How much heat do I really need?

[wantmo]

I have a 35 x 28 (minus a couple of corners) attached 3 car garage. I had an HVAC contractor come in to quote adding heat, and he quoted both a 24k BTU natural gas and an 11k electric heater as two options. I was expecting at least 40k BTU, and online calculators are all over the map. The contractor was a bit concerned with natural gas supply as the gas line enters the house on the opposite corner so is a long line - so he said electric might be better.

Construction: Garage is 14' tall. Approximately 35' of wall is common with the house, and there is a bedroom above part of the garage. Walls are brick, with a 1" rigid fiber board insulation and 2x4 construction. I plan to insulate walls (hopefully spray foam), ceiling (on section without the room above (unknown current insulation) and drywall. Garage doors face north and are being replaced with insulated ones, and there are man doors to the house and back yard and a set of windows.



Goals: Keep garage above freezing in the winter, and be able to bring the temp into the 60's a very few days a month when I am going to work for a more extended time. I can handle a longer recovery time.

Climate: Front Range CO. Winter days are moderate, with a larger swing from day to night temps. We do get a few days every winter where the temperatures are below 0 F, as well as several warm days. I've attached a graph of the garage temps this year from mid-Feb to mid-Apr against the daily high and low. Unfortunately I missed the typically colder end of Dec through start of Feb.



My inclination is that I would be happiest with the natural gas solution, but not sure if it is enough BTU's. I'd also appreciate any comments on the location noted.

 

[b-boy]

Is the plan to tee off of the meter to the garage, or tie into the gas somewhere in the house?

I think I'd opt for teeing off the meter with a bigger pipe running directly to the garage.

Hopefully, that should cause fewer problems. I did that with mine. I have a 1" gas line that runs to the garage directly.

It originally teed off right before the boiler, then exited the house through the wall. It ran about 50 ft to the garage. The pipe was 1/2" and I had problems getting consistent gas flow during the winter.

 

[HoosierBuddy]

You definitely want gas as the operating costs will be 1/3 to 1/4 what resistance electric costs will be.

I'm concerned that he limited you to 24,000 BTU so he could use a vent free heater.

Vent free heaters **** in the garage. Literally, the combustion air ***** in every speck of dirt and solvents you have in the area, burns that along with the gas and then blows all of that back into your face. Terrible heating choice.

You want a vented gas heater. If he has to run a new line for it...so be it. Look at the Modine Hot Dawg or similar. I think they start at about 45,000 BTU's...which may be more appropriate for your needs. It will require a vent, normally through the roof. If money is less of a concern...consider the modine effinity line, which are high efficiency (fully condensing) and vent with pvc.

The thing about heat loss calculations is they don't tell you how long it will take the garage to heat up from 40 to 65 some Saturday morning when you want to work out there.

Phil

 

[wantmo]

The quote calls out a sidewall vent on the nat gas heater, and we discussed that it should be vented. His concern was about amount of gas flow over the distance, but he did not run any calculations to support that concern.

Ideally I extend an existing 1" line that is capped not far from the garage that previously serviced a cooktop that was converted to induction. I have a partially finished basement, so adding another line from the meter either means tearing up some ceiling, or a bunch of stuff around the house as this is literally on opposite corners of the house.

I did try to map out the gas lines and try to calculate, but I don't think I did it quite right. Compounding the issue is that I also want to use the line for a grill and burner - I just have a lot of BTUs on a long run.

 

[theoldwizard1]

Walls are brick, with a 1" rigid fiber board insulation and 2x4 construction. I plan to insulate walls (hopefully spray foam), ceiling (one section without the room above (unknown current insulation) and drywall.

Never heard of fiber board insulation. Is this some kind of exterior sheathing ? If so It is likely 1/2" or 3/4" not 1".

Insulation is the most important factor in sizing for heat. If the walls are not finished on the inside, I would add 2x2s and user 6" fiberglass rolls or batts. Even better, apply 2" of spray foam and then 4" of fiberglass.

If the ceiling is finished your in a bad spot. Likely it does NOT have adequate insulation. You really want 12" of fiberglass or cellulose in the ceiling.

"You pay for insulation once. You pay for energy to heat you building every time the thermostat calls for it !"

 

[Marctrees]

Just my thought here... If I was building in a cold climate.. IF budget allows... I would look into pex floor heat as a PORTION of my heat system.

Then, the forced air big space gas heater(s) for your bump up days.

If you have never experienced the comfort of that, I assure you it is awesome.

