My plumber wants to use these on the next job we do. Extruded aluminum flat plate, about 0.050 thickness, with integral ribs running lengthwise, shaped so that the PEX tubing snaps into place. Previously, we have installed the tubing with large staples, fixing it to the underside of the subfloor deck, two runs per 16″ joist bay.
They are 4″ width and sold in 48-inch lengths. Adds some real $ to the job. As much as $2.25 per square foot of floor.
Anyone have any experience with this? Do they work?
Edited 7/27/2004 6:19 am ET by Bob Dylan
Replies
Ouch! As somebody here said, "That's pretty spendy!"
Is this a "staple up" application, or are you installing the PEX in a subfloor layer?
As to whether or not they work: there's no doubt that they'll spread the heat somewhat. The question is- how much, and is it worth $2.25/sq ft?
If you think of a piece of PEX tubing stapled to subfloor, it's pretty much touching the subfloor only where the staples are, and at those points it's only touching along the top surface. If the tubing were to only lose heat by radiation, that would be OK- but clearly that's not the way a so-called "radiant heating system" works. It seems that the intention is for the entire floor to be the radiator, rather than just strips of tubing under the floor. From what I've read here, the water temperatures for an installation without the spreader plates run a lot hotter to give the same feeling of comfort, which reduces efficiency for the heating system. Any heat that goes into the tubing, though, will still end up in the house.
The aluminum wraps around about half the surface of the tube so it's in decent thermal contact with the tubing. The tubing itself isn't all that thermally conductive, but it's also pretty thin. The aluminum's only 0.050" thick, but it's also in round numbers about 1000 times as thermally conductive as wood- in combination with the greater contact area with the tubing, that thermal conductivity will ensure that the plates will do a far better job of spreading heat than a piece of subfloor would. That will reduce running water temperatures somewhat.
Again, is it worth the money? I really want to know that myself, so I hope you get the answer! Personally, I'd rather spend $2.25/sq ft on a floor component that I can SEE rather than one I can't...
I'm using plates, installed in an above-subfloor sandwich, from a different manufacturer, 4" wide, 16" long, with the same omega groove configuration. Might be thinner, but they only add 42 cents per sq ft to the material cost of my job.
Moltenmetal has a good summary of the value of the plates. Lower water temperature leads to all kinds of opportunity for saving on the sizing and type of boiler, types of floor covering, reaction time, etc. I've read numerous times that, using plates, you can run continuous circulation and increase or decrease the temperature of the water, and thus the room temperature, with a combination of outdoor reset controls and modulating boiler.
Initially I'm going to attempt to be my OWN outdoor reset control, and vary the temperature of the water of the output of the water heater that will be my source. I'm at the sleeper/plate/tubing install phase now...I'll let you know how this turns out.
For some real enlightening discussion, try:
http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=1
and:
http://forums.invision.net/index.cfm?CFApp=2
I used a similar product to staple up. I'll check the price tonight, but it certainly was not 2.25$ per square foot. I ran two runs between each pair of joists leaving about 3 inches between each plate. It was simple enough to staple them up using ss staples and a hand stapler.
As to how well they worked, I really have nothing to compare them to. Last winter the floor heated up well on demand. I have had no complaints
I hate to use "rules-of-thumb" or say "typically..." when dealing with RFH, but...
Typically...with today's construction, you don't need the plates.
Stapling two runs per bay, then friction-fitting half-inch polyiso up against the tubing, then using batts (if required) under the polyiso will direct the heat where you want it to go...upwards. Even with the plates you'll still need the PI and possibly, the batts, so they are not an additional expense.
Water circ temps have always worked out to 105-115 degrees with this method.
Some heat-loss calcs (high altitude, high latitude, for example) may require the use of plates. But often they can cause more problems than they supposedly solve.
You won't get thermal striping with the tubing stapled under the subfloor.
However, if the tubing is stapled on top of the subfloor and sammiched under the flooring, yes, you can get striping, and yes, the circulating temperature can be more critical, and yes, you may want some sort of transfer mechanism (Al plates) to help even out the floor temp.
Of course, all this is a rule of thumb...<g>
Mongo: are you saying that you prefer staple-up with polyiso under the tubing, even in new construction?
I infer that Mongo means, "especially in new construction". I certainly would. Becuase in new construction you are going to have a reasonable R-value, not huge air infiltration, decent windows, etc.
All of which means you need less BTU/sq ft. Like 10-30 BTU/hour/sqft. Versus 50-100 BTU/hour/sq ft for your father's Oldsmobile.
