How to figure BTU requirement?

[waksupi]

Since we started taking on the heater situation, one of you guys can surely tell me this. I would need a propane heater, to heat an area of around 600 sq. feet. How many BTU's should I be looking for?

[Steelshooter]

waksupi
It will depend on if it is well insulated, number of windows and doors to get the most efficent heater. What kind of heat are you thinking of using?
Chuck

[felix]

One ton, 12K BTU, should do it nicely if you are not walking on concrete, have steel walls, etc. I assume you live in a well insulated house, like a log cabin? ... felix

[waksupi]

I'd go propane, as I already have the tank. The cabin is a small A frame, very well insulated, double pane windows. When I light off the wood stove, it isn't unusual to have the doors open to keep it comfortable.
I'll take a look at the 12000 size, Felix. No use in getting more than I need. But keep in mind, I have seen minus 47 here before! -20 - 35 below isn't uncommon.

[felix]

Wooden floor? If so, OK. 12K sounds like it would work, but probably would have to be on full time at those colder temps. If you get one that knows how to turn on and off, then get a 1.5 ton, or 2.0 ton if that makes you happy in terms of less noise (if any). ... felix

One more point. If the air is dry, then you'd need the extra tonnage to heat some water. It takes 7 degrees out of your room air to put vapor into it. Any way to pass the air from the heater directly through a constant water fed spounge? Humidifier!... felix

Oh! Forgot. They make an electronic sprayer for normal heater systems. Somewhat expensive, but works well. Actually, I have one on one of my heaters, and set it at 45 percent humidity, as measured as coming into the heater. Make the sprayer shoot into the output air. ... felix

[carpetman]

Ric---Heating and AC is really about the same deal. Same factors--need good insulation and leaks stopped up--unless you want to heat Montana. I went oversize on my AC and found it cheaper to operate than the smaller unit was. Reason is, it comes on and goes off pretty quickly---not running a long time to compensate for the small size. I dont know if you would have restrictions against an unvented space heater. I suspect that will bring all kinds of don't do that comments. I was raised in Texas and in my younger days that's all you saw. Natural gas stove(space heater). Not only that they were supplied by a slip on rubber hose to connect them. (I have heard they are now outlawed). I don't recall one case of someone burning all the oxygen and suffocating---but it can happen. Same with a Coleman lantern in a tent. The sun room I added on is heated by a Ben Franklin looking cast iron stove with artificial logs. It is an unvented space heater. I can open the doors from my sun room into the house and heat the whole house with it. My furnace is vented thus a good portion of the heat goes out the flue. The stove is about 98% efficient. They even make a remote control for it (I don't have that) Don't see the need. Now what would be nice would be a recliner/rocker on a remote controlled trolley so you could move up closer to the fire or back away as needed,but the fire doesn't need to be adjusted all that often.

[MT Gianni]

Ric, it's probably easier to find a heater in the 15-19,000 btu range than a 12,000. I would figure 12,000 btu per hour x effiency rate plus an oversize factor to raise the temperature from a cold start. If your place gets to 32 you need to raise temperatures not just maintain at lowest outside temp. If you use an oversize factor of 10,000 to raise temp you are looking at something in the 25-27k range. The ax-men in Msla used to have the best rates in the state for over the counter sales. PM me if you want a phone call or if you have trouble installing it. Gianni.

[BD]

Ric, I've used a variety of LP space heaters over the years. I would definately go with a direct vented model. 12,000 BTU should be plenty in a small A-frame. Rinnai makes a high end one which is very efficient, has an electronic thermostat and requires 120 V ac to run it's circulation fan. Excellant unit from Japan. Their tankless water heaters are top notch as well. Empire makes a less expensive cabinet model which, (at least on the older ones), used a millivoltic thermostat . Although the empire we owned had a circulation fan which would shut down when the power went out, the heater and it's thermostat would still operate without any electricity and keep the room temp up. Both of these have a double wall vent which goes straight out the back. You must take care to locate this vent, or protect it, so it doesn't get buried in the snow. Intake air comes in the outside pipe and they exhaust through the smaller, center pipe. If the vent gets buried they shut down.