Use the floor heat as the primary system to keep however you like much a bit "above freezing"

Marc

 

[D45]

Do you want heat in the shop 24/7?

Do you want to it to heat up a fast as possible when you go out there to work?

Do not get vent free

 

[Eggman]

A couple of questions for you.

1. How much of the 46' is actually 1" pipe?
2. Does the 1" pipe actually end? Or does it reduce to 3/4" or 1/2"?
3. Can you get to the end of the 1" pipe without hacking into the ceiling?
4. Is there a 3/4" drop for the existing furnace/water heater combo?

Depending on your answers, I would extend the 1" pipe as far as I could, tee off of it and run a 1/2" line to the grill/burner and a 1/2" line to the garage to a 60k btu 92% furnace. The flue pipe can then be ran through the side wall.

If you have a ranch style house, one could always tee off of the line after the meter and go across the house in the attic, quite possibly directly into the garage.

My $0.02

 

[ReggieR]

Man if I could get that shop stabilized at 38-45F with little humidity in the bad times I'd be a happy camper. Obviously on warm days the doors come open and the beers get flowing.
40 degrees is good working weather. Welding attire, bibs, respirators or whatever. Not so good for paint but thats why they make heat lamps and curtains

 

[wantmo]

Never heard of fiber board insulation. Is this some kind of exterior sheathing ? If so It is likely 1/2" or 3/4" not 1".

I am probably using the incorrect term here. The wall construction does not use a traditional wood/particle board sheathing. It is a 1" thick rigid insulation of some type with a silver facing on both sides. This is 30 year old construction. I have only been able to see what it actually is because the PO put a couple of holes in it...

Insulation is the most important factor in sizing for heat. If the walls are not finished on the inside, I would add 2x2s and user 6" fiberglass rolls or batts. Even better, apply 2" of spray foam and then 4" of fiberglass.

I am big on insulation, thanks for the suggestion of adding 2x2's. I'll look at that compared to just spray foaming the entire 2x4 depth.

If the ceiling is finished your in a bad spot. Likely it does NOT have adequate insulation. You really want 12" of fiberglass or cellulose in the ceiling.

Ceiling is finished, but I plan on opening a space for me to get into. Part of the space above the garage is a bedroom and that room is consistently cold in winter as it has 3 outside walls. Figure this is the time to improve both the room and attic insulation and then seal it back up.

"You pay for insulation once. You pay for energy to heat you building every time the thermostat calls for it !"

Just my thought here... If I was building in a cold climate.. IF budget allows... I would look into pex floor heat as a PORTION of my heat system.

Then, the forced air big space gas heater(s) for your bump up days.

If you have never experienced the comfort of that, I assure you it is awesome.

Use the floor heat as the primary system to keep however you like much a bit "above freezing"

Marc

I'd love in floor heat, but it's an existing garage with a floor in good condition.

A couple of questions for you.

1. How much of the 46' is actually 1" pipe?
2. Does the 1" pipe actually end? Or does it reduce to 3/4" or 1/2"?
3. Can you get to the end of the 1" pipe without hacking into the ceiling?
4. Is there a 3/4" drop for the existing furnace/water heater combo?

Depending on your answers, I would extend the 1" pipe as far as I could, tee off of it and run a 1/2" line to the grill/burner and a 1/2" line to the garage to a 60k btu 92% furnace. The flue pipe can then be ran through the side wall.

If you have a ranch style house, one could always tee off of the line after the meter and go across the house in the attic, quite possibly directly into the garage.

My $0.02

All of the 46' of pipe is 1", including the drop to the furnace and hot water heater. The 36' portion is largely in the finished portion of the basement, but I can't imagine it would be any smaller since that is the connection to the meter. Two story house so going up to the attic has challenges as well.

 

[wantmo]

I really appreciate the responses so far, and tried to clarify as much as possible.

I am wondering if the right way to look at this is if I extend the gas line as shown, how large of heater (vented of course) could I put in and not starve for gas? Not sure what happens if the heater is not given enough gas either.

I suspect that at least I would be able to keep the garage above freezing and the variable would be how fast it would heat up on days I want to work in there for a substantial amount of time.

 

[matt_i]

The natural gas utility, if you can speak to "engineering", they can give you a total number of BTUs that your meter can reliably support.

