Hi,
I hope someone can help me with this.  Originally I had planned on installing geothermal to heat using forced air and the sizing (based on man j) indicated the need for 36,000 btuh and a three ton unit (water to air).  When I decided to go with radiant floor heat using a staple up retrofit I am told I will need to provide 50,000 btuh and a 4 ton unit (water to water).  This difference is due to floor characteristics.  The floor is 60% oak floor so ~1.5 inches thick with subfloor, 30% linoleum and tile that is .75 to 1 inch thick, and 10% wall to wall carpeting.  My question is whether to go with the 3 ton or 4 ton unit.  It sounds like it makes sense to me but I want to get second opinions from the experts out there.  Thanks in advance.

FYI, my floor loop design is 8" on center with 5/8" thin wall pex and thin aluminum plating with 100% coverage.  Loops are all ~170' and all start on outside walls.

a 3 ton unit won't provide 36kBTUs of heating, and a 4 ton unit won't do 50kBTUs. are you sure of the sizing? it does not make much sense that heat load would go up for radiant.

Heat load won't go up, but delivery temperature needed and heat pump output will go down if the floor is too poor at delivering heat.

ah, that's a good point jonr. and with thin aluminum plates he's not in ideal range... especially not if they are from a supplier that sells "thin wall" 5/8" pex, then they don't even fit very well.

but I'd wonder if it's even worse than that... has anyone even figured what the water temperature requirement is here? water to water can only do about 120 max and you really don't want to run it that high. this install method typically requires hotter water than that.

Thanks for the input guys. I will re-visit the sizing, I was under the impression that 1 ton = 12k btuh.

Rob when you say I would not want to run as high as the 120 max that I will get out of geo are you suggesting that I should not be running the geo at that max capacity?

There is a newer water to water model that ClimateMaster is putting out that can deliver 140F. Should I be checking that out? This makes me wonder if geo simply cannot support a staple up retro fit. Is there more I should be doing in my loop design?

I'd appreciate any suggestions.

geo at 120 is not very efficient, and the COP goes up quickly as you lower the water temps (roughly a COP per ten degrees of water temp, the geo guys tell me). we run 110 as our target maximum for geothermal systems.

Just because you CAN doesn't mean you SHOULD.

most geo units are rated for cooling capacity. for heating it de-rates between 10% and 20%.

for geothermal systems in staple up retrofit, the answer is relatively low heat loads and heavy gauge plates. With a wood floor you max out around 15-20 BTUs/sq ft at a 70 room temp with 110 SWT in heavy plates. that's a good envelope... not amazing, but good, not leaky, well insulated, modern windows, not too much glass, etc.

over that, and you need supplemental heat, and it's worth it, to keep your water temps down. radiators, radiant wall, radiant ceiling, etc.

I have a staple up radiant heat system with home-made aluminum plates, also going through 1.5" of subfloor+hardwood (1/2" pex on 1 foot centers). A thermostatic mixing valve controls the temperature going to the radiant system. My heat source is a wood gasification boiler. Walls are SIPs. I don't run the system 24/7, but when I do run it, I set my water temperature between 130 and 150 degrees. I realize your tubing and transfer plates are better than mine, and your system will be on more frequently than mine, but I still think you're going to want 120 degree water if you want it to be responsive to thermostat calls for heat. You could go with a lower temp with tubes in concrete (I have some of those too - they work an order of magnitude better than the staple up portion).

I have two gshp units too... but my gshp's are tied to the duct work, and not the radiant heat system.

As Rob indicates I should expect 110F water from the geo.  I'm about half done with installing per the design I described.  I think I will install additional 16 on center loops over the existing which will give me three 'lines' in each joist cavity.  Thats the best I can do.  I will still have my oil fired boiler hydronic baseboard system for the really cold days if the radiant can't keep up.  Any thoughts on whether its worth running the additional loops?

the additional lines won't help you much. plates can't touch.

reducing water temps would be "helping". more tubing in joists doesn't really help much. once you get a couple of runs in a regular 16"o.c. joist bay, you're about maxing out what you are going to do. it will not speed up your response or lower your water temp much to add more runs.

you will want to use plates, good ones, if you are doing geo unless your heat load is very low indeed. and you might need to add more than that, if the heat load isn't low.