2-Stage Forced Air Gas Heating 2+ Car Upper Midwest; anyone using ?

[Junkycosmos]

Hi All,
Thanks for the awesome forums here. I did a few searches but no luck so posting for input here please.

We have a lovely (read original) 2+ car attached garage in upper midwest (as in -10F degree winters). Its 23 feet x 23 feet with 12 foot ceilings. All exterior walls are unfinished, no ceiling but does have a 3:12 pitched roof by way of typical late 1970s trusses. Would like entirely re-build but it is just not in the cards anytime soon and in the meantime it is good space.

So for medium term solution I am looking to install a natural gas heater and then insulate and sheetrock the walls (2x4).

What I am curious about is anyone with roughly the same size setup (500 sq feet @12 foot) is using 2 stage hanging forced air shop heaters. The idea of 2-stage has come up in planning for meeting heating needs now while not insulated yet not short-cycling after we do our best to insulate. Also seems it might be nice for "quicker warm up" when setting the dial up from 45-70f or when the main door is opened on -20 degree days.

Questions; any input is appreciated.
*Brand: Hot Dawg (is the fan loud?), Reznor, ADP, ?

*Sizing: Heat calculators online range at lot! from 30K-125K but of course the insulation is a huge factor here. Few free estimate guys have suggested a 60K BTU would work well for "when its insulated" but be OK now too (suspect?). Anyone else up here in the norther US using gas heater in similar size have feedback?

*Anyone else running 2-stage natural gas equipment? Seems the price diff for most of the "2 stage" units is a bit steep when the only different item is indeed the gas block/valve.

* Sealed combustion chamber (with outside air intake) or not ? While we do not plan anything that dictates the need for SC does seem to pick up some gains by burning outside air as the temps drop.

* Happy to hear any ideas on super insulation with 2x4 construction too..

Thanks for any input!
Junky

 

[BadgerBoilerMN]

Buy or rent temp heater, insulate, do heat load, buy a heater that fits the load. Hint: There are very few gas-fired space heaters made for loads that small. A direct vent tube heater (available in two stage burner) gives you infrared heat with fresh air and no moisture. The are available for residential garages and our second choice behind radiant floors and ceilings.

I like the infrared heaters for comfort and no-blow quiet, but they do create moisture. I use these for temp heat when taking down boilers and other spaced heating equipment to keep the customers and workers warm during construction. You can get a space heater that operates on LP, NG or both.

The only advantage to a HotDawg or any unit heater type is initial cost.

 

[DPelletier]

Ok;

- As the previous poster said, do not get a larger permanent heater for use while your garage is being insulated; you will end up with not enough heat for construction or too large a heater when you're done.

- two stage heaters aren't any more efficient despite any marketing attempts to the contrary (law of conservation of energy - there is no "cold fusion" here!). What they will do is prevent over and undershooting of the temperature set point making for a more comfortable space....but that isn't really necessary in a garage/shop IMO...and then there is the added complexity and expense.

- The only way to properly determine your heating load is to do the calculations, plugging in all the appropriate information. My gut feeling is that your 60,000 BTUH heater is way too big....probably twice as big as you need.

- I don't like ventless gas heaters because they degrade indoor air quality and produce moisture

- I prefer sealed combustion type gas heaters, whether radiant tube or unit heaters that draw combusion air from outside, but even natural draft vented heaters are far superior to the unvented ones....just be careful with negative pressure or exhaust if you are going with natural draft/atmospheric vented appliances.


Bottom line; insulate and vapor barrier using temporary construction heaters then size a proper permanent heater keeping all the above in mind.

2 pennies.
Dave

 

[BadgerBoilerMN]

P is right but, staging heaters is more than just a comfort thing. One of the main reasons you want a proper heat load before choosing your heating equipment is because an over-sized furnace or boiler will cycle more often than a properly sized unit.

Remembering that all space heating equipment is designed for the coldest few days of the year, all other days are by definition not "design" and therefore will not require the output needed in the coldest weather. When a furnace or boiler can't get rid of its full output it simply shuts down, cools down and starts up again.

Condensing boilers modulate flame/output and need to cycle less often. Multi-stage equipment benefits from the same simple idea of closely matching output to load. The more a gas-fired appliance cycles, the more fuel it will use.

More times than not we find equipment grossly over-sized and cycling even on the first stage or off the bottom of a modulating boilers minimum fire rating.

This cycling is exactly the same as highway vs. city driving. The more you stop and start the more fuel you burn and the sooner you get new brakes, starter, etc.

 

[DPelletier]

Yes, I agree with all of the above. Most of our work is institutional and it is common for us to use full modulating burners for many of those reasons.

The only thing I would caution is that many manufacturers suggest that 2 stage or multistage furnaces are more "efficient" and while technically that's true overall (usually because of high efficiency 2 spd motors used in most 2 spd furnaces), the actual AFUE doesn't change and the same amount of gas is used therefore, while overall efficiency is usually slightly better, it is NOT because less gas is used (again, the AFUE is the same). I recommend two stage or multi-stage equipment all the time for comfort reasons and there ARE advantages to a longer run time and less cycling as noted above.

For more dry reading;

http://efficiency.lbl.gov/drupal.fi...s Furnaces_Do They Save Energy_LBNL-59865.pdf

....unfortunately many suppliers and salesmen will simply say "two speed furnaces are more efficient" without explaining (or understanding, in most cases) why and people ASSUME that because they are talking about a gas appliance that the gas efficiency is better....which isn't the case.

I totally agree that it is very important to use the right size appliance and that short cycling is bad for both comfort as well as longevity.
Dave

 

[BadgerBoilerMN]

Right on Dave.

I think the salesman don't know that much about it.

I also think they should continue to push multi-stage burners since most are double size to start with and most will be bumping off low-fire in the shoulder months constituting a net gain in AFUE over an over-sized fixed output...hhhehehehhee