has anyone that did a heat loss calculation ever been wrong?

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Old 10-03-08, 01:42 PM
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has anyone that did a heat loss calculation ever been wrong?

I'm not trying to stir the pot, i sincerely want to know if anyone that ever used the slantfin program was wrong and undersized their boiler? I have enough postings on here about my specific situation so i wont clutter this thread with it. but I just want to know if the calculation, assuming i entered the data right, is 100% foolproof.

And how does a layman like me know if i over or undersized the boiler after it's been installed anyhow?
 
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Old 10-03-08, 02:41 PM
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Heat loss calculations are always wrong and especially so when it comes to existing homes. The good thing is they always err on the side of oversizing the heating system.

They are wrong because it is impossible to correctly ascertain so many of the "details" of construction that are so important in determining the heat loss. They have to allow for less-than-perfect workmanship in the insulation, weatherstripping and many other areas so they have built in "fudge factors" that tend to overstate the true heat loss. There are also many other factors that would be difficult to completely incorporate such as the direction and speed of the prevailing winds along with any kind of buffering action from nearby trees or other buildings. There also is a "pick-up" factor to allow for heating the building from the coldest expected outside temperature in a reasonable time.

The bottom line is that the better the figures you have for your particular building the closer you can come to the actual heat loss but you will never be exact.
 
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Old 10-03-08, 03:48 PM
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I thought I made a mistake once...
but, I was wrong.

If the boiler is sized to match the heatloss, when the home is running at design outdoor temp, the boiler will run almost continuously.

If the boiler is oversized, it would tend to short cycle a LOT during the shoulder seasons, and perhaps run reasonably well when running at design temp (if the boiler is oversized by 2X, then it will be running at roughly 50% capacity at design temp, and the length of the cycles is probably acceptable.)
 
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Old 10-03-08, 04:56 PM
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A manual J has about 50% headroom from what we've collectively gathered. I take about 65% of the Manual J now as a good indication.
 
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Old 10-04-08, 11:30 AM
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Originally Posted by NJ Trooper View Post
I thought I made a mistake once...
but, I was wrong.

If the boiler is sized to match the heatloss, when the home is running at design outdoor temp, the boiler will run almost continuously.

If the boiler is oversized, it would tend to short cycle a LOT during the shoulder seasons, and perhaps run reasonably well when running at design temp (if the boiler is oversized by 2X, then it will be running at roughly 50% capacity at design temp, and the length of the cycles is probably acceptable.)
can you dumb down the definition of short cycle with an oversized boiler for me?

and if on the coldest day my current circulator has run 10-11 hours, does that give you any idea if my current unit is correctly sized? Typical other winter days could be 5-8 hours. (i'm using the 'hours' figure on the thermostat)
 
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Old 10-04-08, 11:44 AM
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Originally Posted by Who View Post
A manual J has about 50% headroom from what we've collectively gathered. I take about 65% of the Manual J now as a good indication.
are the slantfin calculations = manual J?

i'm unsure if the terms are interchangeable
 
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Old 10-04-08, 01:20 PM
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Is it really so terrible to oversize? Or is this oversizing thing something to give pros something to seem "pro" about, like a mathemetician with a slide ruler?

What if an older person (especially) who hates freezing is willing to pay for being guaranteed they stay toasty even if normally winters are 15 below, but then once every 10 years there are 2 winters where one winter gets to 25 below and another gets to 45 below?

What if for say a forced air furnace, one can have high btus that allow furnace to put out 180 degree heat where people come over and enjoy sitting on the heat register, with their teeth chattering, from coming in from outside, to warm up and to dry off wet clothes over a chair over the register, as opposed to the place that they only put a 40,000 btu furnace in and the heat output is only 100 degrees and the people say the furnace runs and runs and runs and they feel drafty? Who wants that? Just to satisfy some math whiz? Personally I am willing to pay for heat. Nice hot heat. I hate freezing. Heat is one of the few luxuries I enjoy, and willing to pay for.
 
