I am building a 2,100SQFT south facing one level house in CT with GHP radiant floors. I would like to eliminate cost the ductwork for the A/C system. In addition, I think the GHP is cheaper if it is just a Water-to-Water system. The architect says that fan coil units are used in cities and all over Europe. My General contractor says these systems will create mold in the summer. Does any one have some experience? Thanks- [email protected]

I suspect what the contractor is saying it that a water to water fan coils won't be cold enough to provide sufficient dehumidification, since latent loads are the primary AC load in CT.  If the coils aren't cold enough, they don't remove sufficient water, and humidity will build up to mold-inducing levels (~70%RH+) inside the house.

But he's only right if the designer screws up by either:

A: running them too warm

or

B: not providing for a means of disposing of the condensation on the coils.

The dew point of  75F 60% RH air  is ~60F. 

The dew point of 70F 60% RH air is ~55F. 

If you run sub-50F water in the coils it will be removing water from sticky air.  If you're running 60F+ water, not so much.  Some people might feel pretty chilly down wind of a fan coil running 40F water, but 50F is fine for most.

Europe has much drier summertime air than southern New England, so one can't assume methods that work there also work here, but this is not an insurmountable problem.  Worst- case you can set up a dehumidifier in a large open room (or the basement) and let IT control the latent load, while the fan coils handle the sensible load. But I'd be surprised if you couldn't get adequate latent-load performance out of fan coils with a bit of design attention up front. 

You can also use the radiant floors to enhance the sensible-load performance, but you'd need to incorporate floor thermostats to keep the surface temps above the dew point.  (A 60F floor feels pretty cool to bare feet when it's 75F in the room anyway.) 

I have a similar sized house in MA, and now that I've tightened up the house a bit the outdoor infiltration is low enough that the whole house humidity is maintained all summer with just mid-sized room-dehumidifer in the basement.  My sensible loads are pretty low-I suspect I could do ALL my sensible cooling with floor temps well above 60F, if I had radiant cooling (but I don't.)  My oversized central AC runs fewer than 20 hours/year, so it's basically doing ZERO dehumidification most days.  I suspect that's the case with many/most central AC systems in our area- it's humid enough to require dehumidifcation far more hours out of the year than it's hot enough to require sensible cooling.

Fancoils perform well with open floor plans, and as Dana said with proper water temp they will dehumidify. If cooling will be a once or twice a year event, than a centrally located fan coil or two would be a good option.

For many people in the NorthEast cooling is a must have. Personal preference out weighs data of CDD's and relative humidity.

If you want cool bedrooms each night than you need a fancoil, supply and return piping, condensate piping and electric run to each bedroom. I would assume that if you had 3 bedrooms, Your installed fancoil cost would start to rival complete ductwork

Another consideration is that ductwork will never be as cheap to install as it is now. The benefit of complete ductwork and water coil with GHP and radiant, is that you can use the air to supplement heating on cold days. I hope you and or your designer is familiar with how the GHP will lose both efficiency and capacity as load temps climb, an airhandler/water coil gives you the option send warm water to the floor and the air handler simultaneously(usually staged), which will lower required load water temps, increase capacity, and increase efficiency.

My vote would be for ductwork, keeping it all within the thermal and pressure boundary.

And Dont let your designer miss the need for an outdoor reset control and proper radiant design.

I think that absent the open floor plan mentioned by HOTE above, the fan coil plan could be more expensive than a duct system.
I also agree that controlling humidity is a problem for the HVAC designer, not the builder.
Good Luck,
Joe

Southern New England homes are characterized by short but fairly intense periods of cooling load as has been demonstrated in the past couple weeks. I think I remember reading that Hartford, CT hit 98.

During heatwaves dewpoints reach numbers we continually combat in the southeast. That said, there is no reason fan coils can't deal with the humidity if they are supplied with sufficiently chilled water and proper airflow. Both need to be selected in keeping with latent (humidity) loads.

I don't envision properly managing cooling / dehumidification without ductwork unless minisplits are installed.

Jim needs to ensure a competent Manual J and D compliant load calculation is performed on his project before selecting equipment or method.

Dehumidification is managed just fine in my place (zip 01609) without ductwork or mini-splits- just a tight enough house and a room dehumidifier in an open basement (the coolest room in the house) maintaining a 60% RH limit. My sensible cooling loads are too low to rely upon central AC as the primary means of handing latent load, as would be the case for many/most homes in southern NE. Dew points in the 50s (or even 60s) are common even when there is NO sensible cooling load. Mini-splits or central AC won't provide any more dehumidification than properly designed chilled water coils will, since none of the above will even be operating when there is no sensible load.

