geothermal or propane for new shop heat

[boughtnotbuilt]

We are in the planning stages on a new shop with apartment over it in Bozeman Montana. It gets cold there, periods with -30 nights and highs of -5. We are coming from a house with pole-barn. the pole barn had insulated walls and ceiling (not great) and had radiant in the slab, but the slab was only 3" with 1" of Styrofoam under. We had an electric radiant heater and were spending $500 a month in the winter just to the heat the barn. So, I'm trying to avoid that. It worked very well, the pole barn was quite comfortable to work in, but it was just too expensive.

The new shop will be 40x60 with 6" slab. We have enough room (20 acres) to run ground loops. So, I'm looking at ground source heat pump as the primary source for shop (and apartment over).

One thing the pole barn taught me is that heating the space via the slab is expensive. However, I also realize that if you leave the slab unheated, it will be horrible to work on. Is it unreasonable to keep the slab at something like 55 or 60 and use mini-splits to heat the air in the shop during the day?

Also, we are being told by professionals in Bozeman that geothermal systems are 'hit and miss'. From what I can tell, the failures are due to bad installation. We are also being told we need traditional propane fired forced air as backup (which means mini-splits are not possible). I'd really like to avoid that.

So, my questions is. Will geothermal heat pump be enough to keep the shop and apartment warm at -30? If not, are there ways to augment the heat pump other than propane?

 

[pcmeiners]

What is your electric cost per kw? Take the total KW usage divide the total bill usage, taxes surcharges by it. As is, propane running costs compared to resistance heating is close in cost. Using a minisplit, propane it is 3-4 times the average minisplit running cost.

"were spending $500 a month in the winter just to the heat the barn."

If I had a barn costing me $500, there would be a spontaneous combustion accident.

 

[toyotadriver]

I'm not a big fan of geothermal a lot of times but it does make sense in some cases. In yours....there's a good chance it could be worth it. BUT...you need to know more about your ground temps. How deep will your ground loops have to be? In MT I'm sure they will have to be very deep....6-10 ft deep maybe? That's some expensive dirt work.

Have you looked into geothermal wells? They can work very well under the right conditions and very deep ground loops might be the right conditions.

Even though mini splits have come a long way, I would not trust ANY mini split to heat a building in MT as a PRIMARY heat source. Yes they will work a lot of the time but some times, they will not be enough. Personally, I think even if you go with geothermal and/or mini splits, you should still go with a back up such as propane. Another advantage of propane is....you do NOT want a building like this to drop below freezing. What if the power goes out? Even a geothermal heat pump system will need a very large generator to run it if the power is out. A propane heater/furnace or two will easily run on a small generator. You could install one or two Renzor/Big Maxx etc type heaters as backup for those really cold days.

Another thought...have you thought about heating the slab loops with an outdoor wood burner? Outdoor wood burners have lots of benefits and some negatives but with your cold temps, when it's very cold outside, a smaller outdoor wood burner running nearly full blast won't make much smoke at all (excess smoke when it's not that cold outside is one of the biggest negatives with outdoor wood burners). I don't know about wood availability in your area of MT so it might not even be a good idea. In my area, I'd run an outdoor wood burner on mostly oak and some hickory and maple but as I recall from my time in Bozeman MT 20+ years ago, that type of wood isn't common so no idea if an outside wood burner is a good idea or a bad one. Food for thought..........




Nothing in this post should be misunderstood, interpreted, misinterpreted, or construed to be of a political nature.

 

[PoorUB]

Properly installed geo is great, but expensive. I would have to pencil it out, but often the return for you money is close to the life of the equipment. Then if you get some hack to put it in and they screw out up, it gets more expensive.

Years ago the shop I worked for got called out to "repair" and installation in a government building. They had geo heat pumps heating water for floor heat. They had two 5 ton HPs that would not heat the building. We were pretty much given a blank check to do the repairs, but they wanted a third HP put in.

When we got there the one HP had just failed and we ordered parts for it. We re-plumbed most of the mechanical room and the HP plumbing was too small, and we replaced the circulating pumps with properly sized units. There were some other details too. We got pretty much done with the job before the parts came in for the failed HP and before the electrician got the third unit wired up. We got it running and to most every one's surprise the one 5 ton HP was heating the building. Also, when we got there throughout the building you could hear water rushing in the pipes and the one HP was working hard and noisy. When we got done you could barely hear the one HP running and no water noise. We went from 120,000 BTU to 60,000 BTU to heat the same building with no other changes.

