Joist heat

ALH_4

Depending on your floor covering and the temperature at which you find the floor uncomfortably warm, you may get significantly more than 30 btu/sf with ThermoFin. As far as mixing vs. injection, I'm more of a 4-way mixing valve fan when using a cast iron boiler, though injection also works fine. Viessmann's Dekamatik HK1 mixing valve control is a nice option.

-Andrew

Rick Carter

Joist heat

Joist heat: What’s the best way to do it?....that’s this home owner’s question.

Should tubing be attached directly to the sub floor or leave an air gap? If I have an air gap can I run 180f and no mixing device / direct to sub floor mixing required? Should I use aluminum plates or fins? Are they really worth the significant additional expense? If anyone has experience with Ultra-fins how did you find their response/recovery time compared to aluminum plates that attaches directly to the sub floor?.......and most importantly, tell me what you've found doesn't work in joist heat.

Thanks for the help.

Rick

Rick Carter

Rick Carter

Joist heat: What’s the best way to do it?....that’s this home owner’s question.

Should tubing be attached directly to the sub floor or leave an air gap? If I have an air gap can I run 180f and no mixing device / direct to sub floor mixing required? Should I use aluminum plates or fins? Are they really worth the significant additional expense? If anyone has experience with Ultra- fins how did you find their response/recovery time compared to aluminum plates that attaches directly to the sub floor?.......and most importantly, tell me what you've found doesn't work in joist heat.

Thanks for the help.

Rick

Rick Carter

Joist heat: What’s the best way to do it?....that’s this home owner’s question.

Should tubing be attached directly to the sub floor or leave an air gap? If I have an air gap can I run 180f and no mixing device / direct to sub floor mixing required? Should I use aluminum plates or fins? Are they really worth the significant additional expense? If anyone has experience with Ultra- fins how did you find their response/recovery time compared to aluminum plates that attaches directly to the sub floor?.......and most importantly, tell me what you've found that doesn't work in joist heat.

Thanks for the help.

Rick

Unknown

you need a heat load calc, first and foremost. the rest is answered after that.

Alan(CaliforniaRadiant)Forbes

Staple-Up

Good radiant heating is done with low water temperatures. There are too many benefits:

- less heat loss from piping

- will allow condensing boiler to run at high efficiency

and you are able to transfer more heat into the room with plates. From what I remember, you max out at 19 BTU's/[] with suspended tube at 180° water temperature which is OK for new construction, but doesn't make it for hard-to-heat rooms.

So we almost always use plates.

Alan

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Rick Carter

joist heat: heat loss calc.


I wasn't sure how much detail to get into on the initial posting.

This joist heat will be added to an exisitng Burham non condensing 93 000 btu output boiler. It presently is used to heat domestic hot water and basement slab only. Scorched air on main floor now.

Using the heat loss calc from the Ultra Fin web page, the total heat loss from the main floor is 47,131 btu. The coldest rooms are rooms exposed on three sides plus attic (bungalow), and individual room loss's range from a high of 12,000 to a low of 3,900. BTU/ft2 range is 82 btu/ft2 (three exposed wall with lots of glass) to 27 btu/ft2 in the living room.


Rick

Unknown

well, either that program stinks, you didn't use it right, or you have an uninsulated house in a very cold climate. Your overall load isn't wildly high, but 82 BTUs/sq ft is basically impossible. Are you sure you divided correctly?

Rick Carter

The design heat factors used in this heat loss calc program are: Wall, 0.06, Ceiling 0.03, Window and door, 0.59, floor 0.2. I have no idea what that value represents, but it was produced after I selected the type of insulation, non exposed basement etc. The other factor of particular interest is the design temp of -41F. It gets damn cold up here.

As an example I'll transpose the highest heat loss room, the summary reads:

square feet = 143.75 (breakfast nook, 3 sided, lots of glass), Pipe: 115 feet required, BTU's: 11875, BTU's per square foot: 82.6, Flow Rate: 1.58, 10" ultra fins required= 115, loop spacing = 12"


That's the summary, a small very cold room which scorched air is doing a poor job of heating.

A more "typical" room reads:

Master bedroom: 220 ft2, 126 ft of pipe, 6600 btu, btu/ft2 = 29.9, flow rate = 0.88, 138 10 inch ultra fins loop spacing 24".

But hey, if there is another good heat loss program out there I can surely run it to confirm these values.

I want to get this right.

Unknown

If your design temp is -41, do yourself a favor and forget ultra fin. There is no way it will do the job well.

