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# SlantFin Heat Loss Calculation

#1
01-30-09, 09:03 AM
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Join Date: Dec 2008
Location: Washington PA
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SlantFin Heat Loss Calculation

Hello, I have run the Software from SlantFin in an attempt to calculate the boiler size I need. I currently have a 30y/0 Utica (size unknown because the Id plate is illegible). It is oilfired and uses baseboard radiators.
Using a 10 degree outdoor temp and a 70 degree indoor temp and 100 ft altitude, I come up with 75,427 btu heat loss/hr. If I include heating my 24 x 24 unheated attatched garage, it goes up to 77,197. If I change the outdoor temp to zero degrees, with heating the garage it jumps to 91,233.

Here is my question(s):

Which outdoor temp is correct for my area 20 miles south of Pittsburgh?

Once a final Heat Loss number is arrived at, how much less of a boiler size is needed, than the SlantFin Heat Loss Caluclator predicts?

Lastly: I use an electric 80 gal hot water tank now. How would trying to incorporate a hotwater unit into my new boiler system affect things? does the boiler size change? Can I change the boiler sooner, and add hotwater system later?

thanks

Ed

#2
01-30-09, 11:03 AM
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Originally Posted by 84EdH
Which outdoor temp is correct for my area 20 miles south of Pittsburgh?
Contact your local township office. They should know.

Once a final Heat Loss number is arrived at, how much less of a boiler size is needed, than the SlantFin Heat Loss Caluclator predicts?
Probably 3/4 to 2/3rds of what SlantFin says.

Lastly: I use an electric 80 gal hot water tank now. How would trying to incorporate a hotwater unit into my new boiler system affect things? does the boiler size change? Can I change the boiler sooner, and add hotwater system later?
Yup, figure out where the indirect fired DHW tank will connect to the system and put tees in there for connecting it later.

#3
01-30-09, 06:16 PM
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Design temp for Pittsburgh is between 0-4F.

#4
01-30-09, 06:52 PM
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I have a few thoughts that I'd like to share about Manual J... and open for discussion:

SOME oversizing I feel is necessary.

I'm gonna use my home as the example:

SF says I'm at around 65 MBH Heat Loss.

During our recent design temp days of a week or so ago, I took the opportunity to measure the BTUs I burned. I 'back' calculated my heat loss. Came up with around 35-38 MBH at design temp. It was dead calm, and sunny most of the days.

So, theoretically, you might feel safe downsizing to that size boiler, right?

I dunno... here's why...

WHAT IF:

It was WINDY?

Spousey was cold and wanted to increase the t'stat a degree or two?

It got FREAKISHLY cold for an extended period of time?

There would be absolutely NO RESERVE to deal with those scenarios. Your home would slowly lose heat... perhaps to the point of pipes freezing... then yer really screwed.

Perhaps that bit of 'extra' that SF and Manual J 'builds in' to the calculation is intentional?

I agree that proper sizing is mandatory, but is it really proper to cut that close to the bone?

Of course, oil fired boilers don't fire that low, so it's not a problem or issue for me, I would install one that comes in right around what SF says... but what of the gas burning choices?

And how much might one reasonably expect to save by cutting that close? I for one would be more comfortable knowing I can pull out and pass that tractor trailer if I had to.

#5
01-31-09, 06:51 AM
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My actual heat loss is 33-35k at design. SF says 47k. If I could install a 40k boiler, I'd do it.

SF does do a Manual J calculation. Just with a (somewhat) better interface than pencil and paper.

Man J is a sizing calc for boiler and radiation. So it assumes "wind from all four sides at once," as they say. In other words, it is the worst case infiltration and heat loss for each side of the house, so that all the radiation is distributed evenly.

Long freakishly cold will not cool off the building so much that the pipes freeze. The boiler will be running full tilt all the time. You might be undersized for a short period by 5-15k BTU/hr. One could do the calculations and see how much the space temp might drop over a several-day stretch.

Note also the weather pattern at your (Trooper's) house in NJ that leads to design conditions -- big high pressure, clear skies, lots of radiative cooling at night. When it's windy and/or cloudy, there's not as much radiative cooling, and the atmosphere is much better mixed. You just won't see 0F outdoors with a 25 mph wind for extended periods. The probability is very, very low. Low enough that a few hours every five years with an electric space heater would solve any problem (which I think would probably never happen in the first place).

