Some general background since you want to get a better understanding. It is most cost effective to size a geothermal unit (the tons part) for less than 100% of your needs. Aux heat makes up the difference but it doesn't run enough to outweigh the cost of a larger unit. In the past, installers over sized furnaces to avoid cases of not enough heat (a gas furnace doesn't have any aux heat). Geothermal systems that are sized to provide full heating output tend to be over sized for AC and this creates humidity problems. Occasionally, there are reports of loops not performing well and so there are some business advantages to an extra conservative design (eg, an extra ton).

Based on the data provided:
Harrisburg PA
49460 loss
23510 gain (19759)
electricity .1275
Of the choices offered I would choose..........
A 3 ton unit with 3 tons of loops.
Climatemaster software suggests the TTV038 will cost $605/yr to heat your home and cool it for $128
based oin 20 year weather average and yada yada.....
balance point by the way around 13*f
But can't the 4 ton heat my house for less (you might ask)? Yep, at $594/yr for heating and $133 for cooling
that's $6/year.
Throw in an extra ton of ground loop, your heating plunges to $586 and your cooling to $130.....now how much would you pay?
Here's where it gets interesting as well, all of these numbers include the use of an electric water heater with DSH. Not knowing any better I assigned this sample 4 people with 130* water.
For all heating, cooling and hotwater, the 4 ton with 5 tons of loop is least expensive to operate at $1,265. Next is the 3 ton at $1,270 and finally $1,274 for the 4 ton with 4 ton loopfield.
By the way the 5 ton (since it was suggested you are on the cusp) would cost most to operate at $1,341/yr.

I get the philosophical difference in design, mine is about ROI. Best ROI on these design conditions- hands down- 3 ton.
Footnote:
a TS 036 single stage variable speed will deliver all for 1287/yr. Regarding dehumi the 1 stg 3ton runs 25% of the hour at 77* the 4 ton 2 stage 24%. 3 ton 2 stage is 31% but it will circulate less cfm. Never much for the dehumi arguments in MI or PA.
Hope this helps.

I am not going to suggest you move all local installers out of their comfort zone. Buying a unit 1 size too large will only cost you a little extra once and only stretch ROI a little. It also allows for modest expansion of the home or design parameters (RE you are 80 yrs old and want 80* in the winter because of your blood thinners).
If you'd rather go with 3 ton and they won't sell I will.
Good luck,
J

I've wanted to ask this for a long time.

I understand that 20 year temperature averages are standard for the industry. Does anyone know how accurate these averages have been in predicting the future? Is anyone aware of a source for current averages compared to a period of time in the past, or old averages compared to a more recent period of time?

Is the 20 year average a rolling average where the oldest year drops off and data from the most recent year is then included? Does anyone know which way the trend has been going?

As a homeowner, and for my system, I tend to like the idea of "Slightly" larger systems to be conservative since no one can predict the future. Right now, weather.com is calling for our zip code to be 87f today, accuweather has us for 99!

The 20 year average reminds me of a stock or mutual fund prospectus - "past performance is no guarantee of future results." :-) I understand that the average is better than guessing. Please someone shed some light on this for me...

Joe, thats what I was driving at the 3 ton system would be my choice also as I hate humidity in the summer and the added benefit of higher air discharge temps during supplemental heat operation. And its been my experience over the last 22 years that over sized equipment has a shorter to much shorter life span than does properly to sightly undersized equipment.
Ajspan, Joe beat me to running the numbers for life cycle costs which should have been done by the contractors bidding the job in the first place...

Posted By ilgeo on 04 Aug 2010 12:51 PM
And its been my experience over the last 22 years that over sized equipment has a shorter to much shorter life span than does properly to sightly undersized equipment. 

Is this due to additional cycles over the life of the unit, or something else?  If cycles, I handle this by reducing CPH to 1.  This does increase temperature swing somewhat, but we are totally comfortable with +/- 1f (actually we find lots of on/off cycles annoying.)

Thanks folks. This is helping, but it still sounds a bit like it is "up to me" to choose. I would have thought that based on calculations about my home people would all propose something similar based on some scientific principle. It sounds like the costs for energy use are fairly close. The difference in cost for extra loop and tonnage is neither one is insignificant. Based on cost, frankly, I would rather go smaller. ROI is important to us too, but I really don't want a unit that cannot heat/ cool the home. I was told that undersized units would have to run higher cycles (run in the second stage more) and that could also limit lifespan. It does sound like a consensus that I don't need 4 ton with 5 ton of loop. It is suggested that smaller would be fine 3 ton unit with 3 ton of loop. The question is, would that be sufficient for the home? Another question is why did 4 different contractors (two that did manual J's) propose 4 ton systems?

