How big is too big?

[ranger098]

I have a 30k BTU Reznor V3 in my oversized 2 car (25'D 27'W 12'T). Tonight i am taking it down and going to buy a bigger Hot Dawg because the 30k just doesn't cut it. The question is, should i go 45K or 60K? It's only a $25 difference in price, but i fear that the bigger one will use up too much oxygen and/or turn on and off too much in my garage. The garage is fully insulated with a ceiling fan.

Anyone have an educated opinion on this? Thanks to all that reply.

 

[CNGsaves]

Too LITTLE Information.

Welcome to GJ, but no way to get proper advice with such skimpy info.

How good is insulation?? Size and # of windows and garage door?? Insulated GD??

In Siberia or Florida?? Now would be a good time to Update GJ Profile.

What were reasons Reznor "could not cut it" as likely it was proper size if garage was fully insulated.

 

[ranger098]

Hi, I did say the garage is fully insulated, meaning insulated GD, walls and ceiling. No windows, just a side entry door which is solid core. Also it's attached to the house so 2 of the walls meet up to the home.

I am in MN, where its currently 0 and is set to be -15 tonight. Yesterday (it was 0 yesterday too) my 30k heater was running for over 1.5 hrs and only got to 40 degrees, even with the ceiling fan running. When i said the 30k "doesnt cut it" i mean it takes about 1.5 hours to warm up the garage to 60 even when its around 20 outside.

One would think 30k would be plenty, but i think my high ceiling and very cold MN temps prevent the reznor from warming up quick enough for me.

The specs on a hot dawg say that a 45k btu has a max mount height of 10', which is lower than my ceiling (12'), which makes me lean toward the 60k. however i do not like the idea of it taking more oxygen from the garage, giving me headaches.

Thanks for the response!

 

[HoosierBuddy]

however i do not like the idea of it taking more oxygen from the garage, giving me headaches.

Thanks for the response!

A vented gas appliance doesn't effect the oxygen level in the garage. Assuming it uses ambient air for combustion (some do, some pull combustion air from outside)...it pulls all the molecules that make up "air" through the burner assembly that air mixed with combustion byproducts is expelled through the flue.

If ambient air is used for combustion, this air must be replaced into the building space "somehow". Generally in a garage, I would think most of that air comes under or around the overhead doors.

My point being...the air in the garage's oxygen content is unaffected as air is "replaced" due to negative pressure in the heated space.

This is NOT true with a ventless heating product by the way. Those heaters have oxygen depletion sensors for the very reason you suggest.

Phil

p.s. To the original question: I'd likely go with the 60K unit. It'll give you quicker recovery if you turn it down when the garage isn't occupied and then turn up the t-stat when you're out there.

 

[CNGsaves]

I just discovered that you had other thread about your Reznor 30K Btu being too small.

Does sound like your tall ceilings and cold climate is hurting your ability to heat. Make every effort to close up any leaks and upgrade like you are planning.

You could run the numbers but since you know exactly how long the 30K took, I'd say go with 60K Btu as sounds like you need it.

If you were closer, I'd buy the 30K unit off your hands ! Good luck with install which won't use up any oxgen in garage. Only question you have is normal heater or seperate combustion which draws in outside air (ie best for shop with paint fumes or lots of dust).

 

[ranger098]

^ That helps a ton, I think ill go big after learning that. Much appreciated

 

[Jagmandave]

Just an educated guess on my part, but if it were me I'd go with the 60K too.....I think you'll be happier with more heat more quickly.

A buddy just installed a 17K BTU heater in his shop and it doesn't do squat except heat the area right in front of it, in my garage the same amount of BTUs warms my shop nicely.

Go big, you'll be happier.

 

[jvitez]

I think 60K will be too big. You sound like you want intermittent heat. However, to be able to heat fast in our current arctic conditions, you'll be wildly oversized during milder temps, which means the heater will shut off too soon, never reach maximum combustion efficiency, and feel colder due to heat stratification, though your ceiling fan will help. So, you'd be sacrificing comfort and efficiency for the rest of the year to allow quick warms during Siberian weather.

Instead of replacing the heater, could you add some electric heat? Use both for quick warm ups then just leave your current NG heater to maintain temps?

 

[Jackfre]

First I wouldn't replace the 30. I'd start it up sooner. You are in these extreme weather conditions about 10-15% of the total heating season and you are going to size for that? Not the best idea, imho. You may have to wear more clothes in these extreme conditions, but on balance the 30 will still do a decent job annually.