If you have noticeable infiltration, then you have an infiltration problem and should be addressing it anyway.

More important is to have good radiation distribution and sizing so that the structure doesn't have cold spots. And that you can actually use the full output of the boiler. If you have a 65k boiler and only 45k of radiation, you will never see the benefit of the extra 20k in the boiler.

I think the only reasons to oversize a boiler are to meet DHW loads and for (very certain) future additions of square-footage to the building.

#6
01-31-09, 07:50 AM
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thanks for all the feedback. Boy there is a lot more science to this topic than I would have thought. My home has baseboard radiators with 3/4" copper pipe with little fins. Is there a formula to calculate how many linear feet of baseboard equals so much radiant heat?

also:

I would still like to know what amount of additional BTU might be required for DHW if I decided to join the Boiler and DHW systems later.

#7
01-31-09, 09:35 AM
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Ed, if you knew the manufacturer of your fintube, you could know exactly how they rate it... SlantFin FineLine 30 is around 550-600 BTU/FT at 180° ... there's charts on their website with temp ranges and output, etc...

In the program it will tell you how much baseboard you would need based on the specs... scroll down the display page...

For the DHW, you would have to look at the specs for the particular indirect you are planning on using. Those specs are kinda hard to understand... first hour ratings, etc ... but it will give you an idea. USUALLY one would size the boiler for the indirect because they tend to be the greater of the BTU requirements. Going with a smaller boiler than they recommend will slow the RECOVERY TIME, and decrease the CONTINUOUS draw on the DHW.

Xiphias, thanks! You know I was sorta playing devil's advocate, right? Your point about the radiation... so true... but I'm willing to bet that MOST homes are over-radiated so they _could_ use the extra BTUs... mine is! Even at those design temp days, the boiler fired up to a max of 165° before the t'stat satisfied... still lots of headroom.

#8
01-31-09, 09:36 AM
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Oh yea,

First determine the make of your baseboard. Count the number of finns per one inch. Then see the plumbing supply house, and they will tell you the BTU/Ft.@ 180F, with this data, go back to your manual J data, and each room will be divided by this BTU figure, based on the heat loss for the room will give you the number of feet required. If this is done for each room, the heat distribution will be even across the home to maintain a proper comfort level.
Priority switching is something that would be of concern when keepng the boiler as small as possible, so that when showering, if boiler water temps got low, the heat circulators would drop out and prioity would go to the DHW circulator demand, till it recovered. If your boiler was running continuous at 0 degrees, the calculation would have to have been off quite a bit, because the DHW was figured into it this and this spare reserve, because the DHW BTU's were not being used, (unless you were showering at the time) would take care of any needs. Although this would be extremely efficient, standby losses would be almost negligent. This is what everyone shoots for, but but most are apprehensive to do so.
Three section is about the smallest I think. I ran a 4 section Pesotti for 960 SQFT one level for 13 years will efficient results.When I finished my second floor after 13 years... The boiler handled it without a hiccup. I also have a 40 Gal indirect. To do it over I would get the on demand HW wall mounted unit. Only drawback is I would have to burry an LP tank on the property somewhere. I heard they cost nearly the same as an indirect, but never run out of HW. I run the devil out of HW many times with a high volume water head especially after a hard day in the cold and my feet are freezing... It charges up my internal batteries...

#9
01-31-09, 11:37 AM
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Here is a list of ASHRAE outside design temperatures for areas around the country.
Multiple_boiler Installation
I have used these charts for years and never had a problem. Interesting thing was I did a heat loss this week for a guy out west. I changed the outside design from 7 to -5 to see what difference it made. I do not remember the exact number but it was less than 5,000 btu's.

#10
01-31-09, 02:41 PM
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Great attachment rbeck! kudos

#11
01-31-09, 03:14 PM
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A real ASHRAE table. Most excellent. Thanks!

#12
05-11-09, 01:12 PM
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Join Date: May 2009
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Sorry to dig this up a few months on, but I am trying to get SlantFins Hydronic Explorer (which I had just last Nov.) but apparently it has disappeared off their website.

Anyone know where I can get it now, or why I can't ?

Thanks

#13
05-11-09, 01:23 PM
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Join Date: Oct 2006
Location: USA
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I emailed them a while back asking why it wasn't there any more and was told it was because they had some 'problems' with the software... I thought that was silly... yes it has a few 'quirks', but not so bad that it's not useable.

