heat loss calc

Brad White

for heating and cooling calculations. Not familiar with H-22 and only use Manual J for quick checks. Not in my normal practice, not right or wrong.

The fact that your heated hallways are all surrounded by apartments -new information to me- really goes to "what the heat loss is".

Here is the challenge- Yes, the hallways are surrounded by heated space so technically, there is no transmission loss. May even be some gains if the hallway setpoint is lower than the surrounding space. But what about ventilation you mentioned? How about infiltration? Does the hallway open to a vestibule? Is there a smoke venting system with some openings if not some leakage?

It seems as you say that there a positive ventilation system (to keep the fried onion odor inside the apartments and outside the common areas for example.) That air has to be heated to at least a neutral and preferably 2 to 4 degrees above neutral to preclude a drafty feeling. Do you have such loads?

If you are heating 100% outside air, you want to have stainless steel heat exchangers- these will condense. You do NOT want "direct fired" equipment. These can only be used as make-up in the presence of greater exhaust. (Combustion products are mixed into the airstream but are presumed to be diluted and also exhausted positively right away.) CO detectors are required as well as other local code safeties if this is an egress path. Fire dampers, smoke detectors.

Just want to give a general picture of what you are seeking. Get a local engineer on the case to keep things safe. Not that you are not, but I would rather say that than not.

JBW_2

heat loss calc. reference

Hello all,

I am a service contractor in heating.
My client has asked me to determine the cost of operating a hallway heating system for a 20 story apartment building. The apartments are heated by a hydronic system. The hallway heating system is composed of gas fired duct furnaces that operate in four stages.

Can anyone refer me to a book or software to do this type of calculation - degree days etc

Brad White

Short Answer

Gather the following information:

1) Design Heat Loss in BTU's per hour (BTUH).

2) Design temperature difference, indoors to outdoors on which item #1 is based.

3) Degree-Days (annual average over 30 years is common) for the location. Absent that, let me know the location and we can find something close.

4) Presumed efficiency of the equipment/system as a decimal (i.e. 0.75 for 75%).

5) Heat value of fuel unit used. (Gas = 100,000 BTU's per therm so you are good there.)


Formula:

(Heat Loss) x (Degree-Days) x (24 Hours)

(Temp. Diff.) x (Efficiency) x (Fuel Unit Value)


The product of this is usually multiplied by a Cd factor which is between 0.5 and 0.7 depending on how many degree-days in your area. For my area (Boston/Eastern MA at 6,000 Degree-Days, I use 0.60).

Example:

Say you have a 100,000 BTUH heat loss in a 6,000 degree-day climate at a design delta-T of 70F indoors to outdoors. You have a system running at 75% efficiency (which can vary in any month but let's keep it simple/averaged), using gas at 100K per therm.


I get:

(100,000) x (6000) x 24 (Hours)

(70) x (0.75) x (100,000)

From this I get 2,742.86 gross therms. At a 0.60 Cd factor, my net therms would be about 1,646.

Multiply that number by your cost per therm.

My guess is that the hallways will be allowed to get chilly

You do NOT want the long answer, now do you?....

JBW_2

Heat Loss

Brad,

you are the man.
Efficiency, Im assuming is the overall efficiency of the duct furnaces, right?

Thanks a lot for your help.

Josh W

Brad White_2

Efficiency

is more like total system efficiency, the net output available to heat your space after all energy is expended. Old systems could have 50% or less realized annually. 80% is sort of a default for most garden-variety systems. High efficiency will annualize between 85 and 90 percent even though combustion efficiencies are higher when specifically tested.

It does vary across the season with spring and fall showing drops in conventional heating systems and increases in condensing heating systems. It is all really a guess to get you into the ballpark.

JBW_2

heat loss

Thanks Brad.

JBW_2

h-22

hey brad,

just found the H-22 heat loss calc. guide at our shop.
How do you feel about this manual. Would you recommend using something else?
Would you recommend something else for hot air applications?

JBW_2

heat loss

I guess that the confusing part about this problem, is that you are not heating a space that is in direct contact with any outside walls as the hallways being heated are all surrounded by apartments, which are heated by copper finned convectors.

Moreover, you are heating cold air from the outside directly.

Josh W

Ready for the long answer.

Hi Brad,

I have been using this short form calc for comparison of different fuels for different clients (electric, nat gas, oil, hp). It has come in very handy.
I think that I am now ready for the LONG ANSWER. I am at the point where I am ready to invest in software in order to calculate heat loss/gain for my clients. Do you have a software that you would recommend for heat loss calcs? Have you used Manual J, D? We are HVAC contractors, most of my clients are on hot water heating systems.

I appreciate all your help.

Regards,

JBW

Brad White_191

Hey Josh!

Nothing like putting a thread on ice for over a year, huh?

How many folks will be using the software in your firm? That runs up the cost and expected features.

John Siegenthaler has a nice, simple heat loss program he developed which I use occasionally.

Mostly I use Elite's "CHVAC" but there are other Manual J heat loss programs out there. Write me off line if you get stuck.

Brad

Josh W

heat loss calc

Hey Brad,

Rhvac looks pretty good (we do mostly residential). Seems like Siegenthalers stuff is all for in floor radiant heating (hydronic) stuff...

Thanks for the recommendations.

I will be using the demo in the next couple of days.

JBW

Unknown

Hello

Hi Daphne!