 

[toyotadriver]

I would go with the 60k and then set the thermostat to have a 1-2 degree temp swing in each direction. So, if you set the thermostat to 60*, it will come on when it gets to 59* and shut off when it gets to 61*. That can make up for having too large of a heater when it's warmer in your arctic temps by making it run longer when it's running and then it'll be off for longer periods of time. The heater should be sized for the coldest temps you'll potentially get. Your 30k is clearly too small. Another 15k isn't much more and I don't think that it'll be enough. I heat a 30x40x10 shop building with a 80k btu heater and I live in a much warmer climate. My 80k btu works perfectly.

In light of your other thread, you DEFINITELY need the 60k.

 

[toyotadriver]

First I wouldn't replace the 30. I'd start it up sooner. You are in these extreme weather conditions about 10-15% of the total heating season and you are going to size for that? Not the best idea, imho. You may have to wear more clothes in these extreme conditions, but on balance the 30 will still do a decent job annually.

The OP lives in MN. I bet he sees those temps for longer than 10-15% of the heating season.

 

[ranger098]

I just sold the 30k.. even when the temp was in the 30's (quite warm for MN) it took over a half hour to warm up. Sometimes i just dont have the ability (or want) to turn it on early, and around here we see temps 20 and under for about 3 months straight.

Right now i am leaning toward the 60k mainly because quick heat up and recovery times are important to me, and i plan to have this heater for a long time and take it with me to my next home, which i can assure you will have much more than an oversized 2 car! I will heat up the garage and then turn down the therm a couple ticks to prevent it from coming on so often. I may have to play with the therm a little more than a 45k, but i think that is an OK trade for faster warm up times.

Also, has anyone heard that oversizing is more efficient because the heater runs for less time? An HVAC guy told me this, but i didnt really trust that shady dude.

 

[Jimmy_B]

I'm in MN too and have about the same cubic feet to heat. I have a 75k BTU unit for 14 years without too many problems. I replaced the inductor motor this year and a controller 2 years ago. It heats up pretty quickly and recovery is quick after opening an overhead door. I wouldn't trade it for anything smaller.


My stepson does HVAC work in MN. He says the opposite about oversizing from what you've been told. YMMV

 

[Fun pain]

darn... I was going to say get another 30k... too late

 

[Pate]

I have a 32' x 22' x 12' that's very well insulated (including doors) and the 45K works great.

 

[Shop Specialties]

It is easier to maintain temp than to go up and down with big temp swings. Find a happy medium, set the thermostat and leave it.

 

[dave67fd]

It is easier to maintain temp than to go up and down with big temp swings. Find a happy medium, set the thermostat and leave it.

Agreed, My 150,000 BTU house furnace took over an hour to heat my ~1000 sq/ft house from 45 to 70 one year when I shut i't down for vaca.

My 75k Mr Heater will take that long to bring up my 1000 sq/ft garage from 35 to 65. When you have alot of metal/tools/cabinets etc.. it takes awhile to bring all that up to temp.

 

[ChargedUp!]

I'm in Plymouth, MN and have just under 1000sqft with 12'8" ceiling height of attached garage. When I built the house, I was all ready to buy a 100k heater. The HVAC guy said an oversized heater would not be as efficient and sold me on a 45k unit. I keep the garage between 58-60 degrees at all times and if/ when I open the 18x10 door, recovery time is around 15 minutes I would say to get me back to 60.
I'm surprised your 30k didn't work better for you...

 

[meathooker]

im an hvac contractor. i would install the 60k if you were my client


i have a similar sized gargae with 9' ceilings and i have a 60k

 

[brewchief]

The ideal heater would be a multi stage unit, Reznor and others offer a two stage option.

The 30k is sized properly for a garage that is kept warm all the time if it is able to bring it to temp and keep it there. Bringing the temp up from single digits in a short time frame will require a considerably larger unit.

I would go with the 60k.

 

[James-W]

I have a 24X36 garage with 8 foot ceilings and I have a Hot Dawg 60,000 Btu natural gas heater that is vented outside thru the rear wall. My garage is insulated, walls are R-19 and the ceiling is R30. I have one window, an insulated entrance door and an insulated overhead door. Here in southeastern Wisconsin it can get pretty damn cold but the heater keeps the garage at 50 degrees and when I am working out there I turn it up 70 degrees. It doesn't take very long to bring the temperature up, but the actual length of time it takes depends on how cold it is outside. This is the second Winter I have been using this heater and so far it is working very well. I would highly recommend the Hot Dawg heater to anyone who is looking for a heating system to keep their garage warm.

 

[Yankeefarmer]

Go with the 60K. My shop is 29 x 29 x 9' high and I 've heated it 20 years now with a Williams 60k propane wall furnace. It heats up in a reasonable period of time and cycling is not an issue. ( For reference, insulation is fiberglass in 2x4 stud walls and ceiling is 2x6 joists.)

Edited to add I'm on the Mass/Ct border, so it gets cold here, though probably not as cold as where the OP lives.