I'm a homeowner who's talking with contractors about replacing my dated furnace, air-conditioner and water heater with a geothermal horizontal loop system. The house is in the middle of Indiana. It's a typical 1960s two-story colonial, 2,200 square feet, and quite well insulated from the bottom of the crawl space to the attic. It also has relatively new Pella windows. My question is whether the 3 ton heat pumps recommended by most of the contractors would be oversized. (They say 2.5 ton units are not available for geothermal). As efficiently as these units operate, it would seem that the house could be handled by a 2 ton unit.

Garth, Welcome, this is a great rescource for Geothermal.  (so to speak).  You are right to not want an oversized system in your home.  There are a lot of variables to sizing your system. 

One of the best pieces of advice I read here was to do your own Manual J.  It'll cost you about $50 to download from www.hvaccomputer.com.  It's easier than doing your taxes and you can be sure you are not getting too big a system. 

Good luck,
Mary

Thanks, Mary. Bad time for me to have a Macintosh. The software you recommended works only on Windows.

Bummer, Garth. Do you know anyone who would lend you a lap top or you could try the library. IMHO it's too big an investment to make if you are not confident in your contractor's numbers.
Mary

A couple of ideas in mind, yes. Did you put in a system?

I believe Slant-fin has a free heat loss calc, no idea if it's windows only. It's pretty simple to do in a spreadsheet or by hand, especially if you're just looking to for a reality check and you don't need super accuracy.

Sounds good. Slant-fin site indicates the software will be available soon.

No system in yet. Waiting for a refi to go through. Just got a quote and accepted it yesterday. I found a lot of help on this forum and would have surely fallen for some really bad ideas. You can read about them on a posting that started "The HvAc Guy. . ."

He had originally thought I needed a 3.5 ton system, but when he ran the numbers he found a 3 would be sufficent. Yay.

Good luck finding a program to work with your Mac, and I hope your system works out to a proper size.

Mary

Don't confuse efficiency with capacity.

Bearing in mind the inadvisability of rules of thumb for tonnage, my gut reaction is that 3 tons is appropriate for the house you describe. Your climate is almost certainly heating dominated, so you might wind up a bit oversized for cooling. A little extra capacity for heating can be usefully applied to domestic hot water via a desuperheater option and a dedicated buffer tank.

Hedge your bet and go for the two speed option - low stage is 2 tons, high is 3.

Finally, note also that 3 tons generally isn't 36,000 btuh for most actual system conditions. Horizontal loops, while cheaper to install, generally experience more variation in water temperatures, and that results in some loss of capacity down below nominal.

Thanks, masski.

Appreciate the thoughts, engineer. You're right about the climate. It indeed is dominated by heating. I'm hoping to include a desuperheater with the system. The salesmen seem to agree that a standby hot water tank wouldn't work in my situation because it would be located in a garage, which can get as cold as 40 degrees in winter. However, the salesmen also say these desuperheaters only operate when the house itself calls for heat or air-conditioning. In other words, the heat pump won't kick on just to run the desuperheater. Is that your understanding? If so, it would seem that one would want a smaller heat pump, in this case 2 tons, in order to keep the system operating more minutes of the day. That way the water heater would rely less on its electric backup.

did you call the el. company they have someone on staff who does this. the guy came out to my home and recommended a 5 ton unit my house about 2500 sf.. just my 2c but it always helps to get second opinions. the el company wants plenty of capacity so you can avoid turning on those strip heaters. our company has an 850.00 ton rebate which is huge so make sure you checik into this verly closely and get whast you need.

bob

Good idea.

As long as the garage is warm enough not to freeze pipes it should be an OK location for a desuper buffer tank. Standby losses would be slightly higher, but they aren't much to begin with when tank and lines are properly insulated. You can always add more insulation.

Salesman is correct - desuper runs only during heating or cooling calls. That is exactly why they work well with their own preheat tank - that tank stores the recovered heat for later use during hot water demand. Desuper provides cheap hot water during winter (1/3 - 1/5 cost of electric resistance heat) and virtually free hot water all summer.

Heat available from desuperheater won't depend too much on system size. Desuper tends to be about 10% of unit capacity, so a larger unit running less vs a smaller unit running longer should store about the same amount of energy in the form of preheated water. If anything, a smaller unit running more often in stage two may produce more or hotter desuper since compressor discharge temp is higher in second stage.

Don't let considerations of desuper drive system sizing choice. You can either size for winter heating load and either live with summer oversize (sub-optimal dehumidification) or opt for two speed, OR size for summer and plan for greater use of heat strips or other heat source. Desuper is a peripheral issue.

What sizing works best for the buffer tank and water heater itself? A fairly large buffer tank paired with a relatively small water heater? It would seem that a small water heater could handle its demands easily if it were receiving 100 degree water from the buffer. House has four bedrooms and 2-1/2 baths, but with only three people most of the time.

I don't think size matters much in buffer tanks (more cooler water or less hotter water) unless you actually cycle off the DSH. I tend to stay at 50+ gal and size primary tank the same.
Also make sure you will qualify for tax credit with chosen system.
Beware of utility company designs...in our area, "restructuring" has eliminated many expert designers and replaced them with younger (cheaper) techs who tend to size by rules of thumb. Case mentioned above suggest 2 sizes larger for just 300 more SF.
Good Luck,
J

My experience has been the exact opposite

I find older techs will say Ive been doing this 40 years I know what it needs

also in my area older techs have 0 schooling as they where grandfathered in for licences

The best advise I have heard is sub out an independent load calc or buy the cheap 50 calculator

to at least confirm what you are being told .

Is there merit to going with a tankless water heater in this type of system?

gas has draw back , mainly if water is to warm to turn on the tankless but to cool to be comfortable
electric has the same draw backs with almost no benifit ( in my opinion )

If the buffer water is about 100 degrees, how much warmer does the tankless heater need to be set for in order to kick on--with either gas or electric?