RHEEM HOT AIR UNIT

MARK_21

I have a Rheem hot air unit with a direct drive blower and was wondering-which is the best setting for the fan speed-When i put it on high the house heats up fast--when its on low-it takes longer to heat up but the air is warmer coming out of the vents!

Either way i get heat! Thanks for any help! MARK

Kevinj

temp rise

The blower speed needs to be set based on the temperature rise of the furnace.

seabee570

FAN SPEED

If this furnace is a newer 80 percent efficient,or a 90 percent efficicent unit you have to follow the manufacturers guidlines for setting the fan speed. Manufacturers require that temperature readings be taken in the supply duct and the return duct.The furnace should have a nameplate that specifies a TEMPERATURE RISE...if temp rise is 30-60 degrees,..example=if return is 65 and supply temp is 125 degrees,the temp rise is 60 degrees,you are within guidelines,but if your supply reading was 130 degrees,and you moved fan speed to high,and you do not come down to 125 you can damage the furnace,and possibly cause the furnace to crack,or overheat causing a very dangerous condition such as carbon monoxide poisioning...so you adjust the fan speed to maintain the specified temp rise according to the manufacturer,otherwise to just set the fan speed without any readings is dangerous,and would void the warranty

MARK_21

I'M NOT SURE I UNDERSTAND YOU!

When my hot air oil fired unit starts-The fan kicks on in 3 minutes (DOENT MATTER WHAT SPEED ITS SET AT) 3 minutes i have heat-So when you stated i can cause posable damage to my unit--you lost me! Its simple-when the unit kicks on-i wait 3 minutes then the blower fan comes on--my question was-should i have the blower on low speed or high speed..

As i said before--Either low or high speed--the blower comes on 3 minutes after the unit starts...

Thanks for any help.. MARK

MARK_21

HOT AIR UNIT RHEEM

I dont think i'm getting across to anyone--Whats the difference what the temperature rise of the furnace is? The beckett gun fires-heats the exchanger then the limit switch turns the blower on at a set temp-so if the set temp was say 130 degs thats when the blower kicks on--what does that have to do with the RPM of the blower??? It takes 3 mins for the exchanger to reach 130degs-then the blower kicks on--Now its my option what speed i pick..Either high or low blower settings the furnace finishes its full cycle...

If i set the blower to high speed-and its 65 degs in the house-i turn the furnace on-fires up-3 mins i have hot air blowing-until the house reaches the set temp of say 70 degs..

The same is done when i set the blower speed to low..

When the exchanger cools down-the blowers turns off...

Thanks.......... MARK

Unknown

Blower Speed

With a temperature on temperature off fan and limit the fan will come on at whatever temperature you set the fan switch for say 130 degrees.

The blower speed has nothing to do with that. It does however matter as to sensible temperature. Because warm air is less dense it makes sense to run the fan at a low speed so that the air temperature sensed at the register is warm. Run at high speed for more dense cold air for Air Conditioning. A higher speed blower speed will give you a higher air velocity and on start and shut down will perhaps blow cold air.

A warm air system should not have the fan running if the supply duct temperature is below 100 degrees (It is actually 95 but I round it off). This also applies with Hydo-Air units.

Many new furnaces today use a Electronic Fan Timer which is time on/time off control and does not take temperature into account. Those same furnaces find temperature rise a critical factor due to possible damage to the heat exchanger if the fan is not brought on with in 30 to 60 seconds. Most modern furnaces in order to get higher efficencies must hold more heat in the exchanger, or in the case of condensing furnaces they actually recirculate the flue products to extract heat. This will cause the flue gases to condensate. Many of these new furnaces blow cold air when they come on and when they shut off. The solution to this has been DC voltage blowers with variable speed.

Hope this helps.

MikeB34

Temp rise

Tim is right that if the fan limit is set 130 thats when it comes on. The heat rise is really a measure of how fast the heat gets transfered into the home.

If your heat rise is too low, you are moving the air too fast through the heat exch to increase the air temp to a confortable level. It may even turn your fan off until the heat exch heats up enough to warrant the fan. If you have a fan timer, then the fan will run and the air will feel much cooler. Although it will heat the house.

