Slant fin heat loss app

Reply

  #1  
Old 08-14-13, 09:11 AM
Member
Thread Starter
Join Date: Aug 2013
Location: USA
Posts: 44
Received 0 Votes on 0 Posts
Slant fin heat loss app

When using the app or any other method for calculating heat loss, do you add basement /attic info?

Ive done my house the first time thru and came up with 39938
Its a brick 2 story 4 square with replacement windows and blow in. Basement joists are insulated w/ 3.5 inch fiberglass and attic currently has 3.5 inch Superfelt and a couple inches of cellulose blown over.
House is 28 x 28.

Does that seem close?

Boiler I have now is 68000 btu 20KW electric and I'm contemplating going to gas.
 
Sponsored Links
  #2  
Old 08-14-13, 09:26 AM
Member
Join Date: Oct 2008
Location: USA
Posts: 2,882
Received 5 Votes on 5 Posts
Here's what I do for the basement: include the portion of the basement walls that are above grade and maybe within a couple of feet or so just below grade. If the room(s) above the basement are heated, then there is no heat loss there. The 2nd floor ceiling, below the attic, does need to be included in the heat loss. Whatever heat-loss program you use will have directions on the various assumptions to use for each type of wall, window, amount of insulation, etc. - the Slant/Fin program certainly does.

You need to run the numbers yourself rather than just going by the size of the existing electric boiler - or going by some horseback number somebody on a forum might guess at.
 
  #3  
Old 08-14-13, 10:18 AM
Member
Thread Starter
Join Date: Aug 2013
Location: USA
Posts: 44
Received 0 Votes on 0 Posts
Thanks Gil for the reply.
I did run the numbers myself but was having trouble knowing what to do with the basement and attic as the ~40 k heat loss seemed low compared to the 68 k heating system i have now. I just wanted to be sure I wasn't missing a big part of the calculation. I did use the ceiling factor for the 2nd floor and floor factor numbers for the 1st floor. But was leaving out the basement as it is above grade on 2 1/2 sides.

Once I added the basement in the figure jumped to 66349 much closer to the 68 K.
I am going to have the HVAC contractors do their own figuring but I wanted to have MY number so i could see who was blowing smoke when they sized the new unit.
 

Last edited by Frank_S.; 08-14-13 at 10:41 AM.
  #4  
Old 08-14-13, 04:46 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
Hi Frank, I'm on horseback myself right now... and the numbers that you've come up with are not unreasonable for the square footage of your home. Based on a very loose 'rule of thumb', 25BTU/SF could be considered 'typical' heat loss.

If the basement is within the heated envelope of the building, it should be included. If the first floor is insulated from the basement, and the basement can go stone cold, I would not include it. In this case you might want to increase the insulation in the 1st floor joists. If you've got plumbing down there though, beware! You don't want to start freezing pipes!

Boiler is in the basement, correct?

The attic is most definitely NOT included. It's completely unheated, correct?

You should know that 99% of boilers installed in the last century were over-sized for a number of reasons that I won't go into.

As a point of reference, when I moved into this home in 1984 there was a HUGE coal boiler converted to oil burner. I ripped it out before the first winter. I have no idea how many BTU it was capable of providing. When I was young and uneducated, I replaced it with a 145K BTU boiler. Ran that until last winter. Now that I'm more educated I had a handle on my home's heat loss and replaced that monster with the smallest oil-fired boiler I could find. I put in a Burnham MPO-84 (not as small as my home needs, but with oil-fired you can't really go smaller). My heat loss is around 45-50K BTU. The new boiler LOAFED along last winter. My home is a bit larger in square footage than yours. No worries.
 
  #5  
Old 08-14-13, 05:44 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,406
Received 23 Votes on 21 Posts
Hi Frank,
It sounds like there is still some low hanging fruit to get that heat loss even lower, although at current gas prices you do have to consider the cost vs savings. There are often some easy places to air seal that will help the basement.