I also question the efficiency of being about to run the tubing at a colder temp. You have a tempering valve in, typically. Until you crank the tempering valve all the way up (i.e. mix in no cold return water), cooler water in the loop gains no efficiency.
Staple-up without plates needs to run hotter than staple with plates. But 120 F versus 95F? You still have your boiler/HWH set EXACTLY THE SAME. The critical thing is that you be able to get that loop hot enough to put out enough BTUs. If that is 95F fine. If that is 120F, fine. If that is 140F, that is probably fine. But if it is 160 or more, than you may have to increase the temp of the HWH or boiler.
That, in turn, may create a scald hazard if FRH and DHW have the same source. If may cause damage to linoleum in a hot spot.
And, if you need to go higher that than, the heater/boiler may or may not allow it.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
David: I would guess that goes double if you already need hotter water to supply old cast iron rads etc. That's my particular case. The existing house will represent the vast majority of the heat loss because the addition will be far tighter and better insulated. Despite the need for hot water, I'll still get some efficiency gain over the existing situation replacing the old thermosiphon reboiler with a modern forced circulation boiler.
If you had a condensing boiler, no old rads/baseboards to need hotter water, and really good sizing such that no zone needed hotter water than any of the others (most of the time), I guess you could keep the boiler water colder with the aluminum plates and get some efficiency gain.
So I guess there's no real benefit to that extra sub-flooring or the aluminum plates in my addition plan. Thanks- that's going to represent a labour and cost savings to me for sure. Partial analysis based on previous advice here had led me to believe otherwise. Maybe I'll spend some of the savings putting in some staple-up zones under the easier to get-to floor areas in the existing house so I can get rid of a few of the bigger , more awkward rads.
Given my situation, in your view is there any benefit to a condensing boiler, or does my need for hot water render that pointless too?
Aways good words from Brother David.
In the retrofitting of older leakier homes, you're exactly correct...or consider new construction with large volume rooms with huge expanses of glazing. Those are two situations where plates may help throw off enough BTUs to allow a staple-up RFH setup to do a better job.
Prefer it to what?
Not meant as a smart-arse reply, but it depends on what you're trying to heat and how often you;re trying to heat it.
Example...a room that needs heat 16-18 hours a day might do better with a high-mass system while a bedroom, or more obviously, a hardly-used guest bedroom, might do better with a quick-reply low-mass system.
Flooring can be a factor as well, though not as much as how the room is to be lived in.
Still, with today's tight construction, if you're asking, yes, a low-mass staple-up can perform quite well. It can even be an advantage in the "tween" seasons...when the temps vary from one day to another. After two cold days, a high-mass system could thermally overload the house if the third day was warmer. Vice-versa, after a few warm days the house might not be warm enough if the third day were to be significantly colder.
Yes, outdoor resets help, but again...in some ways low-mass can be an advantage, and in that regard, staple-up shouldn't be discarded as an option just because it's "staple-up."
With staple-up, the underside insulation is key. 1/2" to 1" of foil-faced polyiso does wonders.
Thanks for the information. My plumber is a thoughtful guy, and seems to be sold on these plates. They are about 4" wide x 48" lengths in extruded aluminum, maybe .050 to .060 thickness, with an "omega" feature down the center, that allows 1/2" PEX to snap in.
Without plates, I had been doing these RFH installations in I-joist floorframes, pressfitting 1" foilfaced rigid foam up against the bottom faces of the top chords of the joists, to give the RFH tubing a nice little insulated cavity, 1-1/2" tall, to live in. No problems, and good performance.
I can imagine some small increase in heat transfer efficiency at startup with the plates, but I feel as you do, that the key is the isolation of the cavity and insulation.
I'll ask him to quote me without plates.
Same technique here. The top flanges of I-joists make great backers for the foil-faced polyiso.
I was looking at these also but at about $2 per 4 foot section. Are you insulating underneath the pex tubing? Using a foil faced insulation will give you more heat transfer up to the subfloor. Also know that this is not really radiant heating but more floor warming. If you are using this up north where the temperature drops below zero and sub-zero wind chills, there are going to be some complaints because this system can't keep up by itself. What I have done is use this approach but add an hydronic air handler for the really cold days up here in the north. I have the ducts located in the ceiling rather than the floor because they will mostly be used for the central air conditioning. I'm not too much into the heating and cooling part of things but I believe these are the correct terms.