I posted a question about feasibility of a geothermal system on the Geothermal Forum and I have some questions regarding radiant floor heating and A/C.   We are building in Tennessee and plan on creating an energy efficient shell consisting of SIP walls and roof, insulated slabs and basement walls, and fiberglass windows and doors.  We would like to use in-slab radiant heat in the basement floor (23800 cf), Warmboard or a similar product radiant floor heat on the main living floor (26,600 cf) and we are currently undecided what type of heat will be provided in the unfinished attic (13,000 cf).

We know that A/C will be needed for a few of weeks in the summer most years and don’t want to be redundant by having to install ducts for a typical forced air system and also have the ductless radiant heat system.  I have been looking at the high velocity/mini-duct A/C systems and realize that the installation cost is currently higher than a standard forced air system. 

 
I also know that I have to install an ERV or HRV system due to the tightness of the SIP shell.

 I have seen radiant heat and forced air A/C supplied by a geothermal heat pump so I know that it is possible.  I am soliciting suggestions to resolve my dilemma regarding redundant systems and reduce the total cost of the cost HVAC system.

First one has to recognize that these are not "redundant" systems as they perform two perfectly opposite functions. As I often design comfort systems for people all over North America and beyond, I have a few simple questions for them: What are the heating and cooling loads for your home? What are the fuel options and their respective cost per therm? How will you be using your home?

If your cooling season is short the emphasis for efficiency and comfort should rightly be placed on the heating equipment.

Radiant floor heating is a luxury and as such cost more than say a commodity. You are on the right track, focusing on insulation, energy recovery, and lastly on cooling. Low velocity as one might guess is quieter (a luxury I would argue) and high velocity is less obtrusive but for the many 2" holes in every room to be cooled.

A good designer will answer many of these questions after performing a complete analysis of your plan.

Heat pumps can be a great heating and cooling source but do not address domestic hot water that is a significant percentage of the average fuel bill. Most of my customers are less concerned about the cost of the system and more concerned about the fixed cost of operation over the 20 year expected operational life.

Scott, What type of Geothermal equipment are you intending to get? water to water or water to air? With water to water type of system, you can heat the water and chill the water. But how to get the air chilled without using forced air? High velocity can work, but i prefer conventional duct systems. Probably something you didn't want to hear. Remember, having a duct system for ERV is very useful, but I highly recommend an ECM blower with it! Anytime you go with a Geothermal system; we are not talking low cost! I hope this helps. Al bluecastlesystems.com

MA, I realize that the systems are not redundant; I was referring to having to install a radiant system and then going on to install a duct system for the low velocity cold air when a heat strip in the handler would eliminate the need for radiant. Currently costwise from lowest to highest, TVA electricity, propane then natural gas are how things fall out in the area. Oil is not used and natural gas is very high. There is also some coal used in outlying rural areas.

The area is definitely heat dependant and could easily be used September through May and heat will be the focus of the design. The need for A/C is limited to July and August. I currently live in Florida and have had the A/C on since May and will probably continue through October. Last winter we ran the electric heat strip for three nights. My current situation is the exact opposite of where we are moving. As part of the SIP design and package, an HVAC engineer will prepare load calculations and provide options based on type of system that we request.

I am not sure what specific type of system we opt for. From an efficiency standpoint, I really like the idea of a geothermal system. I have also been thinking of a desuperheater to provide a hot water boost so I guess I am looking at a water to water system, but I have a lot to learn. While I was at the IHBS in Orlando this year, I ran across a couple of companies that claimed that they could design a geothermal system to run my radiant floor heat and forced air A/C. That sounded interesting, but at what cost?

Regarding hot water, I also like solar for a hot water source. We have been using a passive solar system for over 25 years now and would not trade it. Moving to a cooler climate will reduce the efficiency and also require an active system to avoid freezing. Also we may consider a gas boiler and large insulated storage tanks to store hot water for the radiant system. Lots of thoughts we need to work through before we finalize the design and build. I still welcome any thoughts and suggestions.

I wrestled with basically the same question when I was designing the ICF house I'm currently building. A very low heat load and cooling load requirement caused me to rule out a geosource heat pump because of cost. I was going to do radiant ceiling and use it for cooling also. Many skeptics out there about doing it, but it can be done, and is being done in Europe all the time!

I opted to go with a Daikin air to air HP because it will do both heating and cooling and will pump out heat down to 10°F with a COP of 3. Essentially it will do for me what a geosource heat pump would do but for a lot less money.

I will have a fully sealed and insulated crawl space and will use it for the supply plenum. I expect to have a quasi radiant heated floor since the air in the crawl space will probably be 90 or 100 or more during the heating season.

If electric is the projected to be the cheapest source of energy for the future, start with hydronic tubing in the floor and an electric boiler, add a Marathon electric water heater and your all set. Solar will be a long payback as you move north, lowering you $30.00 water heating bill by 25% or so.

If propane goes down you can change your mind and go for my favorite; Mod/Con boiler, indirect water heater and hydronic radiant floor. My designs include AC and heat recovery ventilators.

Although I live and do HVAC design in upstate NY, I know TN well.  In TN, cooling is your dominant load.  Although it can get brisk up in the hills, heating is secondary.  If you don't want a complete duct system, consider ductless splits.  I recommend Mitsubishi (www.mrslim.com) for these.  They are quiet, efficient and very reliable (bullet-proof reliable).     

Also, most of TN has the wonderful benefit of cheap electricity thanks to the Tennessee Valley Authority, so geothermal, while nice, isn't going to pay you back like it would in NY -- we pay 17 cents / KwH! (you're probably close to half that) -- so it's not going to kill you to run something slightly less efficient than a gshp. 

That said, if you want to go with geothermal anyway (because it's awesome!), you'll need some duct infrastructure if you want central cooling -- there's no way around it - unless you want PTAC units (those ugly things that sit in front of a window inside hotel rooms) or something along those lines. 

Either way, you have two cooling options with geothermal.  The first is to install a separate water-to-air heat pump for cooling (along with the water-to-water for the radiant heat).  The second is to use the water-to-water heat pump for heating and cooling -- in cooling-mode, sending chilled water to a fan-coil / air-handling unit.  There are a number of manufacturers that have chilled-water coils that are for conventional duct work (and not for high-velocity).  

Cost-wise, both options are close in price after it's all said and done.  Option 1 is the simplest way and arguably better -- with two separate heat pumps (both connect to the same ground loops, however), you'll be able to better match each piece of equipment to their respective loads (heating vs. cooling).   

Option 2 will require you to install a buffer tank for the chilled-water (to prevent short-cycling the heat pump) and gets a bit complicated from a piping and control stand-point -- it will be hard for you to find a very good hydronics contractor in TN since boilers and RFH is less common than it is up north.          

That said, different designers have their preference -- neither option is 'wrong'.

Good luck -- JB (www.enhancedliving.net)