We have a new home with Radiant floor heating on all three levels (basement, main and Loft), ICF foundation with HVAC however we currently do NOT have an AC unit, although the rough in is there for the future. I have been unable to find ANYTHING telling me if it's OK (even preferable) to run the HVAC in summer (all year) and if so, is there anything I should watch for?

Please explain what you mean by "...to run the HVAC in the summer (all year)..." Are saying that you have an air handler (fan) and ductwork installed that allows you to circulate air throughout the house, but there is no cooling and dehumidification provided, as yet?

Clark Yes, that is exactly the set up. With the high humid days this summer (first summer using the system) we keep the windows open at night for the cool air and keep the house "closed up (i.e. window drapes etc.) during the day. I'm confused about the outside humidity entering the house when the HVAC is on??

I don't know the details of your situation, but I can give you some insight based on my own experience. Our house is ICF and built very tightly. We have an HRV (heat recovery ventilator) that is tied into our HVAC duct system. There is provision for central A/C, but it's not yet operational (we're not yet living inthe house.)

I try to lower the air temperature and humidity inside the house, whenever possible, by drawing cooler and drier outside air into the house with the help of a whole house fan. This can usually be accomplished in the early morning hours. I've turned off the HRV this summer because it simply brings in high humidity air from outside. The indoor air quality is not significantly affected, as far as I can tell. Because the house is well-insulated and exhibits low air infiltration, I have been able to maintain an indoor temperature in the mid to upper 70's and a relative humidity around 50%. That's pretty good when afternoon outdoor temperatures are near 90 and relative humidity at 60 - 70%.

As to your original question, I don't think "running your HVAC" does any harm if it's simply circulating air within the house. If it's also exchanging inside air for outside air, it's probably a net negative during the hot muggy days of summer. A whole house fan might be worth considering if you plan to delay installing the A/C.

The AC system will eventually be the "ductless" type. The only ducts in the house are for the HVAC system.
Your suggestion of "net negative during the hot muggy days of summer" makes sense based on your explanation.
Thanks for the insight.
With the ICF foundation, does this high humidity cause concerns for a "damp" basement i.e. mould etc?

smonroe,
You say you have ICF foundation. Do you mean that the basement level walls are ICF, if so what are the main floor and loft walls made of? Also, if you have an HVAC system, that is ductwork and an air handler in place, then you need only add an outside compressor unit for AC. No need for a 'ductless' AC system.
As to running the HVAC system, yes you can run it anytime you wish, to recirculate air in the home. If it is a traditional system, it will not draw outside air into the house while running, simply recirculate the air in the house.

Huh?  HVAC = Heating Ventilation Air Condition system.