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Old 10-04-08, 01:20 PM
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Originally Posted by luckydriver View Post
are the slantfin calculations = manual J?
Yes................................
 
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Old 10-04-08, 01:26 PM
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Originally Posted by ecman51` View Post
Is it really so terrible to oversize?
Yes it is. Especially when it is done with so little thought.

That older person is either going to have the furnace or boiler coming off an on far more often reducing its reliability and efficiency or the temperature inside is going to yoyo more.

Nobody here is proposing undersizing, simply rightsizing.
 
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Old 10-04-08, 03:33 PM
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Oversizing a forced air furnace is not comparable to oversizing a boiler. An oversized furnace with ductwork properly sized for the house (but undersized for the furnace) WILL put out a higher temperature airflow and will heat up the old man just coming in from shoveling the snow. An oversized boiler however will simply short cycle unless everything else in the system has also been oversized and then the temperature swings in the house will be unbearable.

Ecman has previously posted how he has an oversized furnace and he likes the ability to stand over the air register and get that blast of 150 degree (or hotter) air on his cold body. I admit that I also have times when that hot, hot blast really feels good but that is NOT a justification for oversizing heating systems and especially not for oversizing boilers.
 
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Old 10-04-08, 05:19 PM
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Originally Posted by furd View Post
An oversized boiler however will simply short cycle unless everything else in the system has also been oversized and then the temperature swings in the house will be unbearable.

.
can you explain in detail what short cycling is?

i'm guessing the temp swings you are talking about is setting the thing at 68..it heats up and gets the house to 72...then wont turn on again until 65?
 
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Old 10-04-08, 05:37 PM
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Short cycling refers to any mechanical device or system that operates in an on/off mode rapidly. Such action is always hard on the equipment.

Specifically with heating equipment the rapid heating and cooling causes stress within the metal parts and can cause rapid deterioration in any refractory (firebrick type materials) within the combustion chamber. Since it takes a few minutes to attain steady-state combustion the efficiency of any fuel-burning appliance will suffer from rapid on/off operation. Igniters, either spark type or hot surface will fail sooner because of too many cycles of operation along with the rapid heating and cooling action.

Most gas-burning furnaces and boilers these days use induced draft blowers that prepurge the combustion chamber to remove any unburnt gases before admitting fuel. Some units also have a postpurge for the same reason. These purges are necessary for safety but they also serve to cool down the combustion chamber and thereby reduce overall efficiency when they come at a rapid pace.

Comfort within the home will also suffer as the area immediately surrounding the heat emitter will get way too hot while the areas further away will never properly heat. This can cause many more problems with both furnishings and the health of the occupants.
 
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Old 10-04-08, 05:42 PM
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so for a mod-con boiler like the Munchkin 140M I had installed a couple years ago ... how many minutes makes up a short cycling situation? 10 minutes before it shuts off? 20 minutes?

I think it may be oversized, but not horribly. Trying to sort things out.
 
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Old 10-04-08, 05:52 PM
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I can't speak authoritatively about any residential boilers because that is not my area of expertise. I will say that a minimum of a ten-minute burn time is probably okay and I would prefer to see a minimum off time of twenty minutes with that ten minute on time.

What would be bad (in my opinion) would be a two minute on time and then a 30 second off time repeating. Under the worst of conditions I would want an off time at least double of the on time.
 
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Old 10-04-08, 06:36 PM
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hmm i wonder what the problem is with my current setup? 3 rooms are always warmer....dining room is where the themostat is and that room and LR and kitchen are nice...bathroom and 2 bedrooms are always cooler. When you walk in there you can feel the difference. Maybe something is wrong with that loop?

just curious if that could be a sizing issue

edit, please click on my profile under biography to see all the numbers i copied down from my current unit..unsure if they mean anything or not.
 

