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Author Topic: BTU to Square or Cubic Feet  (Read 6159 times)
captain ron
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« on: November 28, 2007, 07:22:29 PM »

How many BTU's does a room need to be really efficient?  Or how many feet of radiant heat base board do I need to not be overboard but have a little more than I need? My bath room is really small so I only have room for a 2 ft. section of base board that will be on the same zone as the bedroom. I have a small 7X7 radiator with a 12 volt fan on it I could fit in there also, Do you think it will be needed? I hate a cold bathroom. I have room for about 20 ft. in my living room area. And about 16 ft in the bedroom.  So far the 8 ft. piece I put in the bedroom can keep it at 70 in this below freezing weather.
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« Reply #1 on: November 28, 2007, 07:25:41 PM »

I'm sure Nick can answer that if you give him some specific answers. A buddy of mine is an HVAC guy, and computed my BTU needs by asking everything about my living space...SF, insulation, windows, etc.

Nick?

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« Reply #2 on: November 28, 2007, 08:43:58 PM »

Capt'n,
     What is the output of your Proheat?  Most hydronic baseboard is capable of between 500 and 600 BTU/ft.  So if you are going to put 45,000BTU out of the proheat and you want it in the bus you'll need 75 to 90 feet of baseboard if that's all you use.  I do believe most buses lose far more heat in the front because of windows and many uninsulated areas.  In my system I have 60,000 btu/h of fan coil on a 45,000BTU boiler  With 25,000 BTU of the fan coil just for the driver & copilot.  I have one 7000 BTU fan coil in my 3 foot x 4 foot bathroom and I have the fan on low or it's too hot. But I'm sure you'll be too cold with only 2 feet of baseboard or about 1000 BTU/h  I also have 14,000 BTU of fan coil for the bedroom and they are also on low fan speed.  But the difference between my 14,000 and your 4 or 5,000  makes me think you may want more in the bedroom too.
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Jerry 4107 1120
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captain ron
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« Reply #3 on: November 28, 2007, 08:59:00 PM »

I could go around my bed pedestal and get another 12 feet of base board but if the 8 feet I have now works I'm sure the other 8 footer should be sufficient. But I could be wrong. I got my Aquastat today so I will get it installed this weekend along with some more base board and the radiator/fan unit. I think it is 600 BTU's
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Jerry Liebler
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« Reply #4 on: November 28, 2007, 09:01:51 PM »

Capt'n,
     I just reread your initial post and think you need to either find room for more baseboard or add some fan-coils.  What you say you can fit, 38 feet is only enough to put out 19,000 - 23,000 BTU.  My bus is only 35 feet long, pretty well insulated and my 18,000 BTU heat pump could only hold the bus at 64 inside when it was 30 outside.  This to me says you won't do much better without some more 'radiator' especially in the front.
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Jerry 4107 1120
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« Reply #5 on: November 28, 2007, 09:11:32 PM »

Capt'n,
     I do think you are fine with the 16 foot in the bedroom, The fan coil is more likely 6,000 BTU and that should do real well in the bathroom.  Where you are going to be cold is in the front.  It would help a lot if you could find a couple more fan coils, or one big one like I have, and put them up front.
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Jerry 4107 1120
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captain ron
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« Reply #6 on: November 28, 2007, 09:15:27 PM »

Jerry, the official temperature here now is 33 deg. I now have only the one 8 footer and my bathroom door is open to the bedroom but not the kitchen. My thermostat reads 69 degrees. I would think that the other 8 footer would be comfortable for the bedroom only. I usually keep the bathroom shutoff to both rooms. I can always add more if I need it.
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DavidInWilmNC
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« Reply #7 on: November 28, 2007, 09:16:55 PM »

Why not add some barrier PEX under the floor in the bathroom (and other rooms) and have radiant floor heat?  It sure won't take up any wall space and shouldn't be difficult to install.

David
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captain ron
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« Reply #8 on: November 28, 2007, 09:21:25 PM »

The rad/fan I have is 7X7 and about 3 inches thick, If I remember right the model # was 600 so I just assumed it was the BTU also
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captain ron
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« Reply #9 on: November 28, 2007, 09:29:46 PM »

David, I thought about that but it would not be that helpful as my bedroom is mostly over the tires and rear portion of the bus and not over a lot of bay space. The bathroom has cabinetry on one wall then the tunnel goes through the middle the the toilet and shower takes up the rest so the only floor space is over the tunnel. It would only be efficient if it were installed internally before the finish floor was put in. If I ever replace my front room flooring and bath I would consider putting it down the center.
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« Reply #10 on: November 28, 2007, 09:38:32 PM »

David, I thought about that but it would not be that helpful as my bedroom is mostly over the tires and rear portion of the bus and not over a lot of bay space. The bathroom has cabinetry on one wall then the tunnel goes through the middle the the toilet and shower takes up the rest so the only floor space is over the tunnel. It would only be efficient if it were installed internally before the finish floor was put in. If I ever replace my front room flooring and bath I would consider putting it down the center.

