Building a new house soon. Is it worth having a underfloor heated garage?

[SageBrush]

I'm a fan of heating the person rather than the environment, so I would approach the car the same way.
That said, if I was going to heat the garage I would definitely insulate it to the same level as the home. Your problem is going to be air leak, so the heat loss is not going to be pretty.

 

[vchadha]

Since you're building, what about some sort of geothermal HVAC system? Not only would that save considerably when heating the living spaces of your home, you could probably warm your garage to around 50F with very little additional cost.

Actually my initial preferance was to have HVAC powered by geothermal. However geothermal is costing $80k ($40k in extra material and $40 for drilling the wells) more than the next best alternative. Based on the size of the house etc this worked out to a payback period of 20 years which does not make sense.

 

[David_Cary]

Actually my initial preferance was to have HVAC powered by geothermal. However geothermal is costing $80k ($40k in extra material and $40 for drilling the wells) more than the next best alternative. Based on the size of the house etc this worked out to a payback period of 20 years which does not make sense.

How does 20 years not make sense? Today's interest rates? You don't expect the house to last 20 years? Honestly 20 years is a good cut off. 30 may not make a lot of sense but 20...

There is a huge additional benefit to geothermal - greater longevity of equipment. That pays for itself in less time. The other issue is preheating the water which often cuts the payback.

But - insulation is usually better. Building a house with R-40 walls probably pays back a little faster.

In the US we have tax credits for geothermal which changes the equation significantly.

 

[brkaus]

How does 20 years not make sense? Today's interest rates? You don't expect the house to last 20 years? Honestly 20 years is a good cut off. 30 may not make a lot of sense but 20...

There is a huge additional benefit to geothermal - greater longevity of equipment. That pays for itself in less time. The other issue is preheating the water which often cuts the payback.

But - insulation is usually better. Building a house with R-40 walls probably pays back a little faster.

In the US we have tax credits for geothermal which changes the equation significantly.

The risk of geothermal with a 20 year payback is that the system will not last long enough to repay. If there is a problem in the loop, the savings are gone.

When I considered geothermal I included the tax credits and came up with just slightly over 20 year payback.

The equipment should last longer, but like everything else, there are good and bad outcomes.

 

[vchadha]

How does 20 years not make sense? Today's interest rates? You don't expect the house to last 20 years? Honestly 20 years is a good cut off. 30 may not make a lot of sense but 20...

There is a huge additional benefit to geothermal - greater longevity of equipment. That pays for itself in less time. The other issue is preheating the water which often cuts the payback.

But - insulation is usually better. Building a house with R-40 walls probably pays back a little faster.

In the US we have tax credits for geothermal which changes the equation significantly.

If we consider money in the bank then yes your right since savings interest rates are so low. However even in a low/moderate risk diversified investment you would average 5% return per annum on average across a 20 year period. So when you consider opportunity costs geothermal does not make sense for me. Hydro Quebec offer a DT (dual tariff rate) which basically reduces your electricity rate from $0.08/kwh to $0.045/kwh all year round giving you 44% savings. But when the temperature goes below -15 your electricity rate jumps to $0.27/kwh. We get less than -15 about 30 days per year. The HVAC system however is designed to then switch automatically to GAS when it reaches below -15. Only thing is I pay a higher rate to charge my cars for those 30 days. However with the savings I make year round I still save loads. I will most probably go down the DT rate option and maybe at some point in the future when solar makes sense I will add that.

 

[SW2Fiddler]

The only negative I see is the energy suck and time to warm

Ditra-Heat TB has a thermal break on the lower side, to reduce the heat going into the concrete substrate. I opted for this TB version at a small added expense even though my heated floor is in Houston, TX, because I want my energy to go into the tile surface and not so much down into a slab. Reduces the time to come up to temperature.
Even with their fanciest programmable thermostat, it's still way less pricey than a hydro based radiant heat solution.

 

[Zythryn]

As for the heating the garage question, keep in mind the owner's manual states:


If I build my own house, I'm definitely putting in floor radiant heating for the house and garage on the table. I'm thinking of a passive home.

Garage floor heat is a huge waste of money and energy.
You need to insulate the walls, attic, slab and garage door to huge levels to prevent the conditioned air loss.

Far less energy expensive to just plug the car in and let it heat the battery pack appropriately.

Radiant heat is an awesome way to heat a house, but not a garage.

 

[SW2Fiddler]

You need to insulate the walls, attic, slab and garage door to huge levels to prevent the conditioned air loss.

Well, I definitely wouldn't put the thermostat sensor up in the air. Embed it in the floor.
Car gets first shot at soaking up the radiated heat after all; the battery's only inches off the floor. Any heat continuing to the ambient air is a side effect/bonus (and soon lost thru walls etc.), but yeah you don't want to run the floor heat to keep the garage air at a temp. Nor do you have to.

edit: for that matter, don't heat the entire floor either, if you know for sure where the Tesla will be parking.
2 meters by 4 meters or so. The heated part won't care about the temp of the slab, the rest of the floor, or the air. And a decoupling membrane keeps surface expansion/contraction stresses from being an issue.

I have central-air-based heat and I hate it, from an efficiency standpoint. The house doesn't need the heat, the BODIES occupying the house (usually sitting down) need the heat. OK, in many places the water pipes need to get some too

 

[Zythryn]

Well, I definitely wouldn't put the thermostat sensor up in the air. Embed it in the floor.
Car gets first shot at soaking up the radiated heat after all; the battery's only inches off the floor. Any heat continuing to the ambient air is a side effect/bonus (and soon lost thru walls etc.), but yeah you don't want to run the floor heat to keep the garage air at a temp. Nor do you have to.

edit: for that matter, don't heat the entire floor either, if you know for sure where the Tesla will be parking.
2 meters by 4 meters or so. The heated part won't care about the temp of the slab, the rest of the floor, or the air. And a decoupling membrane keeps surface expansion/contraction stresses from being an issue.

I have central-air-based heat and I hate it, from an efficiency standpoint. The house doesn't need the heat, the BODIES occupying the house (usually sitting down) need the heat. OK, in many places the water pipes need to get some too

I definitely agree with you about the advantages of radiant heat.
However, if heating a garage slab, you absolutely need to insulate the base of the slab. Otherwise you loose half of that heat as the radiant heating warms the ground under the slab.
If you try to heat just a section, you need to insulate the sides of the slab section, and deal with expansion and contraction of the slab vs the section of slab.
And while it is true you don't need the entire air mass at temperature, heated air near the slab will very quickly dissipate.

You can do it, but it is incredibly wasteful. It is far less expensive, and more energy efficient to heat the battery pack directly by plugging it in.

 

[electracity]

I have a gas heater in my three car garage. We also work in the garage year round. We keep garage at 55 degrees and when we go out to work we crank it up to 65. It heats up in about 10 minutes. If we did not work in garage I would not heat it but it is sure nice to get in a warm car in the winter

This ^^
A direct vent gas heater set at 50-55f. This setup also works well when there is a bedroom above the garage.

 

[ReversePolarity]

Are solar panels a possibility? If your geothermal is 20 yrs payback (which, arguable might not be worth it for a smaller home, and considering the fact that technology gets better and better every year), wouldn't solar + hvac in the garage work? Seems these years, solar ROI is anywhere from 5-10 years.