I guess what I am saying is geo is good if installed properly. Screw it up and you will wish you were running LPG or straight electric. Don't pick the installer on price alone!

Also I am not a big fan of ground loops as they can "freeze up". You have the cold air from above and you are drawing out all the heat from the ground and it can catch up with you . Vertical wells are more dependable, but more money.

 

[boughtnotbuilt]

We can do all the dirt work for the ground loops in ourselves - and yes, probably 6 to 10' deep. I'm going to check temperatures.

 

[pembol]

The expense for geothermal is the ground loop, otherwise it is about the same as any ducted heating system. If you can and want to do the ground loop yourself, then the overall cost comes way down, and there is a 30% tax credit on top of that.

We just had a geo system installed on a new house, and had a Montana like cold snap a few weeks ago - lows around -15F and highs below zero. Electric consumption for heat was around 30kWh/day during the cold snap, which would be $4/day (before solar, $0 after solar). I am happy.

One complexity with geo, particularly if you have an installer that is not super experienced, is that you need to get the load calculations spot on. Unlike gas or even air source heat pumps, there is a major cost associated with oversizing, as your loop field needs to be sized to your heat pump. You really don't want the installer to use the gas furnace technique of guesstimate the load and double it to be safe......

 

[Copymutt]

Don’t overlook the free passive energy. It’s paid huge dividends for both my home & garage.
Link for your area.

Bozeman Passive House — Love | Schack - sustainable architecture and design in Bozeman, Montana and Teton Valley, Idaho

loveschackarchitecture.com

 

[gsmith22]

One complexity with geo, particularly if you have an installer that is not super experienced, is that you need to get the load calculations spot on. Unlike gas or even air source heat pumps, there is a major cost associated with oversizing, as your loop field needs to be sized to your heat pump. You really don't want the installer to use the gas furnace technique of guesstimate the load and double it to be safe......

Don't have your installer design the loop size. they rarely want to do any math or otherwise design/size so don't force them to. your heat pump is sized to the load - ie the maximum heating load on the design day. the ground loop is designed to your system's yearly heat extraction (assuming a heating climate) and has nothing to do with the heat pump size. the ground loop is a big averaging mechanism that responds to the total extraction for a season and is much more closely aligned to the climate and overall sesonal load. the temp on the design day never enters the design equations for the loop but the heat pump size is entirely dependent on that design day.

 

[theoldwizard1]

The new shop will be 40x60 with 6" slab.

Why so thick ? Are you parking heavy equipment (bulldozers, excavators, etc.) on it ? The standard 4" is plenty. Use wire mesh instead of rebar.

We have enough room (20 acres) to run ground loops. So, I'm looking at ground source heat pump as the primary source for shop (and apartment over).

I am not a fan of ground loops. You would have to go down 60" to be sure the loops do not freeze the soil which makes the loop ineffective. Also, with aground loop, you need to "reserve" space just in case the loop is not big enough. With wells, it is easy to drill an extra.

One thing I would do for any geothermal heat pump system is have some way of monitoring the temperature of the ground water leaving the heat exchanger and again as it is returned to the heat exchanger.

One thing the pole barn taught me is that heating the space via the slab is expensive. However, I also realize that if you leave the slab unheated, it will be horrible to work on.

Heating your old slab cost a lot because you had insufficient insulation underneath it. In your climate, I would recommend 4" of rigid foam board. It you install it in 2 layers, tape and stagger the joints you do not need a vaper barrier.

The other thing that most people miss is VERTICAL insulation around the perimeter of the slab. Same thickness, but it needs to go down to the frost line (60"?)

Is it unreasonable to keep the slab at something like 55 or 60 and use mini-splits to heat the air in the shop during the day?

No. With a building that size, you will likely need more than 1 mini-split. Minis with multiple outlets are less efficient.

Some of the mini split companies are now marketing systems that will also generate hot water for slab heating.

Also, we are being told by professionals in Bozeman that geothermal systems are 'hit and miss'. From what I can tell, the failures are due to bad installation.

Likely insufficient ground loop length and/or depth.

We are also being told we need traditional propane fired forced air as backup (which means mini-splits are not possible). I'd really like to avoid that.

You need a backup. A power outage when it is -30F and your pipes will freeze in a couple of hours. (Keep all plumbing away from exterior walls and/or be prepared to drain the pipes in the event of a power outage.) A large propane generator will probably run a medium sized mini-split because they use inverter drive for the compressor. (Inverter drive compressors have a much lower "starting" current draw (LRA).)

