New Build in 2018: Geothermal vs NG/conventional AC

[rngrrymd]

LD1, great catch about the reinstatement of the incentive. It looks like Congress reinstated it in that ridiculous 2 year "budget" they just passed in early February. Good to know, I hadn't paid attention to that. Our local electric company isn't offering rebates for GT right now, might change in future.

You are partially right about GT with no backup heat but heat load is heat load. If your house requires 50k btu's at "0" outside that doesn't change. The only way to avoid backup heat is to oversize your system with a multi-stage or inverter unit since u don't want to oversize the a/c side and have dehumidification issues or I have seen 2 systems installed as well (2nd system is purely for heating capability). Either way requires more pipe/wells, etc for the larger unit(s) which costs more which again makes it harder to get payback. Looks like the rebate is only 30% from 2017 - 2019 then drops to 26% in 2020 and 22% in 2021. GT is definitely the lowest operating cost overall for HVAC systems but payback to justify the extra cost is the biggest problem I see. Once again, I deal with closed loops in wells in my area predominately - the well cost is what destroys the affordability.

Also, A 5-ton top of the line Water Furnace unit has a 60A breaker which is 4 times what u need for a gas furnace if a generator is built into the equation so u can install a smaller gen that uses less fuel. It's cold in the winter - if I put a whole house gen in the house I'm building in 2 years I want full heat not just partial heat. No sense in being cold! Either that or move to Arizona. :laughing:

 

[LD1]

. The only way to avoid backup heat is to oversize your system with a multi-stage or inverter unit since u don't want to oversize the a/c side and have dehumidification issues or I have seen 2 systems installed as well (2nd system is purely for heating capability).

Sounds like the typical HVAC contractor.

Thats the exact reason that MOST people around here that I know of, who have a GT, are complaining that their strips are always coming on and their bills are extremely high.

Yep....thats what happens when you let the "pro" talk you into sizing by your AC load. Makes the system grossly undersized for the heating load.

Heating season around here is 6 months. And many 0-10 degree days. IF you want to keep the house 70........we need a 60-70 degree DeltaT.

Cooling is 3...maybe 4 months. ANd it gets to what....90-95 in the summer. So we need a 20-25 degree DeltaT.

So if we size for AC.....That makes the heating side sized for it it never gets below 40 degrees out. Well....it does. So thats why you have problems.

Forget dehumidification. AC and Dehumidification are NOT how you want to size a Geothermal unit if your heating needs are 2x or 3x what the cooling needs are.

 

[techman]

I have a geo system for 11 years and in that time my electric strips have come of a total of 1 day. It was during one of the coldest stretches we have had in memory.

I installed the electric backup strips primarily as a heat source should I have a problem the geo heatpump. No problems have occurred till now.

paul

 

[buckeyefarmer]

My upstairs heat strip comes on if its in low teens outside, course i usually keep the breaker off until the HP falls too behind.
Downstairs heat strip never got connected to AC, never got close to needing it..
When i took geothermal design course, they taught to use the bin data for your area, (number of hrs/yr you get low temps) and size so the heat strip is covering the few coldest times. Its a cost tradeoff, short term electric heat vs long term electric running a larger unit.

 

[LD1]

My upstairs heat strip comes on if its in low teens outside, course i usually keep the breaker off until the HP falls too behind.
Downstairs heat strip never got connected to AC, never got close to needing it..
When i took geothermal design course, they taught to use the bin data for your area, (number of hrs/yr you get low temps) and size so the heat strip is covering the few coldest times. Its a cost tradeoff, short term electric heat vs long term electric running a larger unit.

Is your unit 2-stage?

2- stage or even some of the newer variable speed units really soften the blow of oversizing.

It's almost like having 2 heat pumps. Ultimately sized to cover all the heating needs. But when it's not frigid enough to need that much, it coasts along on 1st stage.

My 5-ton unit is 2-stage.

On second stage the thing pulls 28 amps.
First stage is 16a.

So first stage is like a 3 ton unit.

Ohio weather. When it's modest....20 degrees and warmer....first stage has no issues.