If the heat rise is higher than the rating, then you are not removing the heat fast enough and you will over heat your heat exchanger. The burners will turn off in a cycle, and keep the fan running to dissipate the heat. This will happen regardless of the control you have.

If you are out of the range for heat rise listed on the rating plate, you are no longer using the furnace within the dessign parameters of the unit, and it WILL affect your efficiency. Also it states in most manuals that the fan speed needs to be adjusted for proper heat rise.

This is an installation requirement. If your heat exchanger fails and its because the heat rise is too high, then its no longer a manufacturers defect, but an installation fault and YOU pay for it all. (read warranty null and void)

When reading the inlet and outlet temperature make sure its done out of "line of sight" of the heat exchanger as radiant heat from it will upset your reads.

So...to answer your question "what does heat rise have to do with it?"

Answer = EVERYTHING!

Measure your heat rise and set the fan speed to mantain that temp.

Regards.

Mike B

Unknown

Mark a little formula

for Gas heating Airflow Measurements:

BTUHs (output) = 1.08 x CFM x change in air temperature.

Transposing the formula for CFM

CFM = BTUHs (output) divided by 1.08 x change in air temprature

An example we have a furnace with 120,000 BTU's output (from rating plate on furnace). Our return air temp is 60 degrees. The supply duct temp is 130 degrees difference is 70 degrees. 1.08 x 70 = 75.6. The 120,000 BTU's is divided by 75.6 = CFM 1587

MikeB34

you can

I use 1.085 to average high/mid efficiency, but same formula. You can find what fan speed delivers what CFM you calculate in the owner/installation manual. Use 0.2"wc static if you do NOT have central air, and 0.5"wc static pressure if you do.

Dennis - Reliable Services

Before your head explodes,

let me bring this down to your level (no offence meant).
On your furnace rating plate, the manufacturer will have a temperature rise i.e. 45-65 degrees.
If you put a thermometer in the return (the duct where the filter is closest to) and you read say 67 degrees, then you put the same thermometer in the supply plenum ( the duct on the top of the furnace) and you read 130 degrees.
You would then subtract 67 from 130 and you would get a 63 degree temperature rise across the heat exchanger.
Then everything is right with the world, as the maufacturer does not want that reading to be more than 65 or less than 45 degrees.

But! if that air temperature rise is say above 65 degrees and you are on low speed then your heat exchanger is too hot and in time may be damaged from metal fatigue.
So you would have to put the fan on high speed.

Or if the temperature rise is below 45 degrees, then the heat exchanger is to cool. This could cause water vapor to condense out of the flue products this would be bad as it could rot out your heat exchanger.
You could fool the manufacturer with the cracked heat exchanger, but not with a rotted out one, either way it would be out of the labor warranty.

If you still don't know what we're talking about then don't worry about it, everything is all right.

bob_25

1.08

The 1.08 is the amount of heat transfered by one cubic foot of air per min for 60 min with a temperature difference of one degree. The weight of one cubic foot of standard air is .075 lbs per cubic foot times 60 min times the specific heat of air .24 btu per lb times one degree temp diff equals 1.08 btu per hour. It's just like one gpm with a one degree temp diff equals 498 btu per hour. 8.3 times 60 min times the specific heat of water 1 equals 498. bob

Steve Ebels

The best setting

The best setting for comfort may not be the best for efficiency. The mirror image of that statement is also true. A low fan speed gives nice warm air from the registers but may leave more heat goin up the stack. If you run it on high you have diminished comfort and more noise usually but the efficiency will be a point or two better.
The only thing that really matters from the standpoint of maximum life for your furnace is staying within the manufacturers spec's for temp rise across the heat exchanger. You will find on the data tag or plate a reference to this. It will be stated; temp rise 50-70*F or 45-65*F. It varies with what each maker designs their product to run at. YOU MUST stay with in those parameters. Below the minimum you run the risk of condensation eating your furnace and above it you will find that the furnace burner shuts off on the high temp safety.
If you have the tools to check the incoming and outgoing air temps it's not a difficult thing to do. You may find that the furnace runs OK at any setting.

MikeB34

MARK

If you haven't noticed...everyone here is telling you heat rise is how to set your fan spped. I hope this was helpful to you.

But do it by the heat rise.

MikeB