Space cfm calculation in a residential house

[317069]

Hello,
A house heating load as following (the numbers are only as example):
- Winter outside/inside temperature design : 0F/70F
- Supply air temperature: 120F
- space load (envelop + infiltration): 60000 Btu/hr
- OA ventilation requirement 100 cfm, (7560 Btu/hr)
the question is about air flow calculation:
1- cfm = 60000/(1.08*(120-70)) = 1111.11 cfm
2- cfm = 67560/(1.08*(120-70)) = 1251.11 cfm
which one is much correct.?
Do we consider ventilation load in space cfm calculation.?

 

[trashcanman]

The outside air load is added to the space load. Total load = 60 mbh + 7.56 mbh for a total of 67.56 mbh. The cfm is usually set by the cooling cfm (at the air handler). Assuming house cfm = 1200 cfm, then temperature rise to account for the total heating load = 67560/(1200*1.08) = 52.13 Deg.F. Then supply air = 1200 cfm, outside air = 100 cfm, and return air = 1100 cfm.

Hope that helps.

 

[317069]

Thank you trashcanman, it is helpful, my concern is if we consider the OA loas as a coil load or a zone load, my HAP software take it as a coil load, but another opinion is to consider it as a zone load, ans as you know the cfm depend on zone load, if we take OA as a coil load then why we take the infiltration as a zone load ( both are the same concept, air coming from outside to inside)

 

[lilliput1]

For commercial design of typical office offset infiltration by allowing additional 0.05 CFM per sq Ft for space pressurization. OA CFM is calculated as either that required for ventilation or the sum of exhaust CFM plus the 0.05 CFM/SF pressurization, whichever is maximum. No penalty is taken for infiltration unless the space is a vestibule or entrance lobby. For these spaces door infiltration is taken into account or typical air change rates to estimate infiltration.
Trashcanman is correct in that for residential use the cooling cfm determines the required fan capacity of about 400 CFM/ton. The fan rpm is reduced during heating so discharge air temperature is about 105 to 120F.
But for your equation question you have to solve for CFM SA Htg as follows:

Q = 60,000 + 7560 = (120 - T mix) x 1.08 x CFM SA Htg

Where T mix = ((100 x 0) + (CFM SA - 100) x 70)/CFM SA Htg

Solving both equation simultaneously, you get CFM SA Htg = 1,111.11

 

[317069]

Thank you lilliput1, but the question is: where did mixing happen? does OA mix with room air in the room space or right before the furnace? the idea of the post is weather to include the OA load in space load or coil load?

 

[lilliput1]

I was thinking of commercial AHU where OA is ducted to the unit. But for residential units OA is not ducted. It just leaks in the house. So case 2 of your calculation will apply. You don't need to be this precise w/ residential unit. Just make sure the heating capacity is more than your requirement and the thermostat will just turn heating on/off as required. Follow manufacturer's instruction on how to wire the fan for heating to give the capacity required.

 

[HiDrPo]

Hap is one of the original load programs and can be used for residential calculations if you know how to do residential loads. Look up "Manual J" for info specific to residential.

To answer your question, outdoor air ducted to the furnace mixes before entering and the resulting mixed volume must be heated to offset space heat losses. Outdoor air at the space level is infiltration leaking through windows and doors and the heat loss must be added to the space transmission loads to determine temperature/volume of supply required to maintain comfort conditions.

I suggest you find a circa 1985 ASHRAE fundamentals or similar manual load calculation manual via google search and perform a complete pencil and paper load calculation. Yea, it is tedious, but you will be able to junk in and not live with junk out of the programs thereafter.

 

[317069]

Hid, don't go to far, before you suggest anything, my post was about adding ventilation load to the space level or to the equipment level, for your information, HRAI, and CSA F280 add ventilation load to the space level. most existing houses do not have outdoor air ducts, recently they start to use HRV because of the local code requirement. did you get my point.