Do not cheap out and get an unvented model. They can have two extreme risks in a small well insulated cabin: CO build up, and O2 depletion. I know of two families in northern Maine who didn't wake up in the morning due to unvented LP space heaters.
BD

[454PB]

My house is a little over three times that size, and well insulated. It has a 130K BTU forced air furnace. My loading room is 12' X 24', and contained at one end of a larger building. I have 16" of blown in insulation in the ceiling and 4" in all 4 walls. This room is electrically heated by a 3000 watt electric, which I believe translates to about 15K BTU. I can see why you don't want a bigger heater than necessary, but it seems it's better to have a little more than a little less. I think MTGianni has the number.

[crazy mark]

C-man,
I thought you had plenty of wool and sheep to keep you warm. Why would you need a propane heater? That block heater in Ric's big truck will probably keep his cabin warm. Mark

[crazy mark]

Ric,
Min 15k to 20K BTU should do fine. I don't think I would go below 15K. 12 K may do it but like was stated above would be a little weak onreally cold days.
I keep 2 teapots with water on my fireplace insert to add humidity. Mark

[carpetman]

Crazy Mark---You have some of it correct--but some of it confused. I don't have propane--I have natural gas. I did have plenty of wood(not wool---that's somebody else---get to that in a minute). I was using wood on my front porch in a cast iron Chiminea,but that was too much work and mess. So I plumbed in natural gas to my front porch and converted the chiminea to gas. I also plumbed gas to my back patio and bought a natural gas Weber Flame outdoor fireplace. I quit smoking in the house and like a little heat when it's cold when I go out to smoke. Now if you recall,Waksupi posted a picture of a woman he was teaching how to checker (maybe play checkers too?--move and I jump you--don't know about that). I heard, to show appreciation for the checkering and maybe checkers she cooked him a fine meal consisting of a ewe he had penned up---which she butchered. Now he has the delima of needing to heat his cabin. Need didnt exist before the ewe got butchered. You are correct his pickup is so big the block heater would heat his house.

[crazy mark]

OK C-man, Just blew soda all over my monitor......

[DLCTEX]

I too grew up with space heaters, but once on a Christmas visit home my wife and I were to sleep in the bunk house, so mother went out and lit the heater. About a half hour later we went out to the bunkhouse and when I opened the door, flames were spread over the concrete floor. As the air from the open door entered, the flames retreated to the heater.
The only damage was a fine layer of soot on everything. We slept in the big house, thankful we hadn't rushed to bed.

[Edward429451]

I do HVAC for a living. They make us do full blown heat loss calcs now but before that the rule of thumb sizing I used is:

30 BTU's per sq ft. (Newly constructed tight house)

35 BTU's per sq ft (Average construction)

40 BTU's per sq ft (Loose construction)

Even with the modern heat loss calcs I still do the ROT just to see where / if it differs. It still works and wont leave you cold. If you really want to do a heat calc PM me and I'll send you a work up sheet and the instructions. Easy but largely unnecessary. Regulations, ugh. This works in Colorado for me where the design temp is 0 deg. You may get colder in Montana and want to adjust slightly.

(ROT for cooling is 16 BTU's per sq ft across the board)

[waksupi]

Keep it coming, guys. Some great information so far, that I have not been able to get anywhere else.

[felix]

Now we are talking money, but correctness. Find one having 3 stages, 12, 18, and 24K, having a user set point for each stage of firing up, based upon temp, and time of day both. Downside? Too much upfront cost. Possibly cheaper to install one small unit, and then add another if required from experience. ... felix

[BD]

I have some experience in the building industry, both commercial and residential. While it's been a while since I built anything the size of Ric's A-frame, I
have built, and even lived in, A-frames in a climate very much like his.

Short-cut "per sq ft" BTU load calcs are all well and good if your're building rectangular boxes. However the basis of any true load calc involves the area of the thermal envelope relative to the volume of the conditioned space contained within as the basis. Think about the shape of an A-frame for a minute. While a simple box with a flat ceiling on the envelope one story high requires about 5.20 sq ft of thermal envelope to enclose one sq ft of living space, the A-frame is using only 4 sq ft of envelope. Add a second floor and the ratios vary even more, In this sense adding loft floor space to the A-frame is nearly "free" relative to the thermal envelope. That is why a standard BTU load calc will will way oversize the heat plant in an A-frame.
BD

[BD]

So Ric, I guess the question is are you including loft space in your 600 sq ft? If that's all ground floor area than 18,000 BTU my be more in line. We heated the top floor of our restuarant in Maine with an Empire DV-25 which is about 17,000 BTU. The floor was 500 square ft. with 7 foot ceilings, built as the firehouse in 1910. It had some blown in insulation in the ceiling but none to speak of in the walls. Stayed warm enough for us. That Empire was about $550 in 1996, made in the US.
BD

[carpetman]

BD---You lost me. Is what you are saying it's the cubic feet not the square feet that you need be concerned with?

