New Build in 2018: Geothermal vs NG/conventional AC

[Old Red]

Since it's new construction. check on the installation price of installing a mini split system for the entire home. Very efficient for heating and cooling.

This is actually another very good idea possibly. In one of our clinics we have Mitshibushi Mini Splits that have the ceiling tile air handlers and wan can control each room independently. No heat/air loss through a duct system and the ability to control heat/air in individual rooms would be the ultimate in low power bills. Plus the inverters on these things are incredible....ony pricing the "tonnage" of what is required to maintain the temp in the individual room. My 1.5 ton mini split in my garage can blast out 1.5 tons, but my garage does not need all that "tonnage" each time it needs to maintain the temperature in here. These variable speeds help maintain a more comortable room and use a LOT less power. But in my house there would be no way we could do this as I have too many rooms.

 

[LD1]

I wish I had exercised more control over our geo install, but at the time we had just had a baby and I barely had time to get a couple pictures of where they bored the wells. I would have liked to have more control over the loop plumbing to be able to isolate things if needed, and have better knowledge of how the lines to/from the wells were routed and buried, as well as exact depth. Several times I have had to dig/trench in those areas and it's been a delicate guessing game. Never hit tubing or seen the foil marking tape (which I know they put down), but I still get concerned about fill over the pipes and frost depth when extreme cold weather hits.

If I ever do it again, I would either install the ground loop myself or be in charge of trenching the piping to/from the wells, and then have some redundancy with a manifold and valves, and perhaps an extra loop or well for backup. To me the labor/time are the big expense, but if you can control that, then it makes sense to go the extra mile with the ground loop and plumbing/controls.

I installed a 4-ton unit in my house, which is 1250 sq ft x2 because of full basement.

House was built in the 80's, and as such only has 2x4 walls and R13. Basement is poured walls and I am sure no insulation outside either. Especially since the band board/ perimeter (that little cavity between floor joists on top of the wall)....none of them were even insulated. Which they are now.

Doing a rough load loss calculation based on all that, windows, doors, etc.....using numbers and formulas, and calculators attained online, With 0*F weather outside and a 72* inside temp...I was coming up with a load loss of ~45k BTU

So I settled in on a 4-ton system. And being two-stage.....even if I oversize.....the inefficiencies of doing so are mitigated by running primarily on stage 1.

And the cold spells we have had.....10 days in a row of below zero nights and single digit days......first stage would run for ~1hr...then second stage would kick in and be done in 15-20 min everytime.

So in hindsight.....probably could have gotten by with a 3-ton......but I for darn sure didnt want to be undersized and have to rely on fixing a fire, or turning on my baseboards for backup. So I would have rather err'd on the high side.

Now that said......for my soil type......600' loop per ton is what was called for if going horizontal loops as I did. But the cost of a continuous 600' roll of pipe vs 800' loop was very little. Dont remember exacts....but something like ~$130 for 600' vs $160 for 800'. So I choose to go 800'. So with 4 loops on a 4 ton system....I actually have 5-ton's of ground loop capacity. All the while heating a house that really would probably be fine on 3-ton.

So in the even that I ever have a leaking loop and have to shut it down.....I am not concerned at all.

 

[buckeyefarmer]

I put my header inside with shutoff for each loop. Also useful to purge with the loop pump instead of renting a purge pump.
I also put temp sensors on each loop.

 

[s219]

I installed a 4-ton unit in my house, which is 1250 sq ft x2 because of full basement.

House was built in the 80's, and as such only has 2x4 walls and R13. Basement is poured walls and I am sure no insulation outside either. Especially since the band board/ perimeter (that little cavity between floor joists on top of the wall)....none of them were even insulated. Which they are now.

Doing a rough load loss calculation based on all that, windows, doors, etc.....using numbers and formulas, and calculators attained online, With 0*F weather outside and a 72* inside temp...I was coming up with a load loss of ~45k BTU

So I settled in on a 4-ton system. And being two-stage.....even if I oversize.....the inefficiencies of doing so are mitigated by running primarily on stage 1.

And the cold spells we have had.....10 days in a row of below zero nights and single digit days......first stage would run for ~1hr...then second stage would kick in and be done in 15-20 min everytime.

