Aluminum diffusion panels vs air space

dan_in_sf

Help!  We are about to start work with a hydronics contractor who is going to be installing a radiant heating tubes in the joists under our second floor.  His method is to suspendthe tubing or cables underneath the subfloor between the joists and then use reflective insulation under the cables, thus heating the air space.  He assures us that this will be more than enough to meet our heating requirements. I happened to be talking to another radiant heating expert/contractor who told me that this will not suffice - that we need to have aluminum panels installed that house the cables and diffuse the heat.  Obviously installing a bunch of aluminum panels adds quite a bit to the price of the overall project so I am reluctant to go that route.  However he said that just relying on the air pocket will mean the boiler will have to heat the water to a much higher temparture and that we still might not be able to get the room to the temparture we want.  I should note that we live in an old, drafty Victorian house in San Francisco (which can get fairly cold) 

Do we need to go for these more expensive aluminum plates? 

Gordan

They may both be wrong.

Did either of them do any heat loss calculations to back up their claims? This is not all that hard to do.



The worst case scenario would be that your drafty old Victorian is not a good candidate for radiant floor heating, period - at least not without supplemental heating. In that case, both contractors would be selling you systems that'll leave you out in the cold. The other extreme, which is considerably less likely, is that your drafty old Victorian has very low heat loss and the rest of your system requires high water temperatures anyway, in which case the suspended tube guy could be offering you the best value for the money.



The contractor who is recommending aluminum heat transfer plates is correct in that those do provide more heat output, or same output with much cooler water. Depending on what the rest of your heating system looks like, this could be a big advantage. Can you tell us about what heats the rest of your house, and about your boiler?

Gordy

Gordan

 Is right on his comments..  I would go for the plates at any rate this will lower water temps if the heatloss will allow it, and save money getting back the cost of the plate investment on your gas bill.   Is this going to be your primary heat source, or is there supplemental? Ask the contractor if he did a heatloss. Also ask if the heating system does not give the needed output if you have to pay him.





An old drafty victorian home is not a good canidate for any kind of EFFICIENT heating, sounds like you need to tighten up the envelope.





Gordy

dan_in_sf

Additional information about the scenario

Thanks for the quick responses so far.



To answer your questions:  The plan is to have the entire house heated via radiant heating.  We are currently completely having the lower floor of the house redone.  This lower floor will have a new concrete slab so the radiant tubing for that floor will be installed in that slab.  That level of the house is also getting all new siding, so we will have the latest in wall insulation, ceiling insulation, and heat retaining windows.  The upper floor has a lot less work being done on it (and we don't have enough money to do a lot of work on it now).  This floor has some insulation already, but in general it still feels drafty (this is my subjective assessment) -  some of that might be due to the fact that until now cold air was coming from the lower level, which until now has been "unfinished" and completely uninsulated .  This is the floor that will get the tubes placed in the joints below. 



The proposal from the contractor that wants to use suspended tubing (through an air pocket) is to run the entire house's heating needs from a TriangleTube Solo 110.  There will also be a LochinvarSSS60 installed for our domestic hot water. 



This contractor's proposal is 2/3 of the person who wants to use the diffusion plates, for a savings of almost $10,000.  I doubt we would recoup that difference in savings on our energy bills alone.  Before this remodel, we had an extremely inefficient old gravity-fed heating system and our bills weren't that high.  San Francisco can get cold, but thats a relative term -  ultimately we didn't use the heating system that often. 

dan_in_sf

Additional information about the scenario

Thanks for the quick responses so far.



To answer your questions:  The plan is to have the entire house heated via radiant heating.  We are currently completely having the lower floor of the house redone.  This lower floor will have a new concrete slab so the radiant tubing for that floor will be installed in that slab.  That level of the house is also getting all new siding, so we will have the latest in wall insulation, ceiling insulation, and heat retaining windows.  The upper floor has a lot less work being done on it (and we don't have enough money to do a lot of work on it now).  This floor has some insulation already, but in general it still feels drafty (this is my subjective assessment) -  some of that might be due to the fact that until now cold air was coming from the lower level, which until now has been "unfinished" and completely uninsulated .  This is the floor that will get the tubes placed in the joints below. 



The proposal from the contractor that wants to use suspended tubing (through an air pocket) is to run the entire house's heating needs from a TriangleTube Solo 110.  There will also be a LochinvarSSS60 installed for our domestic hot water. 



This contractor's proposal is 2/3 of the person who wants to use the diffusion plates, for a savings of almost $10,000.  I doubt we would recoup that difference in savings on our energy bills alone.  Before this remodel, we had an extremely inefficient old gravity-fed heating system and our bills weren't that high.  San Francisco can get cold, but thats a relative term -  ultimately we didn't use the heating system that often. 

Gordan

In-slab radiant and suspended tube

Those two are pretty close to being as different as can be, from the point of view of the supply water temperatures and controls they require. Can you give us details of the cheaper contractor's proposal? What's at stake is not just whether you'll realize fuel savings due to increased efficiency, but whether the system will provide the comfort that you need and do so in a reliable manner. Otherwise why spend any money? The cheapest is probably to keep what you already have...

dan_in_sf

A few more details...

All three contractors that gave us proposals had recommended tubing in the concrete downstairs (one zone), and tubing between the joists upstairs (two zones - one for the front of the house, one for the back).  This is a fairly common configuration here in SF.  I know a few other people that have had this done to their homes. 



