Modulating Furnace Airflow

[srm077]

I have a Lennox SLP98V modulating 110K BTU furnace with IComfort stat installed.

It runs at low 35 to 45% the majority of the time and I am having some issues getting enough warm air to the furthest room in my home. I have adjusted the CFM up on the stat from 650 CFM on lowest firing to 800 CFM on lowest firing per the recommendation of my installer.

I am wondering if my bypass humidifier is robbing some CFM too? Will switching to a powered fan humidifier like the Aprilaire 700 and removing the bypass help? Just want to have my facts straight before I call my installed back out to get this corrected. How many CFM does the 6 inch bypass rob?

Thanks in advance.

[ampulman]

It has been stated here that bypass humidifiers 'steal' some airflow from the supply.

Was your furnace size selected based on a heat calculation?

[srm077]

It has been stated here that bypass humidifiers 'steal' some airflow from the supply.

Was your furnace size selected based on a heat calculation?

No, no load calc, but I'm 4300 or so heated and cooled space with a 5 ton A/C so I needed the blower size apparently. I am probably a bit over-sized. Will the powered humidifier put out more humidity than the bypass?

[George2]

No, no load calc, but I'm 4300 or so heated and cooled space with a 5 ton A/C so I needed the blower size apparently. I am probably a bit over-sized. Will the powered humidifier put out more humidity than the bypass?

4,300 sq. ft. and one system? Strange. You're going to have airflow issues with all that ductwork.

Sounds like you need to zone the system.

[srm077]

4,300 sq. ft. and one system? Strange. You're going to have airflow issues with all that ductwork.

Sounds like you need to zone the system.

House was built in 95- not by me and (1100 of that is a conditioned walkout basement). Ideally it would be zoned with multiple units but that is not currently an option. I have manually adjusted the dampers on the main supply trunk for each room and overall have a pretty good temp balance between all 3 floors and the majority of the rooms. Just trying to get a bit more airflow to the farthest rooms and think a replacing the bypass humidifier might help. I am seeing 67 basement, 70 main level, and 70 2nd floor with the stat set at 69 so I am pretty happy.

[big sky hvac]

House was built in 95- not by me and (1100 of that is a conditioned walkout basement). Ideally it would be zoned with multiple units but that is not currently an option. I have manually adjusted the dampers on the main supply trunk for each room and overall have a pretty good temp balance between all 3 floors and the majority of the rooms. Just trying to get a bit more airflow to the farthest rooms and think a replacing the bypass humidifier might help. I am seeing 67 basement, 70 main level, and 70 2nd floor with the stat set at 69 so I am pretty happy.

My personal recommendation would be to stick with the bypass humidifier you have. I would make sure that the control trasformer for the humidifier is wired to the EAC terminal. With the furnace you have, I would run the fan continously. This will do several things for you: maintain a more even and consistent temp. throughout the whole house, you'll constantly filtering air so your home will be cleaner and with the humidifier hooked up to the EAC terminal, anytime the blower is running the humidifier will run if there is a call for humidity. I would also make sure the water line feeding your humidifier is connected to a hot water line as this will make it more efficient. Running the fan continously on that furnace will cost you very little in electricity, maybe $50 or so a year. As far as gettting more airflow to the farthest rooms, it really depends on the condition of the ductwork. Zoning the system might be the best thing to do, if it's possible.

[George2]

House was built in 95- not by me and (1100 of that is a conditioned walkout basement). Ideally it would be zoned with multiple units but that is not currently an option. I have manually adjusted the dampers on the main supply trunk for each room and overall have a pretty good temp balance between all 3 floors and the majority of the rooms. Just trying to get a bit more airflow to the farthest rooms and think a replacing the bypass humidifier might help. I am seeing 67 basement, 70 main level, and 70 2nd floor with the stat set at 69 so I am pretty happy.

It sounds like you are able to install zoning. The nice thing about zoning is you'll improve your airflow which will then also satisfy the zone that is calling.

It improves the efficiency because you're not heating/cooling the whole house when areas are not occupied.

[George2]

Sidebar:

Are the registers in the ceiling on the 2nd floor? It makes it easier but regardless I would recommend 3 thermostats. One on each level.

[SJProwler]

Actually, choosing "precision humidity control" in setup on the icomfort will do the same as wiring to the EAC terminal. In fact if you aren't running continuous blower and there is a call for humidity it will start the blower. Also, to run the blower 100% of the time you have to choose the "fan on" setting since the "cont" setting can only be programmed to run the blower up to 45% of the time max.

[acwizard]

I would question the sizing of your equipment to the square footage of your home. What is the approx distance and size of your longest duct run. Your blower probably does not have the static capabilities needed to push the air to overcome the duct friction loss, etc.

[George2]

I would question the sizing of your equipment to the square footage of your home. What is the approx distance and size of your longest duct run. Your blower probably does not have the static capabilities needed to push the air to overcome the duct friction loss, etc.

That's why zoning would be so nice. It'll shut off one or two zones (rarely do all zones call at once) which will increase the velosity.

[acwizard]

Zoning is a great approach to load shifting.Zoning has to be done correctly or there will be very little energy savings. I have installed and serviced lots of systems. Our largest home was 18 zones in a 10,000 sq. foot home and it worked flawless. The key is proper duct design. Adding a zone system to a home with poor duct design will just enhance has poorly it was designed from day one. Zoning multiple levels using one system is just asking for problems. Stratification occurs and solar heat gain will alway be on the top floor. The air needs to be moving throughout the home to prevent hot and cold spots and also any humidity issues. I would seriously look into adding another system and splitting the upper floor from the other two floors.

[George2]

Zoning is a great approach to load shifting.Zoning has to be done correctly or there will be very little energy savings. I have installed and serviced lots of systems. Our largest home was 18 zones in a 10,000 sq. foot home and it worked flawless. The key is proper duct design. Adding a zone system to a home with poor duct design will just enhance has poorly it was designed from day one. Zoning multiple levels using one system is just asking for problems. Stratification occurs and solar heat gain will alway be on the top floor. The air needs to be moving throughout the home to prevent hot and cold spots and also any humidity issues. I would seriously look into adding another system and splitting the upper floor from the other two floors.

We don't do a lot of zoning (with dampers) on new homes because the builders normally wants to "zone" with equipment.
However, when I have zoned on new homes (2-story or ranches), I've never had any complainants.

My largest was 8 zones system in a 4,000 sq. ft. (new) home.

What I enjoy the most is zoning existing homes. Most homes in my area can not A/C the 2nd floor.

[srm077]

Actually, choosing "precision humidity control" in setup on the icomfort will do the same as wiring to the EAC terminal. In fact if you aren't running continuous blower and there is a call for humidity it will start the blower. Also, to run the blower 100% of the time you have to choose the "fan on" setting since the "cont" setting can only be programmed to run the blower up to 45% of the time max.

I currently have humidity on "basic", and the fan is set to "on" for all programming times but 1200am to 630am which is set to auto. I was told that setting the humidity control to precision should only be done with powered humidifier?

The registers are all in the floor except for the basement where they in the ceiling. The returns are all high on the walls except for the basement where it is near the floor. Airflow is not a problem as long as the blower is above 1000 cfm. I may turn the humidifier off for a bit today and close the damper to see if there is any noticeable improvement in airflow at the farthest ducts.

[SJProwler]

Nowhere in my manual does it say anything about requiring a power humidifier to use the precision setting and mine has been working fine.

As far as your airflow issues, sounds like the ductwork should have been looked at when the contractor did your install. My ductwork tapers down in size the farther it gets from the furnace to increase velocity and avoid the issue you are having.

Using the DAT sensor to control the blower in my system my blower is moving 300-400 cmf at 35% and the house is the most comfortable it's ever been and the system is silent. I'd hate to have the thing running at 1000 cfm all the time, it'd be like living in a hurricane.

[srm077]

Nowhere in my manual does it say anything about requiring a power humidifier to use the precision setting and mine has been working fine.

As far as your airflow issues, sounds like the ductwork should have been looked at when the contractor did your install. My ductwork tapers down in size the farther it gets from the furnace to increase velocity and avoid the issue you are having.

Using the DAT sensor to control the blower in my system my blower is moving 300-400 cmf at 35% and the house is the most comfortable it's ever been and the system is silent. I'd hate to have the thing running at 1000 cfm all the time, it'd be like living in a hurricane.

Actually, my ductwork tapers down 2 times from the beginning of the trunk to the end....... Contractor did look at ductwork and actually said it looked great.

As for the 1000 cfm- it is barely noticeable, my house has a bunch of supply registers. At 100% capacity it is pushing 2050 CFM- that is a little much but it has only hit 100% once when I forced it too. The circulate and on features for my system have a minimum CFM of 600 which is what it is set at. My house is extremely comfortable, I am just a perfectionist and want the two rooms that are 1.5 degrees cooler to be the same temp- even though we hardly ever use them.