Hmmm, how about in the walls behind the finish wall material, if that's not already done.  Otherwise, toekick units sound like your best bet.
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captain ron
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« Reply #11 on: November 28, 2007, 09:47:34 PM »

My interior is about 90% done and no offense but that is a bad idea. Just think one screw or nail in the wrong place  Angry 
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« Reply #12 on: November 28, 2007, 09:50:10 PM »

IHTH....I have an old 73 Winnie S$S that is 30 feet long.  It is insulated with foam in most of the walls and floor by the factory but I think its only an inch max.  I put shrink plastic on the windows when I use it in the winter.  I have a propane furnace that is rated at 40,000 BTU and it will drive you out of there in 20 minutes from a cold start at 20 degrees ambient.  When in a park I use those 1,500 watt cube heaters with thermostats.  ONE heater will raise the temp in the coach 30 DEGREES.  To me that means I have to be in 46 degree weather to limit myself to 76 degrees inside.  The other cube gives the same result so I guess I am good to 16 degrees using only those little cubes.

The MCI has worse insulation stock than I do and more window area.  The coaches I have seen converted, however, are BETTER insulated than mine.  How much does 1,500 watts work out to in BTU?  I plan to use three of these for heat when parked in a park with utilities but only have two on at any given time. That is a 25 amp load for both.

I may have a built in advantage over most of you.  I want my bedroom kept at the same, approx, temp as the outside.  I don't see anything lower than 20 degrees here and I think I would have to amend that if I were up in Canada or Wy.  The bathroom?  Thats warm as toast.  Point being I think is I would be easier to heat with less.  Oh, and by the way, I have a 10 inch thick Down Comforter that is on the bed 9 months out of the year and the windows are open wide regardless of rain storms and wind.  Like sleeping outside on the porch....I LOVE it.  Wifey has learned to love it also but it was a struggle for her at first.  The dogs?  They are in full mutiny at this time of year and resist comming out from under the covers to even take a walk.  Man's best friend...yeah...sure!

Warmly,

John
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« Reply #13 on: November 29, 2007, 05:47:28 AM »

For reference: Covert watts to BTU/hour multiply by 3.4129. (1500 watts = 5119 BTU/hr)

Since I live in cold country I always design for a good heating system. On two MCI buses (on3 35' and one 40') I used a Webasto 40k boiler. Approximately 30 to 40 feet of radiant baseboard was adequate down to -25 F. I never was in colder temperatures than that so I don't know how low I could have gone. The buses were spray foam insulated and more than half the windows removed.

The radiant fin I used was the standard fin used in houses and was 3/4" copper with 2" square fin rated at 900 BTU/hr.

I only used one zone valve because heat balancing is as simple as laying a piece of paper on top of the fin. Heat output is dependent on convection air circulation through the fin. The more fin you cover, the less heat you get from that baseboard.

Using the conversion numbers above, if you need eight or nine 1500 watt electric heaters to keep a bus warm in +30 F weather, you should close the door.
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« Reply #14 on: November 29, 2007, 07:26:18 AM »


I may have a built in advantage over most of you.  I want my bedroom kept at the same, approx, temp as the outside.  I don't see anything lower than 20 degrees here and I think I would have to amend that if I were up in Canada or Wy.  The bathroom?  Thats warm as toast.  Point being I think is I would be easier to heat with less.  Oh, and by the way, I have a 10 inch thick Down Comforter that is on the bed 9 months out of the year and the windows are open wide regardless of rain storms and wind.  Like sleeping outside on the porch....I LOVE it.  Wifey has learned to love it also but it was a struggle for her at first.  The dogs?  They are in full mutiny at this time of year and resist comming out from under the covers to even take a walk.  Man's best friend...yeah...sure!

Warmly,

John


John, this is one area where I agree with you completely. For many years we have maintained the bedrooom at or near outdoor temperatures. Even when in the bus. It seems as if we never have a cold and are generally very healthy.

The coldest winter we ever spent was in Fulda, MN several years ago. It was getting to -40 at nite and warming up to maybe zero during the day. With the heat off in the bedroom and the window up about six inches it was not uncommon to wake up in the morning with snow on the floor or bed cover. A glass of water on the dresser would freeze overnight.

