Figure heat loss

catastrophy

Hi all, just signed up but have been reading here since last fall.

  I recently read in a thread here about figuring your houses heat loss by using degree days and fuel usage.I am having a problem figuring mine and was hoping someone could help me out. I would like to use the results to compare real time heat loss to the numbers I got using the Slant fin heat loss explorer.

Here is the info that I have

Total degree days for my area for Jan was : 1269

Total gas usage(heat and hot water) for Jan was 193 ccf @ .998 btu factor

Total gas usage for hot water in the summer is 21 ccf @ .994 btu factor

My boiler is an old Crane 14 converted to gas burner.

Clocking the burner with gas meter shows 100k bty/hr

Im guessing the effeciency of the boiler @ 60-65%. Might be more or less i dont really know for sure.

I am coming up with a heat loss of 300k btu/hr @ 0 degree outside temps lol  I know that cant be right.

If anyone can show me what I am doing wrong with the math I would appreciate it.

 Thanks Charlie

Gordy

Heatloss with hdd

With those figures I come up with 13526.87 Btus per hdd.

That is total gas usage minus domestic. to do a heat loss you really need to know the NET OUTPUT of the boiler. And deduct the efficiency difference off of the above figure.



 So if it is in deed 65% then your Bus per Hdd move down to 8792 btus per hdd



 Keep some things in mind, and thats the variables 



This is an average for that month, and really not a good indicator for a design day condition.



You really need to know the exact efficiency the boiler is.



Monthly usage readings Whether estimated or actual are always skewed a bit. By that I mean if they miss the estimated reading, they make it up on the actual reading month. So your usage is really not an accurate indicator off the bill.



 The ultimate way is IF you have a digital thermostat that records the amount of time that your system calls for heat in a 24 hour period.  Use that time to calculate your heat loss per HDD using the NET OUTPUT of the boiler. And doing it under a design day condition, or as close as possible.



  I say this because heat loads go up as you get closer to a design day condition. So heat load goes up linear to outdoor temps going down.

 What I'm trying to say is if you get 13000 btus per hdd on a average day that does not mean you can use that number to figure what your heat loss is on a design day.

So this would give you a snap shot at design conditions to compare to an actual heat load calculator using the design day as its basis. This is what you really need to know to properly size a new boiler if that is what you are doing.



 You will find most heatloss calculators are padded with a CYA factor.



Gordy







 



 

catastrophy

heat loss

 thanks Gordy,

 You are rikght, the thing that I dont know for sure is the net output of the boiler. I know the input by clocking the meter but I can only guess at the AFUE of a system this old. and I didnt know about how the heat load and outside temps made a difference.

  I was thinking you could just add all the btus(actual) used that month and muiltiply by the effeciency of the boiler (estimate) and then divide that by the heating degree days to get heat loss per day.

 And then go a little further and divide by 24 hours. and then multiply that by the diff between inside temps (70) and design temps (I used 0) to get heat loss on a design day.

As I said I was looking for a comparison to the heat loss program I tried using which gave me a heat loss of about 45k btu/hr at 0 degree.

I was wanting to repalce the boiler last fall and was wanting a mod/con for the tax rebates. I had some estimates, but after doing some reading on this site and another decided to wait and run this old boiler another year because I didnt like what they were telling me.. They were wanting to put in boilers that were sized between 80-120k btu even though my boiler is only firing at 100k with 60-80% efficiency at best. They said that was ok because they would be modulating. I even had one guy want to put in a 200k boiler because he read the plate on the gas burner said it fired @ 75 to 200k.

 So I am here reading and learning so I can know when i think Ive found the installers for me, maybe next fall. or I might try to install a regular boiler myself.I just dont feel some of these guys have the expertise that I see on these forums.

 Thanks again Charlie

Gordy

Oversizing boilers

 Even a mod/con is not a good idea regardless of modulation. Some times the heat loss does not always fall in to the ultimate boiler size which is usually a really low loss structure.  But you can get boilers that modulate in the 16K to 80K range, some wall mounts 10 k to 50k they like to work up hill even.

 You start over sizing any boiler, and that leads to short cycling which is hard on efficiency, and components of the boiler. Ultimately you would want a mod/con  boiler running flat out on a design day.



What are the emitters for the system?



Gordy

catastrophy

heat emitters

 The heat emitters are cast iron wall hanging units. There are 7 total and 2 tees capped off where I could possibly add 2 more emitters.The radiators are of varying sizes. If I figured it right , I have 250 sq ft edr. That would equal to 48k btu @180 degree water.The way I see it any boiler that puts out more than the 48k is oversized for me  and i dont really trust the results i got from the heat loss program because that would put me right at the max output of my radiators. I know the system kept the house cozy even before the new windows,insulation,screen doors,fanfold,tyvek etc. I have put on in the last year or so. That is why I wanted to try and figure the heat loss in another way.

 The system is a one pipe system with "mono-flo" tees.Its just one loop around the perimeter of the house with inch and quarter main pipe. The way the system runs right now I have about an 8-10 degree temp drop between the suply and return. Its is set up as a warm start system with the aquastat keeping the boiler water at 180.The thermostat is always satisfied long before the aquastat even on the coldest days  so when the circulator is turned off the burner keeps running to get the boiler temp back up to 180. On warmer days when ther is no call for heat i can sometimes still hear the boiler come on to keep the water at temp. I feel as though i am using a lot of gas just to keep the stand by at 180.

I know it sound weird but what I want is to learn enough so that next fall I can tell the contractor exactly what I want and to be able to explain.why I think its the best solution 

Gordy

Tightening the envelope

 When you made improvements to the envelope, you changed everything. So the person using the rating plate to size the new one is a disregard in my opinion. How many sf is your home?



 That's how a lot of new boilers get over sized to begin with.  Thumb ins. Improvements made to the envelope decreases heat loss, and fuel was cheap back in the day the old boiler was installed. No one ever complains about to much heat unless it can't be controlled. Until the days fuel costs are on the rise.  Installers really need to do a heat loss calc.



  Its pretty hard to use your old boiler as a gauge to determine sizing on a new one also.  The old boiler is bang bang technology. Its over sized for the load, and way less efficient.  So how it acts now will be a lot different from how a properly sized new boiler would handle the load in my opinion.



  I would go back through your heat loss program, and make sure your entries are accurate. little errors can make big changes. See what you come up with.

  Use your degree day calcs as a measure also. see what the difference is percentage wise between the two.  I would guess 20% or more.

 Gordy