So, my questions is. Will geothermal heat pump be enough to keep the shop and apartment warm at -30? If not, are there ways to augment the heat pump other than propane?

With proper planning, YES ! Don't expect to easily find a contractor with the knowledge to design and install such a system.

Maybe you should "expand" your project and go fully off grid with propane backup ! As crazy as it sounds it is possible. Below is a short video snippet about such as system install in a similar climate Solar Power Off Grid with Propane Backup (Search that channel for more details on them building that system.)

 

[gsmith22]

If you want to design your system, the following (free) resources are very helpful:

heat loss/heat gain calcs for building: https://coolcalc.com heat loss/heat gain from this will allow you to size the heat pump

for design of the ground loop size (vertical or horizontal): https://looplinkrlc.com

for design/optimizing the flow characteristics of the ground loop/circulator pump system: https://geo-flo.com/calculators/

all of these calculators are only as good as the information you put into them. I would highly suggest obtaining the IGSHPA's Residential/Light Commercial Design Manual. The most recent version is 2011.

 

[bwoody]

have geo in our new shed house. I would work with qualified installer. we did vertical loops 200ft deep. 6-10 foot might not be deep enough to get heat transfer. We have electric back up. Had geo at old house. Only once did the electric back up kick on in 12 years. Our geo runs our infloor heat on main level, and does forced air and heat upstairs and downstairs.

 

[boughtnotbuilt]

Thanks for all the information. We are doing 6" because this is an automotive/fabrication shop and we want to be able to install lifts safely.

I will look into having the well rig drill an additional hole while they are there. - I'm guessing that will be $15k or so. Would seem more cost effective to use the backhoe already on site to dig trench essentially for free.

 

[fitter30]

Geo is the most efficient system made today. Like any other they have to be design correctly. Loops either type will last 60+ years loops stay and equipment can just be replaced. Look into federal, state tax incentives and electrical supplier rebates.

 

[pcmeiners]

"Would seem more cost effective to use the backhoe already on site to dig trench essentially for free."

You will do it better, less likely to cut corners and save most of the cost of a geo install. You will end up with more of the rebate and FED/state incentive money with not having to shell out for excavation.

"So, my questions is. Will geothermal heat pump be enough to keep the shop and apartment warm at -30?"

There is no reason why not if it has sufficient capacity and is install properly. A low temp air to air minisplit, as a Fujitsu can put out 100% of its rating at -30, so a geo unit using much higher ground temperature can do it as long a the heat capacity at soil depth is maintained, along with ground loop length.
Fully agree the quality of the install is the most critical; of the variable to a quality install, most important is the research involved before anything else.
I have five minisplits, If I had the property size I would have self installed a geo system. My units are 400% efficient, geothermal is much more efficient (unless an older tech unit is installed, which happens often due to profit margins)

 

[jkeyser14]

Don't have your installer design the loop size. they rarely want to do any math or otherwise design/size so don't force them to. your heat pump is sized to the load - ie the maximum heating load on the design day. the ground loop is designed to your system's yearly heat extraction (assuming a heating climate) and has nothing to do with the heat pump size. the ground loop is a big averaging mechanism that responds to the total extraction for a season and is much more closely aligned to the climate and overall sesonal load. the temp on the design day never enters the design equations for the loop but the heat pump size is entirely dependent on that design day.

That's not entirely true... The loop size needs to be based on heat transfer rates. The size will depend entirely upon BTUs needed to be moved in a given time, which is determined based on your furnace size and soil type. Undersizing the loop will cause poor heat transfer. Oversizing the loop will cause a lot of pump loss. Using the soil as a heat storage mechanism like you are describing is only an issue caused by poor soil types that can't conduct heat fast enough to keep up with demand during the season.

My loop temperatures are completely stable over a season. We have a high water table which acts to carry heat away from the loop very quickly.

 

[gsmith22]

That's not entirely true... The loop size needs to be based on heat transfer rates. The size will depend entirely upon BTUs needed to be moved in a given time, which is determined based on your furnace size and soil type. Undersizing the loop will cause poor heat transfer. Oversizing the loop will cause a lot of pump loss. Using the soil as a heat storage mechanism like you are describing is only an issue caused by poor soil types that can't conduct heat fast enough to keep up with demand during the season.

My loop temperatures are completely stable over a season. We have a high water table which acts to carry heat away from the loop very quickly.

