Posted By toffee on 07/30/2009 12:39 PM
Hi guys, please tolerate my newbie questions, first time poster here.

The way I understand hydronic-radiant floor system works is that:

1. water heated by broiler or water heater to say 180 deg.
2. hot water pumped into tubes under the floor though out the house.
3. cooled water return to broiler/water heater to be heated and pump back to the tubes.
4. I believe water returning to broiler ought to have temp higher than 55-60deg?

Questions:

1. How can a traditional hydronic system benefit from geothermal if the return water temp is higher water circulating from the geo system?
2. On hot days, is it possible to circular cool water from geothermal system through the radiant tubes?  If the temp is say 100 deg, cool water ought to cool the room down some?
3. How much can (2) cool the room?
   
4. What about moisture or water condensation under (2)?  So a dehumidifier would be needed?

I am just puzzled.

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First off, it's a b[u][i][b]o[/b][/i][/u]iler not a b[b][i][u]r[/u][/i][/b]oiler (unless you have your radiant cranked up to ~800-1000F :-) ).

Most radiant floors using concrete slabs never see 180F water.  In fact, many radiant staple-ups under wood floors (often retrofits) don't have to see more than 140F even when it's super-cold out, assuming a reasonably insulated house in a non-arctic environment.  Radiant slabs can often be designed to run at temps under 100F even on "design day" and much cooler during average heat loads, making them suitable for geothermal.  The return-water temps are set by-design, a function flow rate and heat given up to the room.  Delta-Ts of 20F or more are common, (120F out 100F return, for an average of 110F, etc) but return temps will always be above room temperature while in heating mode.

Traditional hydronic systems run at much higher temps than radiant in order to use less radiator. With radiant you have a very large surface area built-in, reducing the temp need to transfer the heat into the room.  Old-skool stuff typically ran 160-200F out, 140-160F returns.  Cast iron boilers and ceramic/masonry lined flues demanded that return temps never drop much below 140F in order to avoid potentially destructive condensation, so there was little incentive to design-in more radiation for lower temp returns, but it's almost always possible to get there with 1.5-2x the linear feet of fin-tube baseboard (or cushy panel radiators.)

Stainless steel flue liners can reduce the minimum safe return temp requirement to ~130F for natural-gas or propane fired cast iron boilers, but below ~122F corrosive condensation begins to occur on the heat exchanger plates. (Oil-exhaust condensate is significantly more corrosive- it'll eat almost anything.)  Copper water-tube boilers can safely run down to around 100-110F, but modern condensing boilers (and tankless HW heaters) have heat exchangers designed tolerate arbitrarily low return water temps.  Since radiant [i]output[/i] temps are often lower than the minimum return-water temps of tradtional boilers, copper tube & condensing boilers are better choices for radiant designs than trusty tried & true cast iron beasts (even though there are several ways of plumbing outputs & returns to keep return temps up to the necessary minimums while delivering appropriately low temps to the radaition, if need be.)  At low enough radiation temps, heat pumps work too- most slabs can be made to work with geothermal.

Radiant cooling using floors works, but not nearly as effectively as radiant cooled ceilings or walls, since the cool-air puddles at the floor forming an insulating boundary layer, whereas with radiant cooled walls & ceilings significant convection is induced and currents of warmer air come into contact with the cooling surface.  Condensation is an issue only when the dew points above the temp of the cooling surface.  Since radiant floors have to be chilled significantly more than walls or ceilings at the same cooling load that's where you're more likely to run into condensation issues.

In much of the eastern US the latent load (humidity) is a significantly larger AC load than the sensible (temperature) load, and mechancal dehumidification will be necessary for comfort & health.   For instance, right now the dew point outside my office window is over 70F.  With an indoor temp of 76F that would translate into ~82% relative humidity, which is damned uncomfortable and prime growing condtions for mold/fungus on building matierials.  Radiant cooling surface temps would be limited to 70F right now as well, meaning it's efficacy (particularly on a floor) would be limited.   Here in New England, some mechanical dehumidification would will be required, even more-so in Georgia/Florida, but in Arizona/ Nevada you can chill it to your heart's content without much fear of condensation or clammy-discomfort.  (Evaporative coolers are fairly common there, intentionally raising the RH as part of the cooling process.)

As with radiant heating, how much you're able get out of radiant cooling depends on the number of square feet of chilled surface you have to work with, and the water temps needed to achieve the necessary heat transfer.