conduct and re-radiate back our of the slab long before it could conduct to distant cooler parts of itself.
Felix,
Yes you certainly could take that fluid during the day and use that same fluid to warm the unheated surfaces in other rooms and hold out overheating of that surface / room and others receiving the solar gain . Always good to use something others consider a problem to your advantage . . As far as pump power if it designed by a qualified designer / installer and the loops are sized properly you WOULD CERTAINLY be able to move that fluid using less than 50 watts per hour . Seems that many around the community have looked to no end for people whom are capable of these unheard of feats but price was their only criteria when time came to hire . Luckily for a few of us who sat around because we knew that to do the job right required knowledge and a complete understanding of thermal / fluid dynamics and heat transfer and that you could not do it with equipment and salesmen's BS and / or for free . Several of us now exist and have actually managed to possess the knowledge to do it right AND at a reasonable number . For more assistance with this you may contact me directly by posting your question in a forum where people capable of helping you answer your questions dwell . Here http://www.heatinghelp.com/ or here , http://flopro.ning.com/ . I frequent these and will be looking for your question there . By the way , I specialize in the types of houses that are discussed here .
Richard,
You wrote, "You WOULD CERTAINLY be able to move that fluid using less than 50 watts per hour."
I think you meant to write "watts," not "watts per hour."
Martin wrote:
Felix,
Q. "Has anyone heard of using the underfloor plumbing to move heat around the slab from areas receiving direct solar illumination to other parts of the slab?"
A. Yes, I've heard of it. But as far as I know, the disappointing results don't justify the pumping energy or the necessary controls.
Slabs heat up slowly, and act like flywheels. Moving heat from Slab Area #1 to Slab Area #2 doesn't result in quick changes in air temperature. In other words, this proposed system isn't very responsive.
* * * *
Well we do like to split hairs don't we?
I would consider the money spent on ones' home and the design constraints while only achieving beneficial , energy saving gain on the southern exposures disappointing.
The mention of air temperature does not belong in a discussion about a radiant slab. A radiant slab does take awhile to heat up but it must be said that a radiant slab or any radiant surface heats surfaces in a space as opposed to air, MRT is probably the most desired effect in any building as all of the surfaces will prevent heat loss from occupants. Fact is that water is the predicate device that all things heat are compared to and the chase has been on to do what water can for quite some time . This is Green building advisor and I believe that we should do everything possible to remove as much refrigerant as possible from the built environment , I repeat , there are no green refrigerants.
Furthermore, heat pumps that use mechanical means to extract heat from fluids and air that begin to lose their COP at certain points and use refrigerant to do this are nonsensical.
I urge everyone to further research fluid based systems and their true capabilities when designed , installed and commissioned by truly qualified people . Too many contractors jumped into the arena early in the game without knowing about fluid / thermal dynamics or how these systems were supposed to work and those very same individuals harmed the hydronic industry , I and others knew it would happen but were powerless to stop it . People have to ask themselves one question , does financial success really show one's capability or does it display the consumers vulnerability.
All nonsense aside , there are now side by side real life actual in depth studies demonstrating first cost , operational cost between VAV and Radiant and water based systems and VRF . Check out Plumbing and Mechanical article from March by Greg Cuniff referring to the ASHRAE building in Atlanta or the side by side comparison of the Infosys building in Hyderabad India , or what the NREL labs in Colorado found out in their own building . No more modeling , no more nonsense , the results are now appearing at a regular rate .
Let us not forget about the fact that we should be making these buildings healthy for the occupants . Would you rather be in a hospital with people who have contagious diseases that utilizes air based , ducted systems or one that heats and cools with fluid and has a DOAS system ? Sorry folks there are not enough MERVS on the planet .
Richard,
You have introduced many new issues into a thread that was discussing a specific technical proposal: using tubing embedded in a concrete slab to move heat from the south side of a passive-solar house to the north side of a passive-solar house.
All of the issues you bring up are worth discussing, but they aren't the focus of this thread.
For example, you brought up the question of designing an HVAC system for a hospital that minimizes contagion from one patient to another. While that is an interesting question, it has nothing to do with this thread.
In case anyone is wondering, although Richard implies that I am a fan of ducted heating and cooling systems, a review of my articles will reveal that that is not the case.
My house fits the description of what you are planning.
What I've found is that a well-insulated, tight house stays isothermal primarily though air convection.
That means natural air convection inside the house moves heat around faster than it is lost through the walls and windows. Therefore, you are worrying about a non-problem.
I have a basement that naturally stays cooler than the house in the summer. I tried 24/7 pumping to see if it would cool the house and heat the basement. I couldn't measure even one degree of benefit.
Response to Martin
Martin I neither implied or assumed that you were a fan of anything . Sorry that you thought that .
My descriptions and the way I attempted while although not specifically to do with the topic were included in an attempt explain why the disappointing results you so often refer to came to be the norm . My point is that Hydronic systems and proper design and arrangement and a willingness to consider all things in EVERY kind of building should not be wholly discounted because of past poor performance of guys whom probably had no business performing it in the first place . I think it is a disservice to anyone attempting to gain knowledge and build a truly efficient building .
I think you will find those reports illuminating , I know you'll read them to be better prepared to answer future efficiency and efficacy related questions .