The furnace size does not determine the loop size. I have seen contractors use rules of thumb to equate the two but its not proper and generally creates an inefficient or improperly sized loop. the furnace size is entirely determined by the heat loss on the design day (a single day with the coldes temps) where the largest heat loss occurs. all of the other days, you don't need that furnace size to replace the heat loss from the building.

The loop size is based on the heat loss of your building over a period of time (generally the season) and being able to replace the lost heat with heat from the ground. so loops are based on a combination of weather conditions that create heat loss, your building's heat loss loading/calc, and heat transfer between the loop and ground. so heat loss on many average winter days, a design day, and an above average day all contribute to the loop size.

the ground is absolutely your heat storage - where else would the heat be coming from to replace what is lost from the building? its the basic concept of all geothermal systems, a heat pump moves heat from the ground to your house via the ground loop. most ground loops do not have the luxury of being installed into ground with flowing, saturated soil to moderate loop temps. you should count yourself lucky but I don't think I would use a unique case like yours to generalize for all.

 

[nadogail]

I have read of Farmers heating their barns from the heat generated by piles of decomposing manure. That was years ago when Heat Pumps were emerging technology.

 

[4 FN 27]

@boughtnotbuilt first off don't take advise from anyone who does not have or had a Geo System...the naysayers.

I have 2 Geo systems on our property.

The house system which uses a series of vertical wells. The Shop system that uses directional bored Loop Field. Both have performed flawlessly, The house for 15 winters now with zero maintenance other than your normal filter changes. The system is made up of 3 Water to Air furnaces, 1 for the main floor, 1 for the second floor and 1 for the bonus area above the garage. There is a Heat Pump for all the radiant heat, concrete and tiled surfaces. The Garages (upper and lower) and Basement are 6 inch pours, baths and entryways are tiled.

The is a NG boiler in the system to add additional heat if required when temps drop below the point where the Geo can keep up. Our system was designed to run Geo only until you reach -9°. I have only seen it kick in when we reach -20°.

The shop system is 2 Heat Pumps supplying the radiant zones, 7 of them. Then there is an water to air forced air/AC unit. The forced air heat is to supplement the extreme temp drops if you open a big door to shorten the recovery time. Again there is a backup NG Boiler in the system designed as mentioned above.

The other reason for the NG Boilers in the system is our POCO has a program called the Back-up Relief Program formally called Dual Fuel where they can periodically shut down your Geo System and your system now runs off the NG Boilers. The value: because of this program all the electricity used by the Geo System is on a separate meter and billed at $0.50 on the dollar compared to normal rates. A major savings if your POCO has a program like that.

Find a qualified installer. Talk to their customers and see what issues they have had.

The only issue I had with the Shop System was when the electrician drilled through a Radiant Loop. That side of the system was then back filled with "dirty" water causing an issue with bacteria building up and causing the system to be flushed. It had nothing to to do with the Ground Loop System since they are isolated from each other.

Advise: When filling your system use the cleanest water you can get, The house system we had Glenwood bring in deionized water since there was no water on the site yet. The shop I rented a filtration system from a local water softener company and ran a hose from the house to the shop with the filter in between.

 

[My Old Tools]

In your area I would definitely price wells vs ground loops. Wells should give you more consistent incoming water temps in that climate. Trenching would need to be deep. You could also look at units that supply hot water to use for a radiant slab. I bet a cold slab up there is hell on your feet. My system is a Waterfurnace series 7 with a lake plate instead of ground source. Works down here.

 

[pcmeiners]

The OP has the means to dig for the ground loops himself, and plenty of ground area. As long as his ground is good for thermal transfer (tested) why would he go for expensive well drilling ?

 

[theoldwizard1]

We have electric back up. Had geo at old house. Only once did the electric back up kick on in 12 years.

So what size generator do you have and how much fuel do you have on hand ?

 

[theoldwizard1]

Thanks for all the information. We are doing 6" because this is an automotive/fabrication shop and we want to be able to install lifts safely.

Personally, unless you are working on heavy equipment, I would go with 4". After the floor is poured, then I would cut out the area where the lift posts are going to be and go down at least 12". Anchor bolts should be set down to the bottom and long enough to protrude at least 4" above the floor. Make a template out of cardboard that matches the hole on the foot of the lift post. Use that to align the bolts while the concrete is still wet. Use at least 5000 psi concrete in these holes.

I will look into having the well rig drill an additional hole while they are there. - I'm guessing that will be $15k or so. Would seem more cost effective to use the backhoe already on site to dig trench essentially for free.