Last edited by luckydriver; 10-04-08 at 07:32 PM.
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Old 10-04-08, 11:09 PM
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If you've got rooms that are cooler than other rooms, it's because you aren't putting as much heat into the room as it's losing. Simple as that.

The reasons that you might not be putting enough heat into the room are several.

The room is under-radiated, meaning you CAN'T put enough heat in.

The baseboards are clogged up with dust, limiting their output.

There is air in the system, limiting flow.

Not enough flow into the heat emitters, or too low a temperature water entering the emitters.

Defective construction, no insulation, leaky windows and doors.

I'm sure there are more ...

This is the reason for room by room heat loss analysis, in order to know how much emitter you need in EACH room, as part of the total. It's all about the BTUs ...
 
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Old 10-05-08, 11:26 AM
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Originally Posted by furd View Post
Oversizing a forced air furnace is not comparable to oversizing a boiler.
Yeah. And actually I knew that. This was my big chance to say how I felt about heat, I guess. I should have saved it for the furnaces forum.

...........................................

Per luckydriver's last post last evening - does his type of system allow for an ability to bleed the lines in those cold rooms to see if that is his cause?

And besides from all that NJ said it could be - it also is possible that since the stat is in the dining room that gets warm, that maybe those immediate rooms are somewhat oversized and heat up the stat (and shuts off) before the cold rooms have a chance to heat up. In such situations where I have ran into this, I have turned down the radiators in those (warm)areas to allow the stat to run longer so the cold rooms heat up, by the time the warm rooms do by the stat.
 
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Old 10-05-08, 02:06 PM
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Yes, it certainly could be that that the rooms that heat up faster have more heat emitted than necessary while the cooler rooms have less than necessary. It could be several other things as well as Trooper has written. The plain fact is that heating a house is a dynamic process and the heat loss calculation is static. Simply having people in one room and not another will skew the heat loss calculation and thermostat location is more an art than science. Merely moving the thermostat a few feet may make all the difference in the world.

In my house (forced air heat) the thermostat was originally placed in the hallway about three feet from the single return air duct. The bedrooms were always hot and the kitchen/family room a bit cool. I moved the thermostat about five feet and just around the corner from the return and now the bedrooms run about one degree cooler and the kitchen/family room is controlled to a high degree of comfort.
 
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Old 10-05-08, 04:22 PM
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well the DR, LR and kitchen all have new windows...the rest of the house doesnt...so if the DR heats up and retains heat/turns off the boiler, then the other rooms dont have a chance to get as warm. BUT i admit i never used a themometer and let it sit in each room for an hour or so to make sure my perception is the reality.

Also my huge CRT RPTV heats up the room in winter so i'm sure that helps me keep cozy and warm
 
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Old 10-05-08, 07:42 PM
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ok since we now know i apparently have a 237000 boiler in here, i guess this is all making sense now. The thermostat is in the dining room, as the crow flies the stat is maybe 25-30 ft from the boiler itself. There are 2 turns to make in a small hallway but i'm sure the boiler room emits a decent amount of heat at 237K right? (laundry dries quick in there if i hang in the winter)

So in theory, i could move the thermostat to the other colder part of the house BUT i dont think i wanna do that because then the 3 main living area rooms would be terribly hot.

I wish i could do zoning but unless one of you wants to crawl under my house and do that, i dont think i'm gonna be able to swing that kind of manual labor

i think the solution is just seal up the area farther from the stat (bedrooms) and since i like it colder in there anyway, just deal with it. as far as the one icy cold bedroom, it's a pool table room now and even if i ever got it all cleaned off, just keeping the door open for that time period would be ok. So that just leaves the bathroom being a tad cooler but i have an electric cube for showers.
 
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Old 10-06-08, 12:36 AM
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I don't remember what kind of heat emitters you have but if they are baseboard convectors you can adjust the dampers to decrease their heat output.

As for going into your crawlspace...buck up kid. If this old and decrepit individual with several physical limitations can do it so can you. I have a few jobs in my crawl space that I want to work on this week. No, it isn't fun but nobody ever guaranteed your life would always be fun.
 