When we get 10 degree days and below......second stage kicks in. Which I'd much rather have a second stage to fall back on rather than strips.

 

[buckeyefarmer]

Is your unit 2-stage?

2- stage or even some of the newer variable speed units really soften the blow of oversizing.

It's almost like having 2 heat pumps. Ultimately sized to cover all the heating needs. But when it's not frigid enough to need that much, it coasts along on 1st stage.

My 5-ton unit is 2-stage.

On second stage the thing pulls 28 amps.
First stage is 16a.

So first stage is like a 3 ton unit.

Ohio weather. When it's modest....20 degrees and warmer....first stage has no issues.

When we get 10 degree days and below......second stage kicks in. Which I'd much rather have a second stage to fall back on rather than strips.

Mine are 2 stage, variable speed. It helps a lot if oversized.

 

[s219]

Is your unit 2-stage?

2- stage or even some of the newer variable speed units really soften the blow of oversizing.

It's almost like having 2 heat pumps. Ultimately sized to cover all the heating needs. But when it's not frigid enough to need that much, it coasts along on 1st stage.

My 5-ton unit is 2-stage.

On second stage the thing pulls 28 amps.
First stage is 16a.

So first stage is like a 3 ton unit.

Ohio weather. When it's modest....20 degrees and warmer....first stage has no issues.

When we get 10 degree days and below......second stage kicks in. Which I'd much rather have a second stage to fall back on rather than strips.

What control/criteria do you use for upstaging? Is it based only on the amount of time 1st stage has run, or additional factors?

 

[buckeyefarmer]

I have a monitor on my units, daily and monthly runtimes, data saved monthly.
(low time includes high time)

Upstairs low 10.38 hrs
Upstairs high 2.92 hrs
Down low 7.77 hrs
Down high 0.00 hrs

 

[LD1]

What control/criteria do you use for upstaging? Is it based only on the amount of time 1st stage has run, or additional factors?

Two things can initiate the second stage.

First is a temperature differential.

This is where alot of t-stats don't have any adjustment. They are preset at like 2 or 3 degrees. And this is where alot of people loose efficiency because they set their t-stat lower when they leave, and crank it back up when they get home. The non-intelligent t-stat things that "it's 65 in here and you just told me you want 70....so I am gonna do that as fast as I can"....and thus second stage (or worse, backup heat) kicks on.

My thermostat can be adjusted on this setting, or it has an auto mode. The auto mode learns the habits and does what is most efficient.

I can set the t-stat to away, then change back to home when I return....and second stage won't kick on...unless.......

The second thing that controls staging is time. I can choose how long I want to run on first stage before second jumps in right now it's set at 1 hr max first stage run time. But this setting is also smart and "learns". My temp swing is 1.5 degrees. So it needs to heat my house by 1.5 degrees before it shuts off. At the end of the 1hr max run time.......if it knows it is close.....like it's already done 1.4 degrees in the 60min.....it won't turn on second stage. But if in 1hr....it is only halfway there, it knows it needs to jump on second.

 

[Sigarms]

Sounds like the typical HVAC contractor.

Thats the exact reason that MOST people around here that I know of, who have a GT, are complaining that their strips are always coming on and their bills are extremely high.

Yep....thats what happens when you let the "pro" talk you into sizing by your AC load. Makes the system grossly undersized for the heating load.

Heating season around here is 6 months. And many 0-10 degree days. IF you want to keep the house 70........we need a 60-70 degree DeltaT.

Cooling is 3...maybe 4 months. ANd it gets to what....90-95 in the summer. So we need a 20-25 degree DeltaT.

So if we size for AC.....That makes the heating side sized for it it never gets below 40 degrees out. Well....it does. So thats why you have problems.

Forget dehumidification. AC and Dehumidification are NOT how you want to size a Geothermal unit if your heating needs are 2x or 3x what the cooling needs are.

A "pro" will do a load for both heating and cooling. Design temp indoors vs design temp outdoors. Generally in our area, outdoor temp design for heating is either 20 or 17, and for cooling 95.

Biggest issue with load calcs is garbage in, garbarge out, but if they're done right, they really don't lie.