home heat loss calc sanity check

[1 FMF]

using the taco_hydronics_step_by_step.pdf document online to do a heat loss calculation on my house.
i figured i'd do a rough estimate first and it comes out way low. I thought it would be a worst case scenario since i'm over-estimating. tell me what you think and what i'm missing.

house is colonial-style, a 34' x 24' rectangle inside overall. two living floors, the bottom/basement floor is a half finished basement then utility room then 2-car garage. 8 foot ceilings. i re-sided house couple years ago with all new vinyl energy-efficient windows.

U = 1 / R ( R = rated value of insulation)

infiltration loss (BTUH) = (cubic feet) x (DTD deg F) x (infiltration_factor).
conductance heat loss (BTUH) = area x DTD x U

32' x 24' x 8' = 6144 ft^3 for each floor , 12288 total ft^3
four outside walls per floor total area = 896 ft^2
total outside wall area = 1792 ft^2
total window area = 187 ft^2
front/back door area = 40 ft^2
ceiling/floor area = 768 ft^2

infiltration loss = 12288 * 70 * .3 = 26000 btuh
conductance total wall = 1800 * 70 * .08 = 10000 btuh (didn't subtract window area)
ceiling/floor = 800 * 70 * .1 = 5600 btuh
window loss = 200 * 70 * .4 = 5600 btuh
front/back door = 40 * 70 * .4 = 1120 btuh

sum < 50,000 btuh

is this correct?
that's for calculating based on having inside temp at 70 degrees F with outside at 0.
winter temps are generally 20-40 F.
thinks its worth buying a new boiler since my 20 year old one is rated at 103k btuh d.o.e. ?

 

[1 FMF]

and taco uses 0.05 U-factor for walls as a safety margin, i went higher at 0.08.

taco also says
"Another factor needed for heat loss is the so-called infiltration factor. This is a number that helps us determine the rate of air leakage, and can vary based on how old the house is and how well sealed the structure may be. New construction tends to have rather low infiltration rates, while older homes may leak quite a bit. For our purposes, we’ll use the IBR factors for infiltration: 0.012 for rooms with one outside wall, 0.018 for rooms with two outside walls, and 0.027 for rooms with entries or with three outside walls. By way of explanation, 0.012 represents 2/3rds of an air change per hour, and the BTU’s needed to warm that air up again. Technically, 0.012 is the amount of heat needed to raise 2/3rds of a cubic foot of air one degree."

 

[Donald]

NY has programs where you can get a free energy audit. One of the things they do is a blower door test. You need the info from a blower door test. Maybe CT has such programs.

What kind of fuel?

What is the efficiency of your current boiler?

Is it currently doing its job, heating up the house on a cold winter day/night?

Can you increase the boiler temp?

If you go with a new oil boiler get a Reillo gun. Its significantly better than others.

 

[sleddriver]

As a check, you could do a Manual J load calculation and compare the results. It goes into far more detail, particularly the latest release.

I did this several years ago when I was designing a new duct system for my house. I wanted to see whether the heat pump was over/under-sized and just how much heating/cooling I needed per room and over-all.

I'd also recommend a blower-door test to see how much your home leaks. Good luck with your analysis.

 

[1 FMF]

how do i go about getting the Manual J stuff?
do i have to buy it ?

 

[sleddriver]

Originally Posted By: 1 FMF
how do i go about getting the Manual J stuff?
do i have to buy it ?

No you can borrow it from a library. If they don't have it in stock, put in a request to borrow it from another library. That's what I did.