Bud
 
  #6  
Old 08-14-13, 06:08 PM
Member
Join Date: Oct 2008
Location: USA
Posts: 2,882
Received 5 Votes on 5 Posts
The attic is most definitely NOT included. It's completely unheated, correct?
My comment applied to the heat loss from the ceiling between the 2nd floor and the attic - which most definitely SHOULD be included.
 
  #7  
Old 08-14-13, 06:45 PM
Member
Join Date: Oct 2008
Location: USA
Posts: 2,882
Received 5 Votes on 5 Posts
I'm on horseback myself right now... and the numbers that you've come up with are not unreasonable for the square footage of your home.
But was leaving out the basement as it is above grade on 2 1/2 sides. Once I added the basement in the figure jumped to 66349 much closer to the 68 K.
I think the heat-loss calculation needs to be checked and double checked. With the results bouncing around, there is a bit of uncertainty here. But, with the latest result nearly double, it shows the problem with horseback estimates.
I am going to have the HVAC contractors do their own figuring
You are dealing with more than one contactor, which is good. But, I wouldn't expect any of them to perform a bona-fide heat-loss calculation. The ones that know how to do it probably won't want to invest the time on the off chance they will get the job. Most will just propose a boiler the same size as your existing one. Actually, that might not be too bad, since you may not find a gas boiler small enough, particularly if your first heat-loss number in more nearly correct.
 
  #8  
Old 08-14-13, 07:08 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
My comment applied to the heat loss from the ceiling between the 2nd floor and the attic - which most definitely SHOULD be included.
Yes, of course. I never said it shouldn't... but the attic space itself, of course not.

Most will just propose a boiler the same size as your existing one. Actually, that might not be too bad
I wouldn't think so either. Something in the 60-ish ballpark wouldn't be so grossly oversized as to create any major problems I wouldn't think.
 
  #9  
Old 08-14-13, 07:23 PM
lawrosa's Avatar
Super Moderator
Join Date: Dec 2010
Location: Galivants Ferry SC USA
Posts: 17,809
Received 14 Votes on 12 Posts
I like the slant fins...but there are others out there...

The s 60 would be a good match at 51k D.O.E....

Just for example here...


http://www.slantfin.com/images/stori...entry_s_10.pdf
 
  #10  
Old 08-14-13, 07:45 PM
Member
Thread Starter
Join Date: Aug 2013
Location: USA
Posts: 44
Received 0 Votes on 0 Posts
well yes there are some things to do in the basement but I am concentrating on the attic sealing and insulation right now. The basement is only heated from the presence of the boiler pipes which are uninsulated. Another small project to tackle once I decide about the new system. I also want to zone the system so I can reduce heating the 2nd floor. I keep the radiators almost completely cut off in the winter and its still hot. The current system has 2 gate valves to cut off the left side or the right side of the house but not the upstairs from downstairs.

The windows in the basement definitely need some work sealing around them as does the old wood entrance door to the outside and the sill plate and probably need to strip out old insulation get rid of knob n tube and insulate correctly. It only gas 3 inch fiberglass batts stapled with the kraft paper to the basement side.
Again what a can of worms when you start trying to cut the electric bill...
 
  #11  
Old 08-15-13, 05:24 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 10,406
Received 23 Votes on 21 Posts
LOL "Again what a can of worms when you start trying to cut the electric bill..."

Once you dig into any project it always seems to become several projects. Murphy must have a law for that.

As for residual heat in the basement, it is difficult to account for, but that heat comes directly from your system, so improvements down there that reduce the heat loss to the outside do help. There is also the plus that a warmer basement means warmer floors.

Once your heat loss calc is a reasonable match for your homes performance then you can look at different improvements and see their individual benefits. Whole house improvements like a new furnace are usually left to last for both sizing and cost benefits. The less energy your home requires, the lower the savings for installing a new furnace.

Bud
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: