installing new furnace

haroun

Hi
Please advice
My old furnace is 40 inch high and 17.5 inch width 80k btu, can i replace it with 33 inch high and 21 inch width 90k btu both are the same length 19 inch providing a tapered plenum, so plenum ( a neck between the new furnace and the existing duct) dimensions are 21 X 19 and outlet to duct will be 17.5 X 19 with 6 inch height. will that reduction in cross sectional area compromise the air flow or the furnace has that tolerance in dimensions.
n.b.
There is an A frame a/c coil in the duct.
Old furnace is carrier 58MCA080-12
New furnace is lennox ML 296UH090V48

HVACNUT

Are you the contractor or homeowner?
How old is the AC? Would it make sense to upgrade the AC now, with a matching Lennox cased coil, then transition from the coil cabinet to the existing duct?
Transitioning directly out of the furnace could effect airflow and static pressure.

worldclasshvac

Your can transition anything nicely on site with the right tools and some skills.  Agree on upgrading the coil now if AC is older.  You probably don't need 80k unless house is big, a downsize to a smaller furnace is usually better.  Your average 90k has a 5 ton drive.  

mattmia2

Why are you telling them to replace the AC when the new one is likely to leak in around a decade whereas the old one is likely to never leak?

jesmed1

Whatever you do, be sure to measure total external static pressure (TESP) across the furnace after installation and verify that it is within mfr's spec. Usually the maximum allowable TESP is 0.5 inches water column (IWC). The furnace can function with higher TESP, but the higher the TESP, the harder the blower motor has to work, reducing its lifespan. If TESP gets too high, the furnace can overheat from insufficient airflow. This happened with our church furnace because the HVAC company installed a new furnace with a too-small return plenum that choked the airflow, resulting in a TESP of over 1.0, over twice the mfr's maximum. As a result, the furnace overheated frequently, so we had to lock it into low-fire mode permanently.

Don't let that happen to you. If you're paying someone to install it, make them measure the TESP after the install and verify it is within limits. If it's not within reasonable limits, figure out how to modify the ductwork to increase the airflow.

Better still, measure the TESP now with your old furnace so you have a baseline for comparison. If your TESP is too high now, it's going to be too high with a new furnace the same size or bigger.

mattmia2

You can do the math before you install it to determine if the existing ductwork is adequate.

jesmed1

mattmia2 said:

You can do the math before you install it to determine if the existing ductwork is adequate.

That's true if you know how to use a ductulator correctly. Some HVAC pros do not. Including the pro who installed my church furnace with a too-small return plenum. He only pulled out his ductulator after I pointed out the problem with the plenum sizing, and then I had to correct him when he failed to apply any equivalent length factor for the 90-degree bend in the plenum.

haroun

I feel like a theoretical conversation here, does that means I would buy the furnace, do the duct work and then measure the TESP then what? if it is too high I buy another one and do the test again? or buy another one same size without using a cross sectional reducer, that is not a practical solution.
to make it simple and straightforward: Can I reduce the furnace output air flow by 25% and have the furnace working good.

HVACNUT

mattmia2 said:

Why are you telling them to replace the AC when the new one is likely to leak in around a decade whereas the old one is likely to never leak?

Who mentioned a leak?

If I surveyed for a replacement furnace and there's a 25 year old R22 evap sitting on top of it, I'd be stupid to not add it as an option for upgrade as well. Because next summer when the compressor takes a poop, the client will ask, "Why didn't you say anything when I replaced the furnace?" Now it's gonna cost $$$ instead of $$ for new AC. 

I'm still asking if @haroun is the homeowner or a contractor. Like, who's commissioning this furnace?

jesmed1

haroun said:

to make it simple and straightforward: Can I reduce the furnace output air flow by 25% and have the furnace working good.

That is an impossible question to answer, because you don't even know what airflow you have with the existing furnace/duct configuration, and the only way you're going to find out is to either buy a digital manometer and measure TESP yourself, or pay an HVAC pro to measure it for you.

If the existing TESP is already too high, a bigger furnace with a smaller supply plenum is going to make the problem worse. How much worse? It's impossible to say until you (1) measure the existing TESP, (2) find the manual for the existing furnace and find the chart where they give you the CFM's (cubic foot per minute) of airflow that correspond to that TESP you measured, and (3) compare that to the CFM requirements of the new furnace.

So let's imagine the following:

1. You measure the TESP of the existing furnace and find it's 0.8 inches water column (IWC). Which is already above the mfr's probable limit of 0.5 IWC for that furnace. This often happens because furnace installers know they can get away with a somewhat higher TESP. But if TESP gets too high, say close to 1.0, the furnace is going to overheat.

2. Then you look in the manual of the existing furnace and find the chart that tells you 0.8 IWC = (for example) 1700 CFM for that particular furnace. So now you know you have 1700 CFM moving through your existing furnace at a TESP of 0.8 IWC.

3. Then you check the specs for the new furnace you're thinking of installing. The new furnace specs say
(for example) the max allowed TESP is 0.5 IWC, and at that TESP the blower moves (for example) 1900 CFM.

4. Now you compare numbers from the old and new furnaces. Your new furnace needs 1900 CFM at 0.5 IWC max. But your old furnace was moving only 1500 CFM at an even higher TESP, meaning the old furnace had to work harder to suck even less air through it. Which means your new furnace is going to have to work even harder to suck 1900 CFM through it, possibly wearing out the blower motor sooner, or causing the furnace to overheat and shut down periodically at incovenient times.

5. Then you can use a ductulator to calculate the additional problem of a 25% area restriction in the new plenum, which by the way does not equal a 25% reduction in airflow. Used correctly, a ductulator will tell you the new CFM you can expect with the added restriction, but many people don't know how to use a ductulator correctly.

So now you've figured out (1) TESP and CFM of the existing furnace, (2) max allowed TESP and required CFM for the new furnace, and (3) the predicted reduction in CFM with the new supply plenum. And by comparing those you know whether or not the new furnace is going to meet mfr requirements for CFM and TESP.

Or, you can just roll the dice, install the new furnace, and find out the hard way that it overheats because no one measured TESP first, found the existing CFM, and compared those to the requirements for the new furnace.

Ask me how I know not to roll the dice. I have a furnace at church that can only run at 40% capacity because the contractor did not do the above. Do your homework first, or pay a pro who knows how to do it, and then you won't be guessing and maybe make an expensive mistake.

EdTheHeaterMan

I like the way that @jesmed1 tells the story. It makes sense when you do each step one at a time. Math is fun for many. Not for all.

I feel like a theoretical conversation here, does that means I would buy the furnace, do the duct work and then measure the TESP then what? if it is too high I buy another one and do the test again? or buy another one same size without using a cross sectional reducer, that is not a practical solution.
to make it simple and straightforward: Can I reduce the furnace output air flow by 25% and have the furnace working good.

I think @jesmed1 is saying to do the math first. Not after you make the wrong choice.

mattmia2

Why do you want to put a bigger furnace in in the first place?