New Furnace, Bad Ducts, NCI Specialist
I have a Coleman FC9C (80K BTU/1200CFM) and an old home with poorly sized ducts. It took a long time to get someone to come out but the NCI Specialist came by today for an initial consultation.
He said he would put together a proposal to 1) come out and take readings/measurements then 2) come up with a plan to correct the system.
First impressions were that supply may be OK but return is definitely a problem. He also made a comment about the fiilter. Said given all the air flow restrictions the cheap blue spun glass filters would be better as long as they are replaced monthly.
Initially I passed on the blue filters and went one step up to the white pleated Merv 7 or 8 filters.
Is it ok to use these filters? I have enough problems without adding more to the mix by using an improper filters?
Even the cheep low rated pleated filters will generally have more than twice the pressure drop as the cheep blue filters in an under sized filter situation. I have even seen upwards of 4-5x the pressure drop with one particular national brand of 1" MERV 6 pleated filters.
If he said to use the cheep blue filters, that is what you need to use.
As I recall, you had a mod furnace installed.
Use what the NCI guy recomended.
I actually bought one of those blue filters awhile ago but didn't use it. I was worried that it would let too much dust through and damage the unit. I guess that's not an issue. I'll change out the filter tonight and set the stat to remind me to change it in 30 days.
mark: what's an under sized filter situation? I don't understand what that means. Thanks.
beenthere: Yes, I have the mod furnace with variable speed blower (fortunately sizing seem to be OK). And we manually set back the stat at night the way we used to run the old furnace and fuel consumption has dropped significantly.
The NCI guy seem to install more Trane and Rheem units, less familiar with the York/Coleman systems. I sent him links to the manuals online. When he comes to take measurements would test mode be the ideal way to run the furnace when he checks the ducts?
He doesn't have to.
He can run the blower in A/C mode to get it up to the full CFM it uses in heat to test the static. You'll want the static checked for the highest CFM. (cooling or heating)
The installer should have done that to test it when he installed it though.
If he didn't, you might want the NCI guy to check it though.
The variable speed system you have can be adjusted for airflow, and does compensate a bit for external static caused by poor duct design. It looks at airflow across the heat exchanger. Those systems can be tailored to do many things. What specific problems are you having? typically those can handle using pleated filters, depending on other factors. An undersized return causes your unit to try to get the air required from somewhere to push back to the space. If your return is undersized you will notice the ducts sucking tons of air from any little holes you may encounter. The ECM motor will ramp up to try to get the airflow across the unit to setpoint. Is your return duct noisy? How is the airflow coming out of your supply registers?
Return duct is not noisy, however we have another problem with the (panned) ducts not being sealed and open to the air space between the walls.
I stuck the camera down the duct and took a picture of the joist. You can see that the joist doesn't completely meet up with the foundations. The brick/block masonry is visible through the gap.
Supply is fine at low firing rates, but at higher rates the blower ramps up and the first three supply ducts off the main sound like a freight train. Supply air and noise drops off as you get further away from the unit.
That (noisey close to unit) is a sign of a very undersized duct system.Trust him a give him the chance to find the best solution.
Originally Posted by mike08054
Take a look and see if you have manuel dampers on the branch lines off of the main trunk. In many cases you wont, and they simply throttle with the registers. Sometimes you can add dampers in the branch ducts to allow you to better control airflow in the first few takeoffs and get the rest more even. Unless you have one of those classic setups where the first two come right out of the top of the plenum box and through the floor.