I mentioned that previously that all zones will be sourced from a "TriangleTube Solo 110"  which is the source for hydronic heating and domestic hot water.  There will also be a LochivarSSS60 Indirect 60 gallon tank for the domestic hot water. 



The pipes will be 1/2"  Pex pipe.  They will be in an air gap of 2 to 3 inches.  Below that there will be reflective foil on top of insulation.



Not sure what other details I can provide.... please let me know.

Alan (California Radiant) Forbes

Everything sounds good

except the lack of plates.  We always used plates in our jobs and often lost the work to contractors who didn't use plates.  I prefer plates for reasons stated elsewhere - lower operating temperatures for greater fuel savings, better heat transfer, etc. and most of the contractors here on The Wall prefer using plates.



But that's not to say staple-up without plates don't work.  The only difference may be that on cold days - like today - you may have to put on a sweater.  If that doesn't bother you, go with your first choice.



Alan Forbes

Berkeley

zacmobile

plates

I too have always used plates, they aren't really a whole lot more and the only benefit I can add to what everyone else has already stated is by lowering the upper floors operating temperature you can usually (a heatloss calculation would determine that) run it from the same circuit as the basement, ie: you would not have to run the basement as a low temp mixed circuit and the upper floor an unmixed high temp circuit.



Also, what is a LochinvarSSS60? I'm not familiar with that model, is it an indirect tank?

dan_in_sf

Answer to "what is LochinvarSSS60"

This is how the contract describes it:  60 gallon, indirect, stainless steel storage tank.



In answer to the question about our hot water needs:  We have a family of four (2 adults, 2 elementary school kids), we have a clothes washer that seems to be on a lot, we have a dishwasher.  I don't think we have excessive domestic water needs.

Jean-David Beyer

I have a washing machine...

I have a washing machine that I use twice a week and sometimes three times. I do not count it as part of the hot water heater load, though, because I normally run it with cold water. Once in a great while, I run it at warm for wash and cold for rinse. The formula I got from W-M to size the indirect hot water heater seems to count on running a washing machine on hot.

Gordan

zacmobile already mentioned it below...

Typically, to run such different emitters from the same boiler requires some controls to get it all to perform - not barely, but comfortably. A mixing valve, slab temperature sensor, and a controller to pull it all together, perhaps with outdoor reset and/or indoor feedback - the Prestige doesn't have the smarts on board to deal with a multi-temperature circuit. All these components add cost, so a low bidder might leave them out, but I'm pretty sure that running 160 F (or hotter) water through a concrete slab would not be a good thing to do on a number of grounds and running anything cooler through suspended tube may not do much of anything. Also, with suspended tube, it is absolutely critical to insulate the perimeter of the joist bays or that toasty air will leak through the cracks to the outside. Insulation is of great importance even with transfer plates, but it's also easier to accomplish - you shove it right up to the back of the plates. That radiant barrier on top of the insulation may not do much after awhile - dust that falls on top of the surface will severely compromise its reflectivity - so you should make sure that whatever that contractor is proposing has R-19 value under that foil (or better) if facing a heated room, and more if facing an unheated space.



There are ways to increase the output of suspended tube, but these involve little clip-on aluminum fins (Ultrafin) which will add cost.

Jean-David Beyer

running 160 F water through a concrete slab

"I'm pretty sure that running 160 F (or hotter) water through a concrete

slab would not be a good thing to do on a number of grounds"



Wow! I'll say. In addition to having asphalt tile detach from the slab, messing up hardwood floors, possible cracking of the slab (I do not know if this is actually possible, but I have heard people worry about it), etc., to me the biggest problem with running constant temperature through a slab is that the temperatures in the house overshoot by huge amounts (e.g., more than 5 degrees). There just isn't enough anticipation in the thermostats to manage that. With outdoor reset, my overshoot is too small to measure, meaning I can hold +|- 1F. That would be worth the price of outdoor reset even if it saved no energy at all. For me, anyway.



I assume if I put 160F water in the floor, overshoot would be far worse.

Gordan

I would also expect issues with uneven slab temperature

Too hot at the supply end, too cold at the return end, because the slab would not be allowed to stabilize. If you've got fixed temp and fixed flow, even good anticipation couldn't fix that, at least I don't think so.

Gordy

Gotta Ask

?



Gordy

Gordy

Gotta Ask

 Did they insulate the slab with XPS?



What is your floor coverings going to be on the second floor?



I hope your contractor has a good piping control/ strategy for the very different water temps.  You will be running three different temps. Indirect, staple up, and slab.



Finally it sounds as though you have answered your own question about the plates do to cost.  We  just want to outline what the difference is with out them.  I hope your heat loss for the second floor is not more than 15 btus sf or you will be disappointed. Usually a suspended tube plate less application is about good for floor warming not so much total load heating except in milder climates.



 I have not seen you mention if the contractor did a heatload calc. Please ask him if he did, and what the results were. If you get a deer in the head lights look he is thumbing it in.  What may work in one home does not always work in another.



Gordy

NRT_Rob

San Fran

can sometimes skip plates if the house is well insulated, but typically I find insulation stinks to the level the local climate allows. That is, you often have the same output requirements in warmer climates that you have in colder climates, because the insulation isn't as good.



if your insulation is more suitable for a colder climate, you could probably do without plates.



If not, the lack of plates will result in a significantly less efficient system and may even mean that the system won't keep up.



In any case, you should NEVER do a plateless system without a load calculation, room by room, that shows that it will work, and at what water temps.