Richard
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« Reply #15 on: November 29, 2007, 08:16:35 AM »

On my 40 x 102 transit with very large windows, but with 2.25" sprayed insulation, I have one 35,000btu propane furnace that keeps the bus warm down to 25 degrees (coldest I've been in).  It runs for about 5 minutes then off for about 30 minutes (70 degrees- less running time at 65).  If it got really cold, all three of my roof top airs have heat strips, and they do work if you close off the vents to slow down the air flow.
In above freezing weather, two heat cubes work well also.  Good Luck, TomC
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« Reply #16 on: November 29, 2007, 01:44:14 PM »

Hay Capt. Ron,

I have a program the can exactly tell you how many btu's your bus will need to maintain a certain tempature.

It's is a load calculation that we use for customers to recieve rebates from equipment we install for them so we can

prove to the utility companys that we have installed our equipment to their standards. I usually charge $750.00 to the

customer for this service and they now only get about $500.00 back in rebates.  Angry  [No charge for you]

Just give me a call and we can walk through the questions. "there is alot" Tongue

Nick-
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« Reply #17 on: November 29, 2007, 01:47:55 PM »

A related question,,, does anyone know how many BTU's are available from the cooling system on a stock 671 while OTR?
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« Reply #18 on: November 29, 2007, 02:31:26 PM »

Hi guys,

    A while ago on the other board, I posted an HVAC calculation spreadsheet (picture below).  I can make this tool available again, if you email me (address in profile).

Cheers!

-Tim
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« Reply #19 on: November 29, 2007, 06:20:09 PM »

John Z, if you figure out how many gallons per hour of fuel the 4104 is burning on the road and multiply that by 140 KBTU, you will have your BTU into the engine. Roughly 1/3 of the energy is making mechanical energy, another 1/3 is heat out the exhaust and the last third is absorbed by the cooling system.

This means that you can recover a good part of the cooling system part for heating the inside of the coach.

For example, if you run 60 mph and get 10 mpg, you are using about 6 gph, or about 840,000 BTU. One third of that goes to the cooling system, or about 280,000 BTU.

I expect that 1/2 to 3/4 of the cooling system heat ought to be available for heating in moderate weather, or somewhere around 200,000 BTU. You're going to have to go some to get that much heat out of any furnace.

As the outside temperature drops, the amount you can take from the engine will be reduced until you get to the point that the engine will not stay up to temperature. Then, the amount you can take from the engine will really fall.

We would not willingly give up our bus heat.

Tom Caffrey
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« Reply #20 on: November 30, 2007, 05:17:33 AM »

My interior is about 90% done and no offense but that is a bad idea. Just think one screw or nail in the wrong place  Angry 

That would go, I would think, for any wall, floor, or ceiling with wires or pipes.  It's not necessarily a bad idea, but it may just be one that you don't wish to implement.


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« Reply #21 on: November 30, 2007, 07:07:24 PM »

Thanks Tom. I just bought two bus heaters that are rated 80,000 BTU each at max. So i was wondering how well those would work plumbed inline behind the oem heater/defroster. From you info, it sounds like they should work out pretty well. I am going to save your answer into my '04 notebook.  John Z
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« Reply #22 on: November 30, 2007, 09:00:32 PM »

You're welcome, John Z.

If those are school bus type heaters, the main complaint that I remember of them was that they were usually noisy. If you haven't checked out the noise level yet, you might want to try them out before you install them.

For what it's worth.

Tom Caffrey
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captain ron
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« Reply #23 on: December 02, 2007, 05:15:41 PM »

Well I installed 20 feet of base board heat in my living room/kitchen area and It's too warm in here right now. I have to use the bedroom thermostat to control all of my heat until tomorrow so I have to turn it up a little too high so the front Aquastat don't close before it gets warm up front (have them hooked together right now). My dog is not shivering so it must be good Grin
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« Reply #24 on: December 02, 2007, 05:45:08 PM »

My MCI9 is foam insulated with 8 dual pane windows, 1" insulated windshield cover and no floor insulation. 15300 btu is good for 68-70 degrees with outside air temp of 15 degrees. I hope to improve on this a little with some more detailed insulation up front. Jim
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« Reply #25 on: December 02, 2007, 06:10:45 PM »

Hello

Something else to think about...

It takes a lot more heat to warm up a cold coach, than it takes to keep it warm.

If you can design an efficient system for maintaining temp, with accessory ways of bringing big BTU online for those times you find yourself with a cold coach....

Best of both worlds!

Costs/cycling of running an oversized heating unit may be less than adding the big BTU accessories to a smaller system?

Webasto DBW300 (100 000 BTU) plumbed inline to the stock coach heat sure warms up the 8V71 and the coach in an "adequate" length of time in the northern climates!

FWIW, the Webasto has a fuel return to the tank, some minor warming from that?
Stirring around at the minimum.

happy coaching!
buswarrior
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