Hi,

I posted before http://www.greenbuildingtalk.com/Fo...fault.aspx about my 1873 home and how I want to install a GSHP. That thread ended with my deciding to put off installation until I had some air sealing and insulation completed to get a better sense of my new heat load calculations and ensure a correctly sized system.

Well, the insulation work is mostly complete (except the attic floor, which will be done after the heat pump is put in) and here are my new numbers:

Before 143.2 kbtu/hr for heat  ? cooling

After   63.8 kbtu/hr  for heat   33.5 for cooling (these number factor in the additional attic insulation not yet present)

We still have some minor caulking around windows and some weatherstripping on a few crooked doors, but already we have seen quite an improvement.

Two questions:

Seeing as we live in central PA, where good reliable heating is more of a priority than cooling, based on these new numbers, would you go with a 5 ton or a 6 ton system? Of course I will be asking my installer this as well, I just was interested in a number of opinions.

Also, just curious---how many feet of well do you like per ton? I have been quoted as low as 400' for 7 tons and as high as 750-800' for 5 tons. Is 150' per ton a good start, or is going to 175' or even 200' better (if possible with soil conditions)?

Thanks!

WOW!!!
That is quite an improvement in your envelope, congratulations. How much tonnage you really need is a function of the design load capacity. Fancy way of saying it may be cheaper to install smaller unit to cover a percentage of load and make up the differance with stage 3 electric on a limited number of days. Feet of bore per ton is a function of the grounds conductivity. I would assume rock in your PA location so the cost to drill is going to be high, but conductivity will be good. Pay attention to the design aspects of each bid in regard to the amount of money it costs to drill extra footage vs. the cost of stage 3 electric heat over say 10 or 15 years.
Eric

If money was not a problem, how many feet of bore in rock would be best.  In my area, in granite rock, we try for a little over 200' per ton.  Is the depth per ton different for heating as opposed to cooling?

Believe it or not, I have had clients install an extra well.  Payback on a retirement home is not much of a concern to some people.  They want extra capacity in the wells to avoid using antifreeze.

PA is a big state, what is "central PA", Williamsport?

I would not be surprised if they don't recommend a 4 ton unit... Assuming average rock, 150' x 4 is the min I would go. 175'x4 looks better.

The advise always given here is if you 'need' to oversize something, oversize you loop field and not the system.

How much do you trust the new numbers?
I keep saying, old houses are tricky. In my neck of the wood, 64K heatloss would be 5 tons.
Remind me, forced air?
Nobody should give you advise about your loop size over the internet, because no one knows your ground conductivity. We have done a couple jobs in northern Pennsylvania, and 150ft/ton was just fine. does not mean much for you.

You probably have already, but discuss open/standing column/vertical/horizontal loop pros/cons with the installers. Maybe even DX.

We are south of Williamsport, on the Susquehanna River. I see what you all mean that it depends on the ground how deep the wells are or how many wells we should have. I will definitely discuss all of this with our installer.

Our current heating system is a combination of oil furnace/forced hot water and electric baseboard. Installer is proposing a WaterFurnace Envision dual capacity, 410 A refrigerant with Senergy (we have some hot water radiant floor heat we'd like to keep). Aux heat EAL 20.

The guy who did our first blower door is the same guy who just did all of our insulation work, and then did the second blower door. His company also installs geothermal systems, but they are too far away to make their doing the job worthwhile. So this is the guy who was discussing with me that he would size the system to cover all heating needs so we rarely if ever needed to use the aux strips, and he would recommend a 6 ton for that purpose (we can get single digit nights here and days in the teens). He was also the person who told me his company recommends at least 175' of well per ton, but the ground may be different where he usually works.

I do trust his numbers, mostly because he wanted to get us down even lower and was upset that he didn't do it. So I don't think he was lying, or he would have said he met or even exceeded his goal, not fell short....They air sealed the attic, will be putting R-50 of cellulose up there, sealed the rim joists in the basement and spray foamed two crawl spaces, and put dense pack cellulose in all the of the exterior walls. So we went from just about nothing---nowhere to go but improvement.

Am I understanding all of you that I can go with smaller tonnage if I have bigger wells, and that it is a combination of low tonnage and short wells that can cause trouble? I just don't want to be posting here next winter saying my electric bills are through the roof because my aux heat is always having to kick in.

You can size the geo to provide all or nearly all your heating load, but chances are good that's not the best (lowest lifetime cost of ownership) solution

A six ton unit would be way oversized for cooling, introducing comfort issues.

Unless your electricity costs are very very high, my gut suggests a 4 ton geo unit would be reasonable for the new loads. Aux strips (and wood / coal / pellet stove) would make up the rest.

Extra geo tonnage acquired at great expense and then rarely used is a false economy.

Thanks for the response.

Our electricity rate through Jan 2012 is 8.48 cents per kWh, after that I'm not sure--but it seems most people around here pay somewhere between that and 10 cents per kWh. As we don't have/are not interested in adding a wood/pellet/coal stove, our backup would be exclusively electric.

This all boils down to sizing philosophy. While it is a valid argument to size for 100% geo the economics often fall short when the cost of drilling is high. If the cost of drilling is high, the footage cost for the additional loops plus the cost of a larger unit look more and more un-attractive. This whole concept is just another thing to look at when comparing designs and costs, there is no "right or wrong". Part of the design estimate will show the anticipated cost of the stage three electric usage vs. the cost of larger unit and more loops.
Eric

In my neck of the woods that load would often suggest a 4 ton. A 100% loader might go with a 5. Since your cost/kw is cheap a designed in 3 stage makes way more sense than a 6 ton.
Also in my neck of the woods we use 150' well/ton, however a given well is usually penetrating one or more aquifers.
j

I just met with my installer again; he very patiently answered all of my questions. He still recommends the same size unit (the NDV064T, 5 tons). He said the aux should only be needed on very windy days or days below zero, but that it is okay for it to come on every once in a while--that it would cost more to run a larger system that kept us solely on geo.

He is recommending 5 wells of 150-175 each, somewhere between 750-800 feet total.

Now we are just waiting to hear when they can fit us into their schedule!

I live a couple hundred miles north of you and installed a 4 ton. I went with 2 450' wells. The drillers made a mess of my front lawn. I had to take out a tree, remove my fence and they broke my walkway which they said they would be able to stay clear of. I may have been able to go less depth but I wanted to make sure that I didn't have to go through another drill and the mess. On new construction it may had been different. As far as unit sizes, is there a cost difference? I know on my brand, the 3 or 4 were the same price and on the model I finally chose, the 4 was the smallest. My load calc put me at 3.5 tons not including my finished basement and sun room which I don't currently heat. I managed to go without aux heat all year. I also won't go burning fuels.
My house is all electric. I had no ducts. The thing that really made the sytem work so well IMO is that I worked closely with the designer/ installer of the duct work on every detail. Every detail. I was a pain and paid extra but everyone who knows anything about it, was impressed with the way it came out. Even though I had thermostats in every room with the baseboards, the 2 zones I have now is way more comfortable feeling. Like a luxury car, it just feels better.