There is also another solution: controls. Basically the call for heat at one furnace has to inhibit the call for heat on the other system so they both can't run at the same time. Its a pretty simple circuit, it uses low voltage 24vac controls and relays. Once one furnace finishes its cycle and the thermostat is satisfied, it (electrically) opens the pathway for the other furnace to get the signal from the local Tstat and start working. The utility probably won't like that solution but functionally limiting to one unit consuming gas makes sense....at least to me

 

[Eggman]

It seems that you are at the limit of your 1" pipe for the entire run.

Take a look at this... https://www.edcgov.us/Government/building/documents/Gas Pipe Sizing (Natural).pdf

 

[Jackfre]

Look at a Rinnai EX38.it will fire as low as 13,200-38,000 btu in seven stages.

 

[wantmo]

It seems that you are at the limit of your 1" pipe for the entire run.

Take a look at this... https://www.edcgov.us/Government/building/documents/Gas Pipe Sizing (Natural).pdf

A big thank you, this appears to be exactly the information I needed although it does not appear to be good news.

Even if I eliminate the outdoor grill and burner the 1" line at 134' would only be able have a capacity of 157 cubic feet an hour, which after furnace and hot water heater is about 13K BTU to the garage.

If I skip the garage and include the outdoor kitchen, at 109' the line can support 173 cf/min, which is not enough to support those items with the furnace and HWH.

Granted, there is a level of conservativeness in this analysis, but other variations of this method have significant penalties for fittings and bends - and I have many.

Looks like I need to figure out the least disruptive way to get another line from the meter run

 

[Eggman]

One question that you should ask yourself is how often are all of the appliances going to be on at the same time?


matt_i did have a good idea on preventing the garage heater from coming on while the main house furnace is calling for heat. Furnaces don't run all of the time all day long. Using a relay or two would keep the furnaces in check.

....There is also another solution: controls. Basically the call for heat at one furnace has to inhibit the call for heat on the other system so they both can't run at the same time. Its a pretty simple circuit, it uses low voltage 24vac controls and relays. Once one furnace finishes its cycle and the thermostat is satisfied, it (electrically) opens the pathway for the other furnace to get the signal from the local Tstat and start working. The utility probably won't like that solution but functionally limiting to one unit consuming gas makes sense....at least to me

 

[HoosierBuddy]

OK...I just ran all this through a node analysis on GasCalc and I come out with a total of less than 1-inch pressure drop at your garage furnace with everything on at the same time and the garage furnace at 100,000 BTU's hour.

Follow me here:

7-inches inlet pressure.
48 feet of 1 inch with .380 MCF/HR load drops you to 6.563" at the furnace inlet.
46 feet of 1 inch with .260 MCH/HR load drops you to 6.409" at the tee to the grill
40 feet of 1 inch with .100 MCF/HR load drops you to 6.383" at the inlet to the heater

I guess the biggest difference between how I figured it and the chart is I'm dropping off the load at each point, where (I THINK) your chart is assuming you are taking it all the way to the garage....which you aren't.

YMMV....but if this was mine...I'd say it will work just fine.

Most homes are equipped with an R275 meter that is only accurate to flows of 275,000 BTU's/hour. Suggest you may want to talk to your gas company about getting a 415 installed, which is the next largest common size.

Phil

 

[Matti]

A 45k unit works fine in my 26x24x11 garage which I can easily keep at 70F. Its well insulated (2x6 walls) and temps get down to -30C. I would think you could get away with a smaller heater however the costs savings would be minimal between a 30k and 45k unit. The deciding factor might be gas supply.

 

[Showkey]

Large garage attached to the house in front range climate .......the garage most likely never drops below 32* on the coldest days with no heat. ( if the garage is insulated)

That’s likely why the contractor quoted a small heater.


My well insulated garage attached to the house in much colder climate almost never freezes. On the coldest prolonged below 0* stretches might get water freeze on the out side walls but never on the floor. Insulation is the key..........

 

[finn]

Best is a subjective requirement. There are several popular “value” products, ie Big Max, etc, that people are perfectly happy with for garage heaters, and a wide range of mor expensive heaters that extend into the commercial market.

Mine are both Beacon Morris, which I think is a house brand, medium priced, made by Sterling and marketed by Menards.

Sterling and Modine both have models sold as light commercial, at a higher price, and I assume all the major manufacturers do the same.

The higher end models have heavier, stainless steel burners and heavier steel construction.

Are you a Mercedes or Kia type of guy and what is your budget?

How important is service, noise, etc.

You should probably talk to a couple of local installers if this isn’t a diy project, and go with what they have experience with.