Ruke of Thumb?

Unknown

Unfortunately, too many folks may have and may still use the methods you describe along with other methods such as measuring the basement door and using a boiler that will fit through it! As you and others stated though, there really is NO rule of thumb method. You can have two identical houses on the same street but the heat losses may vary depending on a few things such as who built it, exposure to the sun and elements and construction materials. By the way....here is one of those fellows you just described. I hope his wife and kids didn't get run over!


Glenn Stanton

Manager of Training

Burnham Hydronics

U.S. Boiler Co., Inc.

Ricopp

Rule of Thumb?

Is there a value I can attach to calculating the BTU required per sq. ft. in different styles/ages of houses.
How do you size a circ. for a gravity conversion system?
Edit - found the chart right here - http://www.heatinghelp.com/newsletter.cfm?Id=125

Al Letellier_9

rule o' thumb

Never do this. Every house is different and using a rule of thumb is a shortcut used by those who don't care enough to do it right. Do the heat loss correctly and you'll never have to worry about getting the dreaded " my house won't heat" call.
Using a circulator on a gravity convertion is best done on a pirmary/secondary set up. Use an outdoor reset control, constant circulation on the system side and inject heat from the boiler into the loop.

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hr

Spend the time

to do a room by room heat loss calc. You can download a free one at this site.

And be fair when calculating old homes. If there is no known insulation in the walls or ceilings, enter it in the calc as such.

hot rod

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EdyLogicMstr.

500 BTU's,,

Sq. ft. just like they use to do. But NO. listen to the others. 25-35 is better but without measuring the structure this means nothing. Good heat loss software available at the bottom of this page from our friends at Slant Finn for FREE!
Gravity conversions are fun. We take a huge pipe and well you know.

What the heck is a ruke anyway?

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Brad White_73

Rules of Thumb

I agree with the others that Rules of Thumb are completely inappropriate for calculating a heat loss. Still, I am going to go out on a limb and assume you are looking for "Check Figures" for "sanity checks". Points of comparisons or orders of magnitude. Just remember, everything has a range; areas of glass, quality of insulation if any, air leakage/infiltration can vary widely from a mean.

Larger homes with higher overall SF areas to outside wall area ratios will have lower figures PSF. Conversely, smaller homes have less SF over which to amortize the total heat loss, hence those will be higher PSF figures.

Here is what I use for check figures for detached houses in a climate with design heating days in the single-digits and with indoor temperatures in the 68-72F range:

Older uninsulated Victorians: 60 to 75 BTUH/SF. Conservatories on these can be as high as 100+!

Above but insulated with storm windows, caulked and sealed as best as can be done: 45 to 60 BTUH/SF

Originally partially insulated housing built 1940's to 60's: (Typical ranch type): 35 to 50 BTUH/SF

1940's to 60's with additional insulation (attic usually) and storm windows: 30 to 45 BTUH/SF

Post 1973 housing, fully insulated from the get-go and with thermopane or storm windows: 30 to 45 BTUH/SF

Current best stick-built construction (not SIPS), low-E glass, Tyvek, etc. 25 to 35 BTUH/SF

Modern SIP construction: 15 to 25 BTUH/SF

You can see the range and these are merely to check your totals AFTER you have calculated a detailed heat loss properly.

High glass areas means more framing and less insulation opportunity so beware.

Edward A. (Ed) Carery

Ruke of Thumb heat loss procedure

RUKE OF THUMB PROCEDURE:

STEP 1 Go out to the curb and put your thumb up at arms length, if it covers the entire house, (not including lightning rods and flag poles of course) then you need a little boiler. If you thumb does not cover the house, then proceed to step two.


STEP 2 Go to the middle of the street. Have your wife and children lay down in the road to stop the traffic if it is a busy street. Put your arm out and see if your thumb covers the house. If it does, then you need a medium boiler. If it does not, proceed to step three.


STEP 3 Stand on the side walk completely across street. Be careful not to stand behind a tree or your calculation may be affected. Put out your arm and see if your thumb covers the house from there. If it does, then you need a big boiler.


FOR VERY LARGE OR COMMERICAL BUILDINGS PROCEED TO STEP 4


STEP 4 Have your neighbor across the street open his front and back doors, and you stand on his back porch, looking entirely through his house. Be very careful that you do not accidentally look at his big screen TV showing a picture of the White House, and mistake it for your building. This could greatly affect your calculations. If you can cover it with your thumb from your neighbors back porch, then you need a really big boiler.


This seems very simple to me,,,,,,


BTW, this is just a joke.

I totally agree with all who have posted. Please take the time to do a proper heat loss. It can be free if you care to spend the time to get the program and input the data. Also, converting a gravity HW boiler to forced takes some special talent. Al gave you some good suggestions there.