Also, if that's even close to accurate, you're going to be borderline with any radiant floor. Extruded plates would be non-negotiable necessities, and even then you might need some supplemental heat. The breakfast nook in particular... nothing made today is going to heat that without cooking you.

There is a free heat load calculator on this website (Free heat loss calcs in the sidebar). Use that instead, though of course you need to understand that you are not a pro and so your numbers will be approximate. Of course, a pro's are too, just less so . That calculator is built for baseboard and overstates things a bit, but it'll do a decent enough job to get into a ballpark.

Rick Carter

Do you have a particular brand or style of plate you prefer?

I don't have a condensing boiler, so mix it down to......what temp do you recomend??

Rick Carter

Have you found a plate that works well? Seems like there's a lot of different brands out there.

I assume I'll want something that is going to be attached directly to the subfloor?

Unknown

Yes. Radiant Engineering's "Thinfin" or "Thermofin" product line is probably what you are looking for. But as I said... radiant floor alone may not do the trick here.

Rick Carter

We have to keep in mind that these heat loss calcs are based on the coldest day of the year. We get those temps for 5-10 days a winter.

All I want out of a joist heating system is to heat the main floor for as long as it can, and if it can't keep up the existing scorched air will kick in to make up the slack....I believe you called it "supplemental heat".

I'm expecting the comfort benefits of radiant for most of the winter, and radiant + forced air on the real nasty days.....

Thanks for the help on this, much appreciated. I'll look up the web page for the plates you've reccomended.

Unknown

well, now you're talking two different animals. I'm sorry, I guess I assumed the scorched air was going away.

You can go two routes; "floor warming", or full on radiant heating, if you are keeping the scorched air. Floor warming is just what it sounds like, and for that, Ultra fin or suspended tube applications are fine. Limit their water temperatures as appropriate for your load and flooring, and let two-stage thermostats do the rest. Floor sensors can be a decent idea too, to maintain a minimum temp.

The more you want the radiant to take over, the better the system you'll need to do it. Ultrafin and suspended tube max out around 15-20 BTUs/sq ft, and that's at abysmally high water temperatures. Thinfin/Thermofin get you up to around 30, at generally lower temps, and after that the floor starts getting too warm for comfort typically.

Rick Carter

My preference is to get the boiler to do as much of the heating as possible ie. use a product like Thermofin that'll get me up to the 30 btu/ft area.

You touch on a few things I've been wondering about. What type of thermostat to use, and what type of mixing to use. I've had one wholesaler tell me air sensing is the way to go, while another has told me floor sensing is the way to go. The flooring used in our house is hardwood, linoleum, and carpet.

I hate to show my ignorance, but what's a two stage thermostat?

What's you preference for mixing? From what I've read an injection system seesm to be used more often then 3 way/ 4 way mixing valves?

Jim_65

Underfloor

I agree with NRT.Rob with the out of whack Btu's per sq.ft. requirements. We have had the best track record with the Radiant Engineering plates.

Didn't you mention that the Burnham boiler was already doing your basement slab? You had to mix that down didn't you? So you should be providing high temp for the DHW and a mixed down low temp for the slab.

You also mentioned that you want to get this right and for us to keep in mind that design conditions are only a couple of days a year. If you intend to use the forced air as supplemental you will need to control it as a second stage. Fourty one degree's below sounds pretty darn cold to be assuming.

I suggest that since you are wanting to do this right contact a professional through Find a Professional on this site to assess the system and verify your calcs.

Unknown

we prefer injection mixing for multiple zone systems. relatively cheap, and very effective. many mixing controls also provide boiler firing features, which, if you don't already have them, are a substantial improvement over your existing system.

Two stage thermostats are thermostats able to trigger supplemental heat when the primary heat can't keep up. Tekmar's 512 thermostat is *excellent* and can do this. With it, as long as you limit your water temps appropriately (again, depends on load) so as not to overheat your floors, you shouldn't need floor sensing; the thermostat will cycle the zones so the floor is nearly always warm.

Rick Carter

There's nothing I'd love more then to find a professional to handle this........one problem, believe it or not there are still parts of North America that are tried and true SCORCHED AIR country.

Here's my example. Our houue was built in 2001. I had an air entrainment issue with the boiler from day one that never went away. The contractor couldn't solve it. I had 3 different professionals come and look at it. I'd ask them all the same question, "does it matter that the boiler circulator is upstream of the expansion tank?" they all said that's not a problem........I even took pictures of the boiler to the local government plumbing inspector and he said "it all looks fine, it should work".

After ordering a Dan Holohan book I hired a plumber and told him to move the pump to the hot side, down stream of expansion tank. No more problem.