# How fast will the room heat up?

Posted By EatenByLimestone, Nov 25, 2008 at 3:22 AM

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1. #1

### EatenByLimestone Minister of Fire 2. ```NULL ```

Jul 12, 2006
5,925
683
Loc:
The family cabin needed a new heater. The 150yo potbelly was at the end of it's life. My father and I argued over DV vs vent free for about a year when Lowes had their end of winter discount and had a 20K blue flame unit on sale for \$100. I gave in and said I'd try it. I figured worst case I don't like it, CraigsList it, and I'm probably not out anything. Or maybe I'd heat the shed with it.

So it's now installed in the cabin. The cabin is mostly used in warm weather, but I'd like to use it in the winter too. It's installed in the 16x20 main room. This is the only part of the cabin that would need to be heated. I used the heater in the fall, but it was just quick bursts of heat. It really didn't have to work hard. How could I expect it to react when it's 15 degrees out? How about 0F? The cabin only has a reflective barrier as insulation and wind break, but has Energy Star windows since we replaced them a few years ago.

I've played with the different BTU calculators and the one on this site states I would need between 12,800 and 6400 BTUs. Some of the other sites with calculators figure as much as 28,000 BTUs. I hope the 20K is enough. I assume that the calculator gives the heat required to hold a temperature say 60* higher than outside. So if a room only needs 12,800 to hold a temp, and 20k is pumped in, how fast would the temp rise? Would it take 2 days for the heat to go from 15 to 60? If I wanted it to heat up in a reasonable (couple of hours) time, is there a way to calculate how many more BTUs I'd have to add?

Matt

2. #2

### PaulRicklefs New Member 2. ```NULL ```

Sep 24, 2008
68
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If you are looking for quick recovery you will likely be disappointed with 20k in cold weather with your type of insulation.

Certainly would not recommend vent free. You will have serious moisture build up issues as well as potential life threatening issues. Buy a CO detector if you are using one of them. Vent free is illegal here in Canada.

There are very nice DV heaters out there in the 60k range that would better suit your setup. They will heat up your cabin quickly and safely and most don't require power to run.

3. #3

### EatenByLimestone Minister of Fire 2. ```NULL ```

Jul 12, 2006
5,925
683
Loc:
I fully understand the issues of the vent free. I will swap it out if I don't agree with it's performance. Trust me in that it wasn't pretty going back and forth between the VF and DV systems. If it wasn't for the sale price my father and I would still be going back and forth.

Do you really think I'd need 60K for the small room?!?

Matt

4. #4

### PaulRicklefs New Member 2. ```NULL ```

Sep 24, 2008
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I misread your post. Your only heating 320 sq ft? What about bedrooms and such? If the cabin is 320 your 20k should be enough during cold weather.

5. #5

### EatenByLimestone Minister of Fire 2. ```NULL ```

Jul 12, 2006
5,925
683
Loc:
The cabin is only 2 rooms with a bathroom added on the back. In total the cabin is 16x30 with a 5 foot addition on the back for a bathroom. The cabin is winterized... All water shutoff and drained, etc. There isn't much reason to heat the bedroom in back so I'd have the door closed. In the front room there is the stove, kitchen table, futon, etc... All I would need.

So how fast do you think it could warm up?

Matt

6. #6

### tubbster Member 2. ```NULL ```

Aug 12, 2008
127
0
Loc:
Central NY
Be very careful using vent free when it is cold. Look in the nooks and crannys of your cabin for condensation build up, which, sure as poo will stick to a blanket, will happen. In my house it hit behind books lined up on a wall. New sheet rock.

7. #7

### PaulRicklefs New Member 2. ```NULL ```

Sep 24, 2008
68
0
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That is entirely dependant on building construction and outside temp. Is it 2x4. 2x6? Log? What is the r-value of insulation? Floor insulated? Ceiling insulated and how much? Is the building airtight? Is it windy outside? Too many questions before a firm answer can be given.

All I was saying is that 20,000 btu should be enough heat to be comfortable. It could take anywhere from 1 hour to 20 hours to heat up a space like that depending on all those factors.

Put it this way, I live in one of the coldest areas of north america (regularly -50F in the winter) and 20,000 btu would be just enough for 320 sq ft. in a modestly insulated building (1970's standards 2x4 construction and r-20 in the attic). I regularly size furnaces to 50-75,000 btus for houses of 1100 sq ft.