Best to all

Ed Carey

Ricopp

Thanks everyone

for the info. Most of my jobs are new houses for building contractors (read- the job is mine already) so after being approached to quote, to convert an older home (cast iron) to a modern (Vitodens) system, I was looking for a quick way to size up the boiler with just a few facts (after all there are 3 others pricing the job so who knows if we land it)

Mike T., Swampeast MO

Do your homework! If you haven't done so already, go to the Viessmann US website and download EVERY piece of literature you can find regarding the Vitodens! Study WELL! Most certainly begin with a heat loss calculation. Compare heat loss to DOE Heating Capacity. Resist the urge to oversize! If your heat loss is slightly higher than one model, I'd still stay with the smaller.

Vitodens is not your average boiler. Many system components are proprietary. When you go to look for thermostat connections you'll find that there are none!

Look carefully for modifications that have been made to the system. Things like a zone of fin baseboard or a kickspace heater can significantly impact system design as they will almost certainly require a higher temperature than the iron radiators. Running multiple temperatures adds significantly to material cost.

For the finest comfort and highest efficiency give utmost consideration to TRVs (thermostatic radiator valves) on all of the radiators. With TRVs on all radiators, you may not need the low-loss header and additional circulator(s).

A cleaning/flush of the system is highly desirable. Rhomar makes some good products.

p.s. How are you sizing your new jobs without heat loss calculations?

Tony_23

HA !

p.s. How are you sizing your
new jobs without heat loss calculations?

Most likely the ever reliable supply house method
They're free AND it's never your fault !

Ricopp

heat loss calcs

I use the Wirsbo software for my calcs. because all the stuff I do is infloor heating... Ok there is also some smaller, add or delete rads jobs... you know.
I guess you caught me being lazy, trying to get the pre-job paperwork out of the way quickly, but allow me a few more lines.
When I see an infloor heating job (modern house) I can guess at a rate of 25 BTU/sq. ft. (conservative) and any computerized heat-loss calc. will prove I'm within the limits generaly speaking. So now I go up to a 2 1/2 storey, 80 year old house, and without having to go through every room to measure everything from ceiling heights, window/door size, sq.ft., no. of sections, columns, & heights for every rad. I would like to be able to say hmmm 3400 sq. ft. x 50 BTUH = 170000... after all, the boiler manufactures don't produce boilers with outputs in increments of 1000 btuh anyway, so basically we want a way to quickly determine if we can use the smaller unit or do we need the next size up.
Now if I'da spent as much time on my heat loss calcs as I did trying to type all this up, I'd be further ahead yes

Mike T., Swampeast MO

No need to measure the radiators at least...

You don't have to believe me, but my house is about that size and about 30 years older. Lots and lots of windows/doors and uninsulated basement 50% above grade. Fiberglass in the walls; cellulose in the upstairs ceiling. Original windows with storms. Outside design temp 8°F. Standing radiators with TRVs.

Used the smallest model (6-24) and it's certainly adequate. Unless the insulation/weatherization is awful or the climate is significantly colder it's hard to imagine that you'd need the 15-60 or even the 11-44.

D107

mike you size to doe heat loss? not Net IBR?

or is that just for viessman?

Mike T., Swampeast MO

Long story behind that.

On job I'm referring to (1st Vitodens) with a HVAC-Calc loss of 100 mbh; boiler has a 70 mbh I=B=R rating and DOE heating capacity of 81 mbh.

I know there will never be a problem, but not sure that others are willing to go that low...

Nature of the system with big standing iron rads and TRVs everywhere and driven by a Vitodens certainly seems to contribute.

Even without the TRVs, the system in question is likely rather similar. With a well-conducted Manual-J based heat loss, comparison to the DOE Heating Capacity of a Vitodens would seem generous.

Brad White_9

The debate rages on...

Some of this was also covered in a thread you may find useful if you have not already seen it, David:

http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=37335&mc=5

Ricopp

Hey Mike T.

Did you use a Tstat or do you let it work off the outdoor reset control, and then adjust the slope accordingly?

Mike T., Swampeast MO

No traditional thermostat--TRVs on all of the radiators are the replacement. Remember that the "T" in "TRV" stands for thermostatic so each and every radiator becomes its' own "zone". Multiple radiators in the same room each with their own TRV are NOT a problem. When doing their job, TRVs constantly vary somewhere between "on" and "off" to adjust the radiator output to meet the loss of the space as closely as possible.

Response from the TRVs is ruled by the reset curve (slope). A barely adequate curve gives the best efficiency at the expense of VERY slow reponse if you want to raise temperature. As the curve gets higher, response time gets shorter as efficiency drops. The Vitodens puts the homeowner in complete control of the reset curve, response and efficiency. Curve adjustments are best made in small increments with days in between. Nothing difficult--you just have to pay attention mainly to your built-in thermometer--not ones hanging on the walls...

Unknown

Thanks Ed.

Edit-copy-paste.

That was very well thought out. The White house thing killed me. LOL.

Thanks for the laugh.

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