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Old 10-06-08, 05:37 AM
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i dont know what i have either. Taking off the end cap thing i just see the pipe coming out of the floor and into the baseboard. If you look in from the front 1 inch slot, all you can see is what appears to be a protective 'comb like' barrier. It's like a shelf that was bent back inside the baseboard (doesnt touch the rear) and looks like it's there to protect fingers from actually getting to whatever it is below it. Or maybe that's some sort of heat dissipator? I'm pretty sure water doesnt go thru it since it appears to have been molded directly from the front of the baseboard. But i'm not positive on that. Could be a 2nd piece. Tried using macro on my camera to get a peek but pics didnt come out right

oh and i cant even solder a pipe under my sink, no way would i ever attempt anything to do with heating pipes and zoning.
 
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Old 10-06-08, 08:53 AM
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here's part of a pic i had on my work computer..shows what kind of baseboards i have...will try to remember to post close up shots sometime

http://i371.photobucket.com/albums/o...rdsinhouse.jpg
 
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Old 10-07-08, 06:04 AM
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funny thread title, i was wrong!

i recomputed my heat loss after having made a few errors in the original one.

at 10 outside my heat loss is 106K
at 0 outside my heat loss is 125K

so which do you use? And do you add anything for domestic?

i saw a website link from one boilersite they say for Philly area the 'low' was zero. so use zero?
 
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Old 10-07-08, 07:37 AM
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Here are the heat loss figures suggested by the industry.
http://www.comfort-calc.net/outdoor_design_temps.html
It shows philly at 14 degrees. In Harrisburg I have always used 11 and never had a problem.
 
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Old 10-07-08, 08:17 AM
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So the numbers listed for all those cities are the expected low temps? If so, then when sizing is done, is any additional amount (of btus) tacked on, to cover say those days out of the year where it gets maybe 20 degrees colder, or whatever?

My area shows -11.
 
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Old 10-07-08, 08:41 AM
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i'm guessing you are right. I went to accuweather.com and they said avg jan temp was 20 for me. But the site mentioned above stated they use 13.

my choices are between a 128K and 150K unit and even putting slantfin to zero only shows 125K so i'm heavily leaning towards that at this point. But it's a slow and painful process as those here that have helped me will atest to. I'm also still curious if you need to add btu for domestic usage
 
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Old 10-07-08, 10:18 AM
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Originally Posted by rbeck View Post
Here are the heat loss figures suggested by the industry.
http://www.comfort-calc.net/outdoor_design_temps.html
It shows philly at 14 degrees. In Harrisburg I have always used 11 and never had a problem.
after reading your post about HBG and also running into an article you wrote on the contractor website about piping, i just realized you're only 1/2 hour from me. I should hire you to come up and beat sense into my local dealer And properly size and install my new beast, whatever that may be.
 
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Old 10-10-08, 07:45 AM
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Originally Posted by furd View Post
I don't remember what kind of heat emitters you have but if they are baseboard convectors you can adjust the dampers to decrease their heat output.

As for going into your crawlspace...buck up kid. If this old and decrepit individual with several physical limitations can do it so can you. I have a few jobs in my crawl space that I want to work on this week. No, it isn't fun but nobody ever guaranteed your life would always be fun.
question:

i just had mold inspector in. They need to clean the entire (estimated) 3000 sq ft of the crawlspace. Then he will add r30 and vapor barrier.

i do NOT know the layout of my heating pipes but did see some pipes (stuck my head in the window) that could be heating pipes and they are below the joist level

if he's doing all this work on the joists/insulation, but water pipes do hang down below, is insulating them even more important then? or does him doing all that other stuff mean i dont have to insulate them. since he's already down there i'd probably just let him do it..crawling around in puddles of water doesnt excite me all that much.

also now i need to do every rooms heat loss again..i assume adding r30 on the floor will really seal me up!
 
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