So in hindsight.....probably could have gotten by with a 3-ton......but I for darn sure didnt want to be undersized and have to rely on fixing a fire, or turning on my baseboards for backup. So I would have rather err'd on the high side.

Now that said......for my soil type......600' loop per ton is what was called for if going horizontal loops as I did. But the cost of a continuous 600' roll of pipe vs 800' loop was very little. Dont remember exacts....but something like ~$130 for 600' vs $160 for 800'. So I choose to go 800'. So with 4 loops on a 4 ton system....I actually have 5-ton's of ground loop capacity. All the while heating a house that really would probably be fine on 3-ton.

So in the even that I ever have a leaking loop and have to shut it down.....I am not concerned at all.

We have a 48 kBTU system for a 2700 sq. ft house. Single geo unit with a zone controller, duct dampers, and ECM blower.

If I could do it again, I would increase the size of the ground loop so that I'd have more capacity for extreme cold weather. With two stages and an ECM blower, the system could certainly be run at lower capacity in summer so that it could still give the right mix of cooling and dehumidification. But I am clearly limited by ground loop low temperature limits in real cold weather, and the easy way to eliminate that is more ground loop capacity.

I don't know if we will have another serious cold snap this winter, but if so I plan to monitor entering/leaving water temps and see how the system does with long run times in 1st and 2nd stage.

The way my system is setup, the single geo unit handles two zones via a zone controller, duct dampers, and staging. I have it setup to run at stage 1 whenever a single zone calls for heat/cool, or go to stage 2 if both zones call for heat/cool. If in heat mode and it's run at stage 2 for a certain amount of time (which I can set) then it turns on aux heat (stage 3). In reality, that time limit should change based on outdoor temperature, becoming longer at higher temps and perhaps skipping stage 3 all together above about 25F. I can add an outdoor temp sensor and use that to disable aux heat above a certain temp, which I may do.

Another option is to tell the zone controller to let the thermostats control staging, which they would do based on runtime (typically upstaging after 9-10 minutes for this type of system), but I don't like this as it could initially be running both zones at stage 1 for that time, and in cold weather, or summer heat, that is not enough heat/cool and airflow for two zones.

Anyhow, I will probably start a new thread about this when I get to measuring performance and setting the staging, as I will want input from folks like you. With this setup, it seems like the staging logic and scheduling has a big effect on performance and electricity use.

 

[Rustyiron]

INSULATE! EddieWalker's post is spot on.
Going with a first rate insulation (foam) pkg. that also get's you an unbeatable air infiltration benefit, that will make what ever heating/cooling source you choose a minor decision. That foam will be working for you every summer & winter 24-7 for your next.. heck forever. Depending on your floor plan, I'd highly consider using the ductless mini splits going with foam. Talk to your architect about some "passive" solar including southern orientation for larger glass areas along with overhangs long enough to keep out the high summer sun. It does help a lot with very minor design changes, and it's another "ding" against the importance of you heat source decision.

 

[buckeyefarmer]

Have to agree with good tight insulation. My house leaks too much with batts, despite me trying to tighten it up.

 

[bcjmmac]

Good discussion. Mini splits are pushed fairly hard here - contractors & government. In our climate they're OK for 8-9 months of the year but require supplemental heat sources for the real winter.
To whit; my son's house was built 5 years ago. They opted for 3 mini splits to save ~$10K in installation costs (contractor estimate).
Their house is a bit smaller than ours, insulation levels are comparable (very air tight & highly insulated). His electricity usage is approximately double ours - year round! Some of that is likely lifestyle differences, but major part is heating/cooling (including hot water as our geo unit heats water).

 

[Rustyiron]

(including hot water as our geo unit heats water).

Will G/T make 120* dhw or just do the lion's share of preheating prior to a "normal" hwh?

 

[Erik M]

The GT will preheat the CW as a byproduct of the heating cycle. It still needs an electric HW Heater to finish it, but it won’t take as much power to do so.

 

[techman]

Will G/T make 120* dhw or just do the lion's share of preheating prior to a "normal" hwh?

As stated by Erik M it is primarily a preheat. Typical winter weather my preheat tank will be at 80-85 deg. On really cold stretches with long run times, it can reach 110 deg. Hut remember that every degree above the incoming cold water temp is energy not needed by the hot water heater.

paul