Calculation of CFM need

FitterFred

Ruud makes a fully modulating furnace that has been out for several years. Fully mod gas valve and blower. I have one in my own home. Fires between 40 to 100%.

allen_9

CFM

Is there a standard way to calculate CFM need per BTUs? So if I get a load calc. done, how can I determine the amount of CFMs needed.

Empire_2

Allen?

If you get a Load Calc. performed on your structure, It will have the CFM/GPH if it is detailed. Meaning that the load calc. will give you the total heat gain and loss per room. I think this what you are asking... As to a general generic figure,..It can't be done in our professional sense. Every wall, ceiling, floor has it own loss or gain varies IN EVERY STRUCTURE! Perimeter walls, non or poor insulated barriers have an effect on this total value that you are looking for.

Mike T.

Brad White_202

BTUs to CFM

It all depends on your desired leaving air temperature relative to the desired space temperature.

Typically, I use BTUH / (Delta-T x 1.1).

(The "Delta-T" is the difference between entering supply air and room temperature, so if 70 in the room and 110F air, the Delta-T is 40 degrees F.)

The 1.1 is a constant reflecting the specific heat of air at warmer temperatures expressed in pounds of air per hour by the way. When cooling or using cooler air, a figure of between 1.08 and 1.085 is used to reflect the greater density. For heating the number is a wee bit higher but the difference is academic.

If you have your heat loss, great, but are you also cooling (as in a Hydro Air system)? That may dictate the CFM or at least the higher airflow required.

Often I will heat at a lower CFM, perhaps 2/3 of the cooling CFM, but "it all depends".

The best "Back of a Napkin" figuring I can give you is to assume, if a high efficiency system with lower leaving air temperatures, I would use air not lower than 100 to 110 degrees provided it is very well distributed and not blowing right on people.

Say you have a calculated heat loss of 27,500 BTUH and a design outlet temperature of 105 degrees and a room temperature of 70F:

27,500/(105-70 x1.1) = 714 CFM


Using 120 degree air:

27,500/(120-70 x1.1) = 500 CFM


Using 130 degree air:

27,500/(130-70 x1.1) = 417 CFM

You can see, less air if it is warmer, more air if it is cooler.

Jim Hilpipre

CFM calcs

The calculation is as follows:

CFM = BTUH Output/ delta T X's 1.08 at sea level.

For example:

Lets say you had a 60,000 BTUH furnace rated at 95% efficiency and the data plate on the furnace calls for a temperature rise of between 50 and 70 degrees.

You would take 95% of the 60,000 BTUH input and divide by (60 x's 1.08) which would equate to 880 CFM.

I always take the rated temperature rise and calculate in the middle to allow for either lower return air temperatures or a restricted air filter.

Hope that helps.

Jim Hilpipre

allen_9

Very hepful

So do you think if I have a undersized duct work issue, getting a less efficent furnace would help? Currently I have been told we have a 95% eff, 80,000btu, with only about 900cfm. so I am thinking 80% of 80,000 / 60x1.08 give me 987 vs. 1200 with a 95% eff. ?

Brad White_203

To install

a less efficient 20+ year lifespan appliance to suit the existing ductwork is like holding a lightbulb and turning the house. Why would you do that?

With your 80 MBH input and 95% efficiency, 76 MBH is your net output, let's stipulate. If 900 CFM that forces a temperature rise of almost 77 degrees. I think you will have your less efficient furnace right there! Imagine 70F return air and 147F leaving air... your efficiency will drop, but is that what you want?

The higher efficiency furnaces are that way in part because they move more air. More air over the heat exchanger means greater mass flow and heat transfer especially on the "return air entering meets leaving exhaust gasses" approach, not to mention a cooler average heat exchanger temperature which means condensing over more of the range. Leaving air temperatures of 110 to 120 degrees are normal to high, meaning a 40 to 50 degree rise at most. Some units put out 95-105 degree air.

Such a furnace as yours probably does want that 1,200 CFM but also could well deliver 1,500 or more. (Your actual selection and literature will tell the real specific story of course; this is all experiential).

If at all possible, I would add more ductwork, more branches, larger return or another return path, anything to get that airflow up and the leaving temperature down.

allen_9

very good point.

our problem is a mjority of out duct work is in the concrete slab, under the house. there is some duct work going up to the second floor, but we really don't want to add additional duct work up there, it is only the master br and it is plenty warm.

Brad White_203

I take it, Allen

that you have not yet purchased your furnace? If such is the case, do a good, detailed heat loss and take into account the current (hopefully improved) condition, new windows, insulation, etc.

You may find that you need a smaller furnace hence less air. Essentially your house envelope improved to meet your old ductwork capacities.

Now, you may have trouble locating the right size furnace, meaning one small enough to minimize over-sizing.

There was a posting here a month or so ago regarding a fully modulating high efficiency furnace, York I think. New product so the normal caveats apply. That said, if you cannot get the right size furnace, a two-stage one might be a better bet.

Point being, your ductwork may be just fine for your actual needs, you just have to calculate what those needs are.

allen_9

we have not purchased

We have not purchased the furnace we have a 80,000 BTU furnace that is short cycling and I have been told the problem is undersized duckwork. the furnace is about 13 years old, gas, and has had the high limit switch tripped many times, it has been replaces 6 - 7 times, we just bought the house, we have about 900 CFM of duct work I am being told. We live in North Dakota, approx 2100 sq ft. we are in process of a load calc. I have been told a two stage furnace is better for restrictive duct work, but I have also been told that is a myth?

Brad White_203

Two-Stage

Hi Allen

The 2-stage aspect should be qualified on my part. Many two-stage furnaces have just high and low combustion with a constant fan speed. Essentially you get two temperatures of air. Others have adjustable fan speeds, some even variable to suit the loads. (Check out those with ECM motors...)

Point being, there are more features out there than I personally know. I do not do a lot of force hot air design work to be called proficient, just can elaborate on what I have seen lately and by experience.

Contact your best forced hot air folks and see about that (York?) variable capacity modulating furnace.

Then there is always hydro-air... a boiler of appropriate size, an air handler properly sized and with a variable speed ECM motor... leaving you some hot water to direct to a radiator at the entry or bath towel warmer... basically use your duct system properly and heat otherwise directly with HW. Can be very nice.

When you get your heat loss done, let us know.

Bill_111

Calculation of CFM need

Allen,

Was the original system in the house a heating furnace only...no cooling?

If so, don't waste your money until you plan on revising the ductwork as best you possibly can to accommodate cooling.

Bill