I've been pricing out the cost of doing an over or under floor install for our radiant install (using a WTW gshp). I understand that the lower temps achievable by going over the subfloor increase the system efficiency, but is it enough to justify the cost of an above the floor system? I thought about making my own (cutting plywood strips to space out the straight runs then using a router or jigsaw to make return panels) but that gets kind of labor intensive. Base on experience, do y'all believe the cost of an above floor install pays back in efficiency what it costs to install up front?

First we start with a room-by-room Manual 'J' heat load analysis using radiant specific software. This will determine the design temperature using the various emitter modules available e.g. sub-floor bare-tube, thin panel, heavy extruded panel, the 6 or 7 common "sandwich systems, embedded thin-slab (Gypcrete), radiant ceiling or walls.

If you want to use a heat pump of whatever flavor, you can design house and mechanical to the heat source. The ultimate load, and thus the heat source will depend primarily on windows and insulation. The trade-offs should be better engineered and the results finite.

Focus on what you want and have a competent designer define the compromises, if any.

An important question is "are you going to have a supply temperature than can be lowered with above floor". A simple design that mixes fan coils with radiant floors (as you are considering in another topic) may not. So no (or less) efficiency improvement in that case. To be more specific, what is the btu/hr/sqft that the floor needs to output? What temperature do the fan coils need?

jonr... good to see you over here as well. I figured I'd split the radiant subject over here for the sake of not muddying the waters too much on the geo topic.

I guess you guys have given me my first homework assignments, though... time for some more research.

I fully intend on having a competent designer do the final design, but with all the varied answers I got locally, I've decided to dig in on this (just like the geothermal) and do some ciphering to at least be able to not walk blindly into it.

I'll start gathering some room and floor specific heat loss numbers.

One of my contacts locally so far told me I need to up the BTU/h on radiant above the calculatedl losses to account for edge/perimeter loss in all rooms. Can you expand on that a bit for me to help me understand.

I can't, but I know MA can. For just a rough over/under decision, it probably won't change the answer. The house is about 4500 sqft and 60 Kbtu/hr, right?

o account for edge/perimeter loss in all rooms

That's confusing. If you calculate the heat loss for a particular room that has 12 feet of exterior wall, doesn't the calculation include that? Why would you arbitrarily "up" it afterwards?

Posted By danreed76 on 01 May 2013 10:07 PM
One of my contacts locally so far told me I need to up the BTU/h on radiant above the calculatedl losses to account for edge/perimeter loss in all rooms. Can you expand on that a bit for me to help me understand.

A purpose-built radiant floor heat load program accounts for perimeter losses and a host of things covered and of little importance to the overall design. All is a waste of time until the heat loads are finished. We can tell you if you have too much window, too little insulation or floor covering for the particular heat source you have in mind.

Dan, your contact was likely suggesting that you might want to do the room tube layout design such that more BTU/H/SF is applied to the perimeter area of room where you may have higher heat losses (e.g., from fenestration) than from the interior area of the room.

This is accomplished by immediately routing the tube from the associated manifold station to this perimeter area. Since the hydronic fluid will be at it's highest temp in this initial run of tube, it will transfer more BTU/H/SF in this perimeter area than in the interior area during its subsequent run back to the manifold station. Doing room tube layout design in this manner is very common and considered best practice.

Of course the overall heat gain provided to each room should still be in accordance with the overall heat loss for each room as determined by accomplishing a proper heat loss analysis as Badger suggested.

Variable tube spacing is an advanced design skill and like "raising the tube" rarely needed or justified.

For particular applications where the magic 86° surface temperature must be exceeded to meet design load, e.g. large glass, narrow spacing may be required. We find the technique useful for outside corners, more especially in bathrooms with large windows and may even break the near-religious and mystical 86°F maximum surface temperature. But please don't tell anyone as we don't want to loose our key to the tree house.

If you have a proper heat load, you can determine if special attention (materials and labor) are justified.

Ha Ha. That means we did a lot of "advanced design" while we were actually laying the tubing.

Installer: "Hey, do you want it warmer over here in the picture window corner?"
Me: "Sure"
Installer: (readjusting tubing) "I'll just run it a little closer here...Space it out a bit more along that wall you're doing over there...."

Yes. You can't have too much PEX in any floor...especially if you're selling it.

Is it correct that if you have a mix of hydronic conditions/radiators such that some of them require high temperatures, you lose the efficiency advantage from the HP producing low supply temps? Eg , one corner room with carpeting and big windows (requiring 130F) and efficiency for the entire house drops substantially?

It is all climate dependent as "design" conditions may last but a few days as it is here in Minneapolis. The mix is between comfort and economy. It is why the room-by-room is so critical. We model system emitters with various floor coverings to come up with the perfect balance. Some systems require tile at the perimeter providing higher floor temperatures output at a heavy load area (big windows) while allowing low output solid would in the field of the room.

We like to get the first set of plans from the architect and design radiant heating systems around them.

your contact was likely suggesting that you might want to do the room tube layout design such that more BTU/H/SF is applied to the perimeter area of room where you may have higher heat losses

That was what I originally perceived, however when he told me I would need to upsize my HP above the manual J to deal with the perimeter/edge loss, I got concerned. Before going into any further discussion with him or dismissing him entirely, I wanted to do a bit more legwork on it. For a tight envelope, his Manual J was just a hair short of mine, but he also insisted that we use an "average" envelope rating and go with a larger system. So between upsizing for average construction and upsizing for perimeter loss, we were going to be seriously upsizing the check.

Thanks again guys for the education. I like this alot better than going back and figuring out what went wrong.

I'm going to guess that perimeter loss comes from the fact that a wall with 20F on one side and 70F on the other loses slightly less heat than a wall with 20F on one side, 70F on most of the other side and 100F on a small strip where the wall contacts the floor. Probably less of an issue where you have exterior foam (less thermal bridging in that area).

I see no reason for you to build (or design as if you built) an "average" envelope. On the other hand, installers make more profit and take less risk* when they oversize. Also consider that occasional heating shortfall is easily addressed. Cooling shortfall isn't much harder.

* - think "grandma moved in and this defective system can't maintain 80F during record cold spells".

I'm going to answer my own question and say yes, the efficiency of a geo/hydronic system is going to be driven by the highest (lowest in summer) temperature that any radiator in the house needs. One room can spoil it for all of them. Balance is critical.

I can imagine a complicated multi buffer tank control system that would help, but I don't see any references to anyone doing it.