I don't know where it can be had now, sorry.

#14
05-11-09, 03:02 PM
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Join Date: Sep 2006
Location: Connecticut
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It is available on their website but buried in a very inconspicuous place. I used it last week. I'll try to find that link again. I think the issue is SF is trying to incorporate some new functions into the program to handle radiant floor designs.

#15
05-12-09, 10:27 AM
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Location: Connecticut
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Went back to SF and now it looks like the buried link is gone too. If I come across it again I will post. Getting back to question about how to size system from man j calc..... from what I have read the man j computes heat loss for near worst case scenarios. It doesn't add in a safety factor for unknown system inefficiencies or 100 year cold spells. There is a range of opinion as to what safety factor to multiply the heat-loss by... anywhere from 0% to 50%. My own opinion is to use the man j figure as-is. This is because even after a system is installed there are always ways to increase efficiency or to improve the insulation in the building. So over time the system becomes oversized just thru home improvements.

#16
05-12-09, 07:03 PM
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Not so. Manual J heat loss is super-super-super conservative. It is by design a very much higher number than actual. The "safety factor" is built in, and then some. There are fudge factors for everything, and worst case assumptions about infiltration, insulation efficacy, etc. etc.

Nearly all Manual J calcs are 15-30% OVERestimates of the actual heat loss.

Suggestion. Take the Manual J heat loss. Reduce it by 25%. That's your heat loss. Now find a boiler with a DOE output (NOT IBR, which has a 15% fudge built in) that's close to the reduced heat loss. Done. You could even push that a bit further and go with an upper 80s AFUE boiler and size to the gross input.

Design temperatures are by definition achieved about 1% of a typical heating season. In the colder U.S. climates such as the Northeast, a heating season is around 4000-4500 hours long. So in a typical year, you see design conditions for a total of 40-45 HOURS. Those hours will be at night, when most homes are running reduced thermostat temperatures anyway. The space temperature might drop a degree or two. Big deal. Put on a blanket or wear a sweater. The other 99% of the time, the heating need is less. Most of the time, a lot less.

The optimally-sized fixed input boiler is one that is running 100% of the time at design conditions. The rest of the time, it's oversized.

#17
05-12-09, 07:16 PM
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Just to be an additional devil, don't forget about the kids. You know, the ones that hold the back door open going "Mommy, kitty won't come in. Here kitty, here kitty. Mo-om," (while still holding the door wide open in 30* weather, "here kitty... kitty, (which at this point the cat isn't coming near the youngster as it has had it's fair share of being picked up by the tail), "Mo-om, kitty won't come in."

Mom: "Yes dear, please close the door, we aren't heating the neighborhood..."

There is more then one way to throw a heat loss calc out the window. Eerr, or is that out the back door.

Al.

#18
05-12-09, 07:23 PM
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So you'd size a boiler -- a 20+ year investment -- based on a couple times a kid leaves the door open? Smilies or no, that's ridiculous. Partly because kids don't stay little and cats don't live forever, but mostly because the boiler size has essentially nothing to do with the radiative output potential of the heat emitters. You could have a nuke plant in your basement, but if you only have 10 ft of baseboard, it's only going to put out around 6000 BTU/hr at 180F.

#19
05-13-09, 07:02 AM
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Wow, didn't mean for it to be so controversial. There are many other factors involved. As a family having more then one kid. The ages of the kids spread in 10's of years (yes, some people have kids that have graduated college already and their youngest is still in elementary school).

When that cat passes away another usually replaces it.

Most older homes that require boiler replacement have an abundance of radiation. No issue with replacing the heat that went out the door. As long as the boiler can provide it.

An open door is the biggest loss of heat anywhere. Kids (and adults) going in & out all day creates a large loss. Not to mention the kids holding the door open.

Just maybe some of this is why heat loss calc's tend to come in high. I don't recall anyone talking about the heat lost through open doors yet. All the talk has been about the envelope. Which is pierced every time a door is opened.

Al.

{edit: I just noticed that you had posted right before me (8:03 PM with my post at 8:16 PM). Which means that I was typing while you were hitting the post button. I never saw your post until now. It wasn't there when I hit the reply button. }

#20
05-13-09, 04:00 PM
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I bet those transient open doors don't contribute nearly as much as you think Al... it's the slower, constant 24/7 that adds up...