BTU calculations help please

[SteveGH]

You guys are a wealth of information and I need some help. First a little backround. I'm a 50's homeowner that is handy with my hands but without a lot of heating or hydrology experience. We built our home 10yrs ago in Lakeside, AZ at about 7,000ft with 2x6 well insulated walls, dual pane lowE windows. There is an uninsulated crawl space and the 1st floor covering is part carpet part laminate. There is a great room that is open to the second floor with a wall of those windows. We did it for the view but didn't realize the inefficiency. I'm converting the house from LPG forced air to a Garn WHS1500 with wall panels and baseboards. From your experiences I can tell this will be quite the undertaking but I feel I'm in good company. My first issue is calculating my BTU needs. I used two approaches but come up with different numbers.

We use LPG for heat, DHW and cooking. A friend told me that cooking is a very small part of total usage and I could really forget about that part. I've gone back through past bills, my wife says I'm a geek, to get gallons used over past 7 years. I plugged that info in, days between fills, averaged gallons per day. Then I used a conversion of 1gal LPG = 95,563BTU's and came up with the highest usage of 972 kBTU/day during 2/08. Our furnace is about 90% efficient but then I was also told the BTU conversion doesn't take into account altitude so maybe that's a wash.

Then I found a BTU calculator online at Juggle Frogs, that included factors as room type, north facing, windows, exposure etc. When I measured the rooms and did the calculations I came up with a BTU need of 694k. I don't think we use that much DHW to make up the difference. I tend to look at actual usage as being more accurate, and perhaps use the figures from the calculator as sort of a proportional way of figuring room heat needs. Then I can use that to figure out what size panels or how long baseboards. But then I saw Heaterman's post talking about having a one loop system with each panel/baseboard having a TRV.

But then this brings up another question. I wasn't planning on running pipes to the upstairs figuring I'd be losing heat from the great room. The upstairs has a loft and two bedrooms. If I need to I could run pipes up to the two bedrooms if I need to. In fact I might just preplumb those junctions just in case. That's one of the many great ideas I've picked up from this group.

Am I on the right track with my calculations? Oh by the way we also have a hot tub that is heated electrically now and I want to convert that also and not sure how that heat loss should figure into my calculations.

I've got so many other questions but will take this one step at a time. The good thing is I get a lot of answers by cruizin' this forum. The bad thing is I usually have more questions the more I look. I look forward to learning.

 

[DaveBP]

I don't see why you couldn't get a good estimate of your heating needs from your LPG usage. As long as you're sure you have all your receipts. You could double check with your supplier, they may keep record for years for customer estimated usage/delivery scheduling. Lots of different opinions, of course, but I think all the gas ends up as useful heat except what goes down the drain. And even that keeps the septic tank warmer in winter. The heat of cooking ends up in the room and is space heating, too.

The problem with spread sheet calculators is that air infiltration and solar input can be a bigger factor than the insulation in the walls and there's no free way to get those numbers other than guess. Or hire pros to come in with their blower doors and photocell data loggers. If it were a new house with no heating history it might make sense to pay for their services but I really think you have very good numbers in your LPG heating history. All the fudge factors are accounted for that way.

How you convert BTUs from your gas boiler to the BTUs from any particular wood boiler is probably the key factor. And I will defer to others with more experience on that. Hopefully a Garn Geek has good numbers for you.
You might need a bit more output factor in sizing the wood boiler if your wood is cheaper than your propane (my last delivery here in S. Maine was $4.30/gallon). I think you could end up keeping the house a little warmer if wood is cheaper than what you're used to. And don't forget to add a little for that hot tub.

 

[Como]

As you want a Garn, then I doubt you would be looking at a larger one.

Just a question of how long your storage will last. How many sq ft do you have? I am a bit higher with 5500sqft, much worse insulation and I reckon mine at about 400,000btu best case.

 

[SteveGH]

Sorry, I guess that little tidbit would have helped. Our home is about 2600sqft. Also I double checked my data and found that I made that first mistake for the year. The actual max BTU/day used was 871k during 12/07. Glad I got that mistake out of the way before I started the install phase. But don't know why I'm so far off from Durango. How did you come up with your estimate? I can't imagine two mistakes in one year but yet for half the sqft I'm coming up with over twice the BTU.

 

[Como]

I must change my name, but my location is colder than Silverton.

I am not on my computer and have forgotten the name of programme I used. I put in my current situation and then pur in all the best practice I could install.

My guess is that you could fire once a day in the worst of the winter.

 

[Duetech]

whtmtnbiker,
For your btu calculations I would only calculate the lpg amounts used during the actual heating season for my average. Warmer season consumption is there but the time you need heat the most will be more specifically oriented to your heat loss displacement via fuel consumption.

 

[DBoon]

What is the size, in BTUs, of your current LPG heating appliance? Has it managed to keep the house warm during the coldest days of the year? How old is your current heating appliance and/or do you know the efficiency of it? Have you ever had problems running out of hot water for anything while you are trying to heat the house?

If you existing heating appliance is working fine in heating your house, then you don't need anything larger, but you may be able to use something smaller that is size for more efficient use with solid fuels.

 

[Floydian]

whtmtnbiker,
If your max btu/day are 871k that averages out to just over 36,000 btu/hour over 24 hours. If memory serves the Garn 1500 can store 920,000 btu from 180* to 120* F. Looks to me like one firing a day is reasonable if you size your emitters for 120* water.

I am planning on using all panel radiators and sizing for 120* water. This obviously means more cost in panels up front but thats the cost of being able to fire once a day at design temp. I have also considered sizing my radiators for 120* water based on my average winter temperature knowing I would need more than one firing per day at design temp(0*)which doesn't happen often and usually only lasts for a couple hours in the early, early morning. I am not sure if this is a smart way to size panel radiators but hopefully someone with more knowledge can shed some light on this as it seems pertinent to your situation as well.

Another issue for using lower water temps could be your heat exchanger (HX). Make sure this is sized correctly.

Good luck,
Noah

 

[SteveGH]

DBoon - My existing LPG furnace is 10yrs old, 75kBTU and 90% efficient. It has managed to keep us warm and we have never run out of DHW, although have had to turn the temp up when lots of company arrives.

Noah - I'm still trying to figure out the radiator vs panel thing. The master bath's widest free wall is ~14". I was thinking my wife would like a towel warmer but haven't found one that small. The only thing that might work would be the HeatingEdge or similiar baseboard design. I believe it was Heaterman that promoted the use of a single loop system with one pump and TRV's on all radiators/panels. I can't find that post but wonder if that was what you had in mind.

Happy peep hunting to all!