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Is Short Cycling an Issue with Gas Garage Heaters?

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  • Is Short Cycling an Issue with Gas Garage Heaters?

    I have a single car garage that I want to heat during what can be a very cold winters here in Canada. I am down to deciding on an MDS30 or MDS45 from Modine which are 30,000 BTU and 45,000 BTU heaters. Since the MDS45 is only $35 more I thought I'd buy it because it would work less hard to heat the space, could be moved to another home with a larger garage in the future, etc. Is this a bad plan because of the short-cycling affect, or does that apply in this situation?
    thanks, Andrew

  • #2
    Re: Is Short Cycling an Issue with Gas Garage Heaters?

    When sizing a garage or a shop heater for a customer, how they intend to use the heater is just as important as the load calcuations. If they are going to leave the thermostat turned down or off, then expect the heater to warm the garage up in a short time so they can work, I'll usually select a heater a little bigger than the load calcs call for to speed up recovery time. If they are going to keep an even heat, or maybe even turn the stat down 5-10 degrees when the garage is unoccoupied, then I'll size real close to the load calcs.

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    • #3
      Re: Is Short Cycling an Issue with Gas Garage Heaters?

      Thanks for your reply.
      I have a single car 26' x 10' garage with a 10 foot ceiling. There is currently R30 in the ceiling and when I am done the walls will be whatever I can do in the 2x4 frame (likely R12) and I will insulate the garage door as well. I want to keep the shop at a temperature that will keep my tools from damage - thinking 50degF but don't know for sure. I don't mind some time to heat up to comfortable working temp, which for me is 65degF. The winter months in Ottawa can get as low as -30degF or worse but more likely in the +20degF to -20degF range.
      So, based on this information would you recommend a 30k or 45k BTU heater? (the output is 20% lower)
      Andrew

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      • #4
        Re: Is Short Cycling an Issue with Gas Garage Heaters?

        Based off the info you have posted I come up with a heat loss of 14,182btu's. 4493 is for the exterior walls at R-13, 669 is for the R-30 ceiling, 4459 is the heat loss through the slab on grade floor with no perimeter insulation, and 4471 through air infilatration which is based off of several averages and is a decent guess. This does not include any heat loss through doors, windows, and any additional infiltration those doors and windows might impose. These numbers are based off of a 65 degree indoor temp, and -13 outside which is the ACCA recommended design condition for your area. If you'd like more info on that check out this thread.

        http://www.ridgidforum.com/forum/showthread.php?t=13277

        If you want to post up window and door types and sizes we could probably get a little closer. It will only take a couple minutes.

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        • #5
          Re: Is Short Cycling an Issue with Gas Garage Heaters?

          I have a one-piece single garage door that I plan to back with rigid foam. Not sure what R value I can achieve.
          I have an average size entrance door that is wood, doesn't seem to be solid.
          And finally a single pain plexi-glass window about 36"x24".

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          • #6
            Re: Is Short Cycling an Issue with Gas Garage Heaters?

            Still not awhole lot of info on the overhead door and that can be a major source of heat loss. Adding that window, a 3'x6'8" wood entrance door, and a R-5 7x9 overhead door took the heat loss up to just under 18,000btu.

            Throw in the dynamic that heat rises and this heater will probably be mounted close to the ceiling, and I'd have no problems oversizing it to the 45k.

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            • #7
              Re: Is Short Cycling an Issue with Gas Garage Heaters?

              The door is a old Stanley one piece steel door with a supporting metal frame-work on the indside. Currently without any insulation, and I'm trying to figure out the best way to insulate it. Thanks for your advice on the heater size. Andrew

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              • #8
                Re: Is Short Cycling an Issue with Gas Garage Heaters?

                I hung some solid foam insulation on my garage door (about 14' x 20'?) and found little difference myself. Since you have 10' ceilings, one thing I would also recommend is ceiling fan(s). I have 14' ceilings in my 3 car garage and put in 2 ceiling fans in the one deep bay where most of my shop is confined. As Ruudacguy said, my heater is mounted close to the ceiling and the fans help to circulate the heat. Before fans my heater circulated about every 10 ~ 15 minutes keeping the shop at 60 degrees. After the 2 fans it cycles about every 30 minutes now. [This is all when it is 10 to 20 degrees F outside or colder].

                JMTCW...
                Still enjoying all 10 fingers!

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                • #9
                  Re: Is Short Cycling an Issue with Gas Garage Heaters?

                  short cycle= lots of money spent on gas. the ceiling fans are a must, plus i allways jumper the fans on the unit to run continuousley to help as well. if you or your wife are in and out allot a tube heater is better due to the fact that it will heat objects not air. this will protect your tools, and it makes the concrete warmer to stand on, with 10' ceilings you should be okay as long as you dont have a tall suv or a truck with a canopy
                  how is it that so many answers are in the instructions

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                  • #10
                    Re: Is Short Cycling an Issue with Gas Garage Heaters?

                    As proplumb stated, short cycling is bad as far as efficiency goes, you will spend more of fuel bills and not be as comfortable (constant heat).

                    Regards,

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                    • #11
                      Re: Is Short Cycling an Issue with Gas Garage Heaters?

                      I understand the short cycling is bad. The question is, am I going to create a short cycling scenario with a 45,000 BTU gas heater in my particular garage? One HVAC expert has already weighed in and said no. I interested in hearing counter arguments.

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                      • #12
                        Re: Is Short Cycling an Issue with Gas Garage Heaters?

                        Being in Ottawa, I dont think you will have a problem if you are using it occassionally.

                        If you are wanting it to maintain a constant desired temp, a 30,000 btu heater would probalby be more efficient. Although I dont see a problem with the 45,000 other than the efficiency.

                        Regards,

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                        • #13
                          Re: Is Short Cycling an Issue with Gas Garage Heaters?

                          The other side of short cycling is having a heater that is too small which runs all the time and never gets the garage up to temp. That'll waste more gas than the 3000 btu difference in loss between the 30 and 45.

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                          • #14
                            Re: Is Short Cycling an Issue with Gas Garage Heaters?

                            Yes I agree. The concern over short cycling is emphasizing one side of the problem. I assume that the location and controls on the thermostat are going to influence things quite bit as well. On your standard thermostat when does it decide to turn on the heater and then turn it off again? Does it turn off as soon as the desired temp is reached without delay? And what temperature controls the turn on time?

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