Temperature Rise vs. Temperature Drop in forced air gas heating

furnaceNewby

The spec plate on my existing furnace (circa 1988) lists a temperature rise of 65F-95F. New furnaces being offered to me are specified with 25F-55F, or perhaps 35F-65F temperature rise. Using a thermometer I have measured the input (return) air and the output (supply) air in the main ducts near my furnace. I read around 140F air in the supply, and 65F air in the return, giving me a 75F rise across the furnace.

Next I measured the air temperature coming out of some of the farther vents in my system. I have a main 17x8 duct that runs about 16 feet, then a 5" duct (maybe 4") that runs 33 feet (through a crawl space) to the far master bath. At this register I measure 80F or 90F air temp, depending on how long the furnace operates in a given cycle (usually 10 mins). This is at least a 50F temperature fall, and this is where I am concerned.

Are new furnaces (with lower temperature rise specs) designed for systems that have much lower temperature drops across the system? i.e. the temperature drop of 50F for the far registers in my system would obliterate the rise of the furnace being 55F, and I might only get 60-70F air at those far registers.

Do newer systems have limits on the temperature drop to the (farthest) registers?

icesailor

The design of the furnace is fine.

Its the design of the system that is the problem.

furnaceNewby

Thanks!

What temperature drop should my system be capable of?

Is there a particular air temperature that I should shoot for at the registers?

icesailor

More experienced air heads can help you with your issues.

furnaceNewby

Thanks.

In my unfinished basement where most of this ducting runs, the ambient temperature might be around 60-65F.

Is it correct to say that when the air sent into the system is 140F, the registers in the rooms will see a much larger temperature drop versus when the air sent into the system is 110F?

Right now i have about a 60F temperature drop across the ducting that runs between my existing furnace and the farthest register. If I can get that drop down to say, 30F with my existing furnace, then when I install a new furnace that puts lower temperature air into the system, would I see even less of a temperature drop?

Harvey Ramer

The temp rise you have across the furnace indicates 1 or all of the following problems;

Reduced air flow due to,
A, Undersized ducting
B, Dirty fan
C, Blocked or restricted filters
D, Blocked or restricted HX
E, Blocked or restricted AC coil

Or oversized furnace.

Verify or rule out what you can from the above list then have someone check the External Static Pressure and compare it to the unit's blower ratings.

icesailor

Harvey Ramer said:

The temp rise you have across the furnace indicates 1 or all of the following problems;

Reduced air flow due to,
A, Undersized ducting
B, Dirty fan
C, Blocked or restricted filters
D, Blocked or restricted HX
E, Blocked or restricted AC coil

Or oversized furnace.

Verify or rule out what you can from the above list then have someone check the External Static Pressure and compare it to the unit's blower ratings.

F, Uninsulated ducts. The biggest theft of heat in a WA system. Unless you want to heat the crawl space to conditioned space temperatures.

If you go outside on a cold day, you wear a coat.

Jack

Uninsulated duct is only a part of the problem. If it is not insulated it is probably not sealed either. the DOE estimates that the average US home looses between 18-42% of its energy in duct leakage. Crawl spaces are such fun. That run and the others should be sealed with an approved mastic (DO NOT use Duct tape. It is good for everything other than what it is named for). The contractor will likely want to replace that run with flex duct. It is pre-insulated, but you end up with a bit less air flow. There is blanket wrap that could be installed on the current metal duct.

EBEBRATT-Ed

All duct work should be insulated and sealed. the temperature rise should be measured at the furnace because that's the temperature the heat Exchanger sees. The "new" condensing furnaces usually would like to see a 50-55 temp. rise. You may need to change some ductwork to get the airflow you need

icesailor

Jack said:

Uninsulated duct is only a part of the problem. If it is not insulated it is probably not sealed either. the DOE estimates that the average US home looses between 18-42% of its energy in duct leakage. Crawl spaces are such fun. That run and the others should be sealed with an approved mastic (DO NOT use Duct tape. It is good for everything other than what it is named for). The contractor will likely want to replace that run with flex duct. It is pre-insulated, but you end up with a bit less air flow. There is blanket wrap that could be installed on the current metal duct.

I mentioned insulating ductwork because it wasn't on the list.

JUGHNE

If I understood you right you have 33' of 4" or 5" round pipe in your crawl space. Just shooting from the hip but that length sounds like it is too far for 4-5-or even 6" pipe. Do you have more heat runs that are small (4-5") and more than about 10' long? Ducting round runs sound pretty small. The 17 x 8 sounds right but it should go farther to the end of the house with 1 or 2 reducers along the way.

Note: there is a new discussion on the wall concerning the difference between PRICE and COST. Well worth the reading; IMHO. It is posted in the Strictly Steam section but applies to all purchases.

icesailor

The smaller a round pipe is, the greater surface area of the pipe to whatever is flowing inside. A 1/2" water pipe has a greater surface area ratio of surface to fluid than a 3/4" pipe. Therefore, the 1/2" pipe not only carries less water/energy than the 3/4" pipe, it looses more heat energy.

ProblemSolver

That S/A run should never be 30 feet long, nor should any S/A run be 30 feet long. That is way too much resistance for air to travel in a low pressure duct system. The first thing to do is seal all your duct seams. The law of physics says, high pressure goes to low pressure - every time. Inside the ducts the pressure is higher than outside the ducts. So the higher pressured air inside the ducts have no choice but to find every available exit it can to go to the lower pressured air outside the ducts. Once everything is sealed, then the volume of air you get at the registers increases. If you have no choice with that 30 foot run, but to keep it 30 feet; then what you can do is increase its pipe size. If that run is 5 inch pipe, then cut in a 6 inch pipe at the trunk-line, and run the 6 inch pipe 20 feet then reduce down to 5 inch pipe for the last 10 feet.
I had told you in a earlier post that the blower cannot push anymore air than what it can pull. Your high temperature rise is mostly due to the return air side of your system being undersized, or any of the suggestions that Harvey Ramer gave you.
Very important: First you need to know if the furnace is correctly sized for the size of your home and for the size of your duct system and number of S/A openings. If it is, then any problems you may have with air flow or room temperatures, has nothing to do with the equipment; air flow and room temperature problems are 100% due to a poorly designed duct system.

furnaceNewby

The temperature rise I measure across the existing furnace is within the specs on the furnace label. I measure 75F rise, the spec plate says the furnace is rated 65F-95F.

What should the temperature rise be once I fix all of the issues listed above?

furnaceNewby

JUGHNE said:

If I understood you right you have 33' of 4" or 5" round pipe in your crawl space. Just shooting from the hip but that length sounds like it is too far for 4-5-or even 6" pipe. Do you have more heat runs that are small (4-5") and more than about 10' long? Ducting round runs sound pretty small. The 17 x 8 sounds right but it should go farther to the end of the house with 1 or 2 reducers along the way.

The crawl space has three or four runs but most are 20' or less, and most are 6" round duct. None of my duct work has insulation, including the main 17x8. I can measure a 20F drop across 20' of the main 17x8.

Our plan is to insulate a section of the main and see how much we affect the temperature drop.

I read the post on cost/value, and I agree 100%!

furnaceNewby

The temperature rise I measure across the existing furnace is within the specs on the furnace label. I measure 75F rise, the spec plate says the furnace is rated 65F-95F.

What should I expect will happen to the temperature rise once I fix these issues?

>> A, Undersized ducting
>> B, Dirty fan
>> C, Blocked or restricted filters
>> D, Blocked or restricted HX
>> E, Blocked or restricted AC coil
>> F, Uninsulated ducts