Take this morning, for instance: At the house around 7Am the dew point was ~59F, the outdoor temp was 61F. That's about 90% RH outdoors, and ventilating the ~70F house with that air would raise the interior RH to ~67%- on the verge of mold conditions. Outdoor temps at the house probably won't break 75F for more than an hour or two today, and with the windows closed the house won't get any warmer than 73-74F inside, but without active dehumidification the interior RH would likely hit 70% or higher, since the outdoor dewpoint & temp will rise faster than the indoor temp, thus ventilation using outdoor air increases rather than decreases the interior RH. Neither a mini-split, central air, nor a chilled water coil is going to handle this very real latent load (but central dehumidification + limiting ventilation air does.)

Running the AC under dehumidistat control on days like this would leave us uncomfortably cool, yet days like today are far more common than days when peak temps break 85F- a sufficiently substantial sensible load that the cooling-AC could provide the bulk of the dehumidification.

... and to confirm, looking at online data it seems the outdoor temp didn't even hit 80F this afternoon before temps began to fall, but the dew point rose to 65F by mid-afternoon. (Haven't been home to check the temp at the house, but I'll bet it's under 75F.) That's no sensible load to speak of, but a significant latent load- probably similar to conditions all over southern New England.

With no sensible load to trigger the cooling system, with no active dehumidification the ventilation air would have brought the indoor RH up to ~77%(!)- well into the mold & health hazard territory. We can't count on a mere COOLING system to dehumidify tight, well insulated homes (or even kinda-tight imperfectly insulated homes like mine) in this region. Plan on handling latent loads separately, but design the cooling system to run the coils on the cool side to maximize dehumidification.

Hmm...I don't disagree with you, Dana. We get dewpoints in the mid 70s for months at a time down here, sometimes unaccompanied by commensurate sensible load such as during cloudy rainy weather. The OP was wanting to have no ductwork and I couldn't envision how one could meet both latent and sensible loads without either of minisplits or ductwork connected to either of fan coils or direct expansion.

I have no objection to chilled water systems - in fact I rather like the idea of controlling chilled water temp based on humidity just as outdoor reset can be used to control radiant water temperature in winter - heat or cool no more than necessary to meet loads for max COP / EER.

I'd like to see central systems with much more active control of humidity via dynamic regulation of CFM per ton. Have a humidistat interact with the ECM blower controls to make it so. On high latent / low sensible days I see no reason why CFM couldn't drop to the low-mid 200s per ton as long as a discharge air temperature sensor guards against evaporator icing.

Jim, I believe we have designed a system for your home. The design included three fan coil units manufactured by Multi Aqua company. With the type of home you are building and with proper ventilation moisture in  your home will not be a problem. The layout of the equipment (fan coils) is a big factor in the design. The floor plan was very open which lends itself to these units. Please feel free to e-mail or call to discuss further. Tony can provide my phone contact. Thanks, guy

Posted By engineer on 01 Jun 2010 10:39 PM
Hmm...I don't disagree with you, Dana. We get dewpoints in the mid 70s for months at a time down here, sometimes unaccompanied by commensurate sensible load such as during cloudy rainy weather. The OP was wanting to have no ductwork and I couldn't envision how one could meet both latent and sensible loads without either of minisplits or ductwork connected to either of fan coils or direct expansion.

I have no objection to chilled water systems - in fact I rather like the idea of controlling chilled water temp based on humidity just as outdoor reset can be used to control radiant water temperature in winter - heat or cool no more than necessary to meet loads for max COP / EER.

I'd like to see central systems with much more active control of humidity via dynamic regulation of CFM per ton. Have a humidistat interact with the ECM blower controls to make it so. On high latent / low sensible days I see no reason why CFM couldn't drop to the low-mid 200s per ton as long as a discharge air temperature sensor guards against evaporator icing.

I like it too!  It wouldn't likely do the trick in sub-500CDD New England, but in humid 1000CDD+ climates that sort of control would max out both efficiency & comfort.

My climate is ~350CDD-sensible, the latent load is the ONLY load to speak of most days.  Most of CT is also well under 1000CDD, with the same air masses as mine (but lower altitude=higher average temperature, 500-800CDD would likely be typical of the case in question here, not more.)  We get dew points in the 70s maybe a handful to a dozen days/year (usually accompanied by some sensible load) but most of the summer we just have a lot of slightly sticky/hazy but not particularly hot weather.

I have this setup in my house with a First Co fan coil unit. You can dial down the airflow low enough to provide dehumidify without major cooling. I run my GSHP at about 40-42 F when cooling. It works well. I'm in Michigan and get horrible sticky days here. When it's cloudy yet humid, it's a good time to run the AC with low flow rates.