Also, to clarify, we have a gas water heater now and were going to replace with gas, and then a nonenergized electric tempering tank for the desuperheater water (which would then feed the gas heater). I was told that was more efficient, but higher up front costs.

With a large enough aux heat (and global warming, but that is a tiny factor), you will never be too cold, even with 3 tons.

> why did 4 different contractors (two that did manual J's) propose 4 ton systems?

One can only guess, but perhaps they aren't looking out for your best roi?

It is the contractors job to do the following:
1 assess the structure and perform a manual J
2 inspect air distribution system and consult manual D
3 consult with building owner to establish lifestyle and usage as in large parties lots of cooking in frequent but important high load. desired level of control over room by room temp, humidity control, future plans, set back and recovery, other heat sources
4 consult selection software and compare the different equipment based on life cycle costs and operational features
5 present to home owner the options and explain the costs and benefits.
6 properly design, layout, install, and commission the system
7 maintain and service equipment
very few do all these well.
ajspan, I believe you have at least 2 experienced contractors recommending 3ton 2 stage with a properly designed 3ton field.
I wish contractors would learn to spend more time explaining geo to the customer. just not in my area..I'll do that...Eric

Eric, I wish someone did all that in my area...

ajspan, I can say that it probably wont be the least expensive nor does it have to be the most expensive....I'm sure some body does it nearby..... to be honest I have a hard time fitting it in every time. I know there are other contractors all over the country that try and do these things on every job. try here......http://www.igshpa.okstate.edu/directory/directory.asp......without seeing your house first hand I would recommend a 3 ton with a 3.3 ton field just because I'm cautious and a 5 to 7kw aux resistance heater.
geome, how was your equipment sized to your design temp loads...Eric

Gme,
re 20 year weather average, with no crystal ball we can only predict the future with data from the past.
To give you some highlights, I can tell you as a career heating guy (for data my bank deposits will bear this out , when the calculator says we usually don't have temps lower than -8 in my area, they are dead on.
Last few years we saw negative teens but it was the first time since '92. Hardly a blip on the 20 year average scale.
'course we all recognize it could happen....thus the aux. coil.
J

Posted By ilgeo on 04 Aug 2010 09:12 PM
geome, how was your equipment sized to your design temp loads...Eric

On paper, it was very close to 100% (I'd give an exact figure, but load calcs from different quotes were not very consistent) and 17f winter design temperature.

What was the range on load calcs?

To me it seems the greater the spread in summer winter design temps then it makes more sense to under size equipment

As Engineer mentions, there is currently no cooling equivalent to the aux. heating coil, so "undersized equipment" makes sense in heating dominated climates.
I don't care much for the choice of the word "undersized". Further when we discuss the merits of "100% load" it implies that persons who design 3rd stage in based on calculations of benefit, somehow are not satisfying 100% of the load which of course is rubbish.
Cost of loops offers real savings ops for folks who don't drink the "aux coil is the antichrist" water.
In OP's case 3 ton might be cheaper to operate, but reluctance to ignore the 100% loaders bidding the job is understandable as they are all on the same page.
It's a shame, but will only cost a slower ROI vs undersizing which might offer no ROI.
j

your right joe in reality proper sizing has as much to do with local as it does actual load. ajspan the proper size as far as Im concerned is 3 tons for your application. Not only on ROI but also on comfort and equipment lifespan.Heck maybe you can fit a 3ton horizontal field in your yard and save a lot of money or is it to rocky in your area....Eric

Posted By ilgeo on 06 Aug 2010 03:23 PM
your right joe in reality proper sizing has as much to do with local as it does actual load. ajspan the proper size as far as Im concerned is 3 tons for your application. Not only on ROI but also on comfort and equipment lifespan.Hecj maybe you can fit a 3ton horizontal field in your yard and save a lot of money or is it to rocky in your area....Eric

But didn't all the the local guys suggest 4-ton units?

On lifespan - can you offer any specifics as to why this may be the case (other than, possibly, unit cycles which may be able to be addressed through thermostat settings)?

Geome just because the local guys say 4 tons .....I work in an area that is close to the same climate . not everyone wants a large deadband thats associated with a low cph setting. and that setting doesnt limit you to 1cph it only increases the deadband. Eric

Oh, maybe you meant locale instead of local...

The CPH works on our thermostat when set for 1 stage equipment. I've timed it - yes, I was bored at the time. :-)

Do you have any thoughts on unit lifespan related to sizing? You've mentioned it twice. Thanks

Some reasons why equipment that cycles often may have lower life:

1) thermal cycling stresses metals, increases wear (tolerances change)
2) lubricant is designed for full operating temperature/pressure and doesn't work as well cold
3) windings rub as they thermal cycle
4) discharge pressure still present causing hard starts
5) oil-refrigerant mixture flashing into foam on startup

On the other hand, some people say that fewer hours of operation cause equipment to last longer.