Heating Requirement Calculation

[Zeroman]

Goodmorning All,

I am trying to calculate a rough estimate of heating requirements so I can size some electric heaters. I’d appreciate it if you could tell me if I am heading in the right direction.

1.
I have calculated how much heat will be lost from the building assuming that the walls are R15 and the ceiling is R20. Using the coldest design day of -34*C (Northern Alberta) I calculated the heat loss using Q1=U*A*deltaT where Q1 is heat loss through walls and roof, U is 1/R and A is cross sectional area.

2.
The building requires 6 A/Cs during emergency conditions (does that make sense, that is what it shows on the existing equipment P&IDs). I used this 6 A/Cs and multiplied it by the volume of the building to determine the CFM required. I then took this CFM and calculated the amount of heat that would have to be added to it to keep it at 5*C from the coldest design temp of -34*C. Q2=CFM*(T2-T1).

Using the quick calculations that I did above would it be sufficient to design an electric heating system? Would you think that I would be off by plus/minus 10% or way off with the assumptions that I made. Using Q=Q1+Q2, I’m hoping that I can just call a vendor and give them my Q (Q+25% to be on the safe side) and order a heater. If I am totally off I’d like to hear your input.

Thank you

 

[MintJulep]

Zero,

Q=UAdeltaT is correct. Need to do this calc for each type of unique construction (walls and roof in your case).

A is surface area. Not sure what you mean by cross sectional area.

Also need to deal with doors and windows, and heat loss through the floor, if appropriate for the space.

You need a constant in your Q2 calculation. 1.04 if T2 and T1 are in degrees F and flow is CFM. Degrees C and CFM are a bastard combination. Make sure you are consistent with your units.

 

[Zeroman]

Thanks MintJulep,

I know what you mean about keeping your units in tact, can be a pain in the a$$, especially with switching back and forth from SI to Metric.

Once I take into account the doors and windows should I be fairly close to having the right heating requirements, so I could just add 25% or so and give that info to the vendor? I don't want to analyze the whole building like I've got a PHD or something, I just want a good estimate.

Thanks again for replying.

 

[lilliput1]

Where are you putting the electric heaters? Will it have a unit mounted thermostat? In addition to transmission losses, include infiltration and floor perimeter heat loss. Is there are central ventilation system with outdoor air that needs heating?

 

[Zeroman]

Electric heater location is inside and they will have unit mounted thermostats. A central ventilation system will be supplying the building (large room) with outdoor air.

Thanks lilliput1 for replying.

 

[IRstuff]

I would recommend that you simply provide the HVAC supplier with the size of the facility, heat sources, percentage of make-up air, etc.

They've already have standard tools for making this calculation.

The more you try to design it for them, the more likely it is that you'll get something wrong, particularly if you have little experience in doing so.



TTFN

 

[Zeroman]

IRStuff,

You are probably right.
But it is always nice to know how they have made their calculations so that you can cross reference your calculations with theirs, even if mine is somewhat uneducated.

 

[IRstuff]

I would try to get at least 5 suppliers and have them explain in detail their calculations.

Based on my own un-scientific survey, 4 out of 5 will give you an overkill system. The remaining one will give you a straight answer. The problem, of course, is that the variance is about 25% between those two answers.

TTFN

 

[walkes]

Zeroman,

Just a quick question for you. When not in an emergency how are you ventilating? Do you have a two-speed fan or a secondary means of ventilation?

The ventilation rate is the only component that the air handling unit manufacturer would need to know. The skin heat loss will be comparatively small.

 

[Zeroman]

Wow, you guys are great.

That makes a lot of sense walkes. That is what my calculations show as well. Thanks !!

 

[quark]

If you go with a recirculating system, then after initial heat up, it is Q2 + fresh air load. You can have savings in operating cost though installation cost remains same.

 

[CESSNA1]

ZEROMAN:

I suggest you do the calculations based on ASHRAE methods. Windows and air infiltration are big sources of heat loss in cold areas. (I am assumming it gets cold, like -34, in Alberta). ASHRAE has a publication that goes through the calculation methods. Their website is

http://www.ashrae.org

Good Luck
Regards
Dave

 

[lilliput1]

Hire an engineer. Do not stick your neck out. You will not want to be sued if you install units that do not work.