The problem with ground loops is, if the calculations are off and the loops are not long enough OR they are not deep enough, your screwed ! You may have the acreage to install more loops, but loops that are too shallow are a real problem. With wells, you can usually drill and extra well. You might even consider this when the drillers are there. Might be good insurance.

This goes back to what I said before. You need to know what temperature the water is when it leaves the heat exchanger and what temperature it is when it return ! Most system are designed assuming about 50F for the rturn water. If it is even 40F, you have a BIG PROBLEM !!

 

[theoldwizard1]

The OP has the means to dig for the ground loops himself, and plenty of ground area. As long as his ground is good for thermal transfer (tested) why would he go for expensive well drilling ?

The problem with ground loops is, if the calculations are off and the loops are not long enough OR they are not deep enough, your screwed ! You may have the acreage to install more loops, but loops that are too shallow are a real problem.

 

[pcmeiners]

"As long as his ground is good for thermal transfer (tested) why would he go for expensive well drilling ? "

That is why I included "thermal transfer (tested), which is critical.

"With wells, you can usually drill and extra well. "

Like drilling a well is not expensive, or they can be impracticable to use.

 

[Jackfre]

Just to add a bit of flavor to the discussion consult a local hydrologist on your local wells. There are areas where water flows underground vs just being a pool of water. If the local characteristics offer flow you may be able to use a down hole HX (DHH). If the DHH is not possible you need another well to re-inject the water used. Oregon Inst Technology in Klamath Falls, OR is a great resource for you.
As pointed out, get the design right and have an experienced reputable contractor do the work. On principle, Geo systems are pretty simple. BUT, you have the ground/well work, the piping/circs, the HP(s), ductwork and let us not forget the controls. It is a lot. Get a meathead in the mix and you will suffer.
As this is a new design I would definitely cover the roof top with solar collectors. They pay and are cheap to install today.

 

[theoldwizard1]

Like drilling a well is not expensive, or they can be impracticable to use.

The problem with ground loops is, if the calculations are off and the loops are not long enough OR they are not deep enough, your screwed ! You may have the acreage to install more loops, but loops that are too shallow are a real problem. With wells, you can usually drill and extra well. You might even consider this when the drillers are there. Might be good insurance.

This goes back to what I said before. You need to know what temperature the water is when it leaves the heat exchanger and what temperature it is when it return ! Most system are designed assuming about 50F for the rturn water. If it is even 40F, you have a BIG PROBLEM !!

So tell me what you are going to do if your loop system return water is too cold ?

There are a lot of videos online about people jetting their own wells. You need a high volume pump, water supply and bentonite and of course the pipe.

Yes, they are under 100', but 4 or 6 of these would probably be adequate.

 

[pcmeiners]

So tell me what you are going to do if your loop system return water is too cold ?

That is the point of testing, as I noted. My point, the OP has the equipment available to dig, Montana average ground temperature is sufficient, why drill well if you do not have too.

"There are a lot of videos online about people jetting their own wells. You need a high volume pump, water supply and bentonite and of course the pipe"

I drilled at least 100 wells, a few by jetting, most by drilling, jetting it is not as easy or practical as they want you to believe. Aside from that, geo wells need a lot of surface to exchange heat. Unlikely you are going to drill anything halfway deep by jetting without hitting rock or large gravel except in shore areas, once you hit that jetting is more difficult or impossible, you definitely not going to jet wells hundreds of feet. Likely you will not have the pressure or volume to sink more than a shall well. The last jetted well I did I had access to a NYC hydrant, at 100psi via a 3 inch fire hose; got to 59 feet with a 6" casing, stopped by a rock.

 

[theoldwizard1]

The last jetted well I did I had access to a NYC hydrant, at 100psi via a 3 inch fire hose; got to 59 feet with a 6" casing, stopped by a rock.

What did that cost you in $$$ and time ?

Sure, you might have needed 4 - 6 of those, but that still might be cheaper than an excavator and a bulldozer.

 

[pcmeiners]

"What did that cost you in $$$ and time ?"

Actually that well was for my mom, during a drought in Staten Island NYC. Took 12 straight hours and (60) ft of 6" and 2" PVC. It almost cost me a trip to the hospital for a concussion, as the elbow at the top of the 2" pvc, used to jet the well came unglued at 100psi, and caught me under the chin, saw stars for about 5 minutes during broad daylight, ....kinda rushed the glue a bit.