[MT Gianni]

The overall square footge of wherever the heat loss occurs. In other word the sq ft of each wall with inuslation factors, the sq ft of the floor and sq ft of the ceiling all factor in as do windows. I used to figure in 8" log walls as r-16. The rinnai is a great unit that used to come in 2 sizes 22,200 btu and 38,800 btu. It does modulate the flame up and down as it approaches the thermostat setting and the smaller one used to be about $1400-$1500. An Empire or Cozy in direct vent should be about 1/2 that at the ax-men or from a Spokane warehouse. Direct vents can be more prone to pilot outages in the wind so try to set it neither into or directly away from the prevailing wind. Gianni.

[felix]

Yeah, you heat and cool cubic feet. Hot and cold tend to layer, and the more wind you have within the space, the more mixing you get and the "harder" it is to manipulate the overall temp, requiring more BTU. Layering air tends to act as insulation. ... felix

[Edward429451]

I'd have to disagree with that Felix. More air circulation would provide a more even temp throughout the house. Less stratification of the air means less fiddling with the t-stat by ones who come out of the cold room / area (hence less gas). And maybe he likes to sit (down) in a chair in his underwear (while the heat rises!). Ceiling fans would even help. Most of the highest efficiency furnaces on the market today have variable speed blowers which are to be set up to run constantly (and ramp up & down) for this reason.

If you got some kind of multi burner unit around 30 or 35K btu's, you could likely plug off the far burner's (from the ignitor) gas orifice and be sized for mild cold, but if the once in 20 yr blizzard rolled in...you'd be set and able to double the out put if called for.

[carpetman]

Every possible variable has been mentioned except the most important one. This is the one realtors use. Location,location,location. Ric your log cabin is uphill from the Pacific ocean and on West side of Continental divide. All you have to do is disassemble the cabin and make it into a roller. Hook that Huge pickup up to it---in fact you might be able to just load the thing in the back of that HUGE pickup. Anyways tow or haul it to the Pacific ocean and reconfigure it into a raft. Start sailing West. When you get to Hawaii keep going. If you make it to the Philippines you went too far,turn around. What you are looking for is Guam. Re-erect the cabin on Guam and you absolutely wont need a furnace ever. Some folks really like to complicate simple things.

[BD]

"Is what you are saying it's the cubic feet not the square feet that you need be concerned with?"

Yes. in a simplistic sense. You have a given volume of space for which you're trying to maintain a temperature differential in comparison to the world outside. The square footage of the envelope needed to enclose that volume of space, and the resistence of the various elements of that envelope to thermal transfer are the basis of any real load calc. Load calcs give you the sort of information that we get from an exterior ballistics table, it's math. Now, take the structure and do a blower door test, and factor that in, and you've got the kind of information you get from actually shooting the gun at the various ranges, it's the real world. The blower door test has always been the bain of those who tout the "R Value" of log structures. They do not generally do too well in air infiltration testing and they get worse over time. It takes a methodical builder to build a tight one, and continual maintenance to keep it that way.

I build houses for clients who tend toward banks of mutiple boilers controlled by a computer program feeding radiant hydronic systems supplimented by tempered air ERV systems. For myself I live in well insulated houses which face south and have no automatic central heat at all. If it gets stuffy I open a window. Within a year or two of my daughter graduating college I think I'll go back to building only the sort of houses I believe should be on this earth, rather than the ones which pay the most, and cost the most, to build; which is what I'm doing now.

I've gone back and forth with my wife over the years about whether we should put in a rinnai or a moniter in our house in Maine. In my view it's really a discussion about whether we want to rent it when we're not there, or sell it to someone who wants bank financing. To date I still put wood in the stove if I'm feeling chilly.
BD

[waksupi]

Well, i already have an Empire in the bath area, as that is where most of the plumbing is located. That was my first object when I put it in, to keep the water from freezing. That room stays pretty nice, but the living area is generally too cool without auxilary help. This place is small, so discounting the room the first heater is in, I have about 3200 cubic feet to heat, if my math is halfway close.