And by the way "vent free" and "99.9%" is a misnomer as you still need some sort of fresh air to replenish the depleted oxygen supply in the building. Where do you get that? Better keep a window open. There goes your efficiency. I'll say it one more time vent free is dangerous and you will ruin your cabin with moisture. Of course you already know that but it feels good to "vent".. get it? hah Okay time to go..

8. #8

### EatenByLimestone Minister of Fire 2. ```NULL ```

Jul 12, 2006
5,925
683
Loc:
Building construction is 2x4 frame. Outside is plank with a cove on top and rabbit on the bottom. If I remember right it's Dutch something or other? I should know this one... The building is far from airtight. The addition pretty tight, but I won't be heating that. Inside the studs there is an aluminized foil barrier under OSB. No insulation/drywall, etc. The ceiling is sheetrock over 2x4s. No insulation either. I want to put something up there, but haven't rec'd the ok from my father to do it. (Cabin is his generation's now and he is against the idea.) The floor is not insulated since water comes up into the cabin some springs. We've had it 1 1/2 feet deep inside one year. It's windy outside, we are on a peninsula out into the lake.

9. #9

### EatenByLimestone Minister of Fire 2. ```NULL ```

Jul 12, 2006
5,925
683
Loc:
Camp front

http://i167.photobucket.com/albums/u124/EatenByLimestone/CampFront.jpg?t=1227660329

Parking area behind camps drops down to the lake.

http://i167.photobucket.com/albums/u124/EatenByLimestone/camp-truck-bamboo002.jpg?

Looking down lake in front of camp.

http://i167.photobucket.com/albums/u124/EatenByLimestone/Picture001.jpg?t=1227660540

With the old potbelly we didn't have to worry about heat. That stove could pump it out. If I don't go up in winter we still don't have to worry about it. But I want to go up a few times a year during the winter.

Matt

10. #10

### PaulRicklefs New Member 2. ```NULL ```

Sep 24, 2008
68
0
Loc:
I've installed DV heaters in cabins like that and I'll tell you 60,000 btus was just enough. 20,000 btu would be underrated for that install. You would likely not get it up to a comfortable temperature if it were colder than 0-10F. Your BTU heat load at 0F is approximately 22,000 btus. (If it's not windy) So you would be spinning your wheels regardless. Probably looking at days to warm up that space.

Keep in mind most DV heaters are about 75% efficient so you are only truly getting 45,000 btus of usable heat. Your 20,000 is technically 99.9% efficient but you are still only less than half of what you need for recovery, and you still have problems with gassing yourself out.

Nice cabin BTW!

11. #11

### Wet1 Minister of Fire 2. ```NULL ```

Apr 27, 2008
2,529
9
Loc:
USA
Since you obviously already have the 20k gas heater, just go with it and see what it will do for you. The problem will be getting the place up to temp in some reasonable amount of time when the outside temps are really low.

Years ago my family had a similar cabin (although a little larger), all we had was a small wood stove in there. The little stove heated the cabin, but it took a long time to get it warm enough for any level of comfort, and sometimes 24 hours before the place didn't feel like it was radiating coldness. We lived a couple hours away so it wasn't like we could swing by and pre-warm it before living in it. When we did use it, it was often for the weekends. Unfortunately, it took a good portion of the weekend before it became comfortable, and then it was time to leave again. Far from an ideal setup. It actually made it pretty miserable IMO.

I'd good ahead with the little gas heater, but I don't think you'll be happy with the results unless you can go up a day or two ahead of time and start pre-heating the cabin. I think I'd also look into replacing the wood stove (with something more efficient than the old pot-belly). Even if you don't get a new stove, I'm sure you could find something suitable and cheap on CL. This way you could walk into the cabin fire up both the gas and the wood and get the cabin up to a reasonably comfortable temp in a matter of hours instead of days. Besides, you almost have to have a (wood) fire in a cabin...

12. #12

### tubbster Member 2. ```NULL ```

Aug 12, 2008
127
0
Loc:
Central NY
With no insulation, you will have very cold walls. The vent free will make a mess of that in short order.

13. #13

### Wet1 Minister of Fire 2. ```NULL ```

Apr 27, 2008
2,529
9
Loc:
USA
Good point...