CFM problems in old hot air system

Smith19

Hello all,
I have a problem regarding my own home. First some background. I live on the Front Range of Colorado in Boulder. My home, like many here, is a split level ranch built in 1972 during a significant construction boom that took place at that time. These homes are of extremely poor quality. In 2006, when we moved in, the house had just been beautifully remodeled. The ugly split entry had been taken out and a spacious kitchen added to the back. New windows and reconfigured eaves made a significant difference in the architecture - the renovation was pretty thorough. Nearly a total gut job. We love it. However, it wasn't so thorough in the HVAC department.

The original portion of the house has the two steel duct trunks running down the middle, accompanied by a mystery section of a third duct which doesn't seem to be connected to anything. These oversize trunks were once connected to a huge atmospheric gas furnace, but that was replaced in 2002 with a Coleman hot air furnace. Next to this is an extremely low efficiency Rheem gas water heater.

Under the addition is a different story. A second Heil condensing furnace was added to heat said addition and resides in a pretty big six foot high "crawl" space. This unit works okay, but the ducts are uninsulated and very leaky.

The "old" furnace delivers horribly low CFM to the registers, many of which see no heat at all during the winter. The ducts in the original part of the house are also uninsulated and I assume are dumping heated air into the walls. What I would like to do is to zone the original ducts in two in order to more evenly deliver pressure to where it's needed, along with adding insulation. But since the ducts are buried in drywall I assume that that would cost an arm and a leg. I have also pondered installing a Lochinvar Knight boiler and running some zones to two air handlers which would be in place of the furnaces. However, all of this would be crazy expensive.

Any ideas on how we can make this work? We have had the ducts "scoped" and cleaned for dust and found no obstructions.

Jamie Hall

Ah yes. The joys of a low budget scorched air system.

Several thoughts -- and I'm sure others will have more.

First thing to do is to figure out how air is returned to the furnace. Sounds backwards, but if the cool air can't get out of a space, then warm air can't get in. An extensive remodelling job -- particularly if partitions were moved, removed, or added, can mess up the return air something fierce. (one has to wonder -- was that mystery duct part of a return which got discontinued?)

Second, you can put dampers in the two main ducts, if they aren't already there -- but you may find (after you have fixed the returns!) that you get better results and balance from dampers on the individual outlets.

There's no harm to oversize trunk ducts, except that they take a lot of space. Just means the velocities will be lower (and hence the blower losses and noise, by the way). However, there is a lot of harm in leaky ducts -- and they should be sealed wherever you can get at them. And insulated. Ductwork in the walls can be a real problem -- as you point out, if they are leaking (which is moderately likely) they are hard and expensive to fix. Depending on how extensive they are, it may not be worth it. On the other hand, if they are really seriously bad, have you thought of snaking a smaller flexible duct in them? Depending on the size of the original ducting, that might be an option. I would note that you can get a handle on the leakage, once the main ducts are fixed, by closing the dampers on the registers and seeing what air flow you have...

I think I would try and optimize what's there, before I considered going to air handlers and a boiler -- particularly since that wouldn't address any leakage in the walls.

EBEBRATT-Ed

Draw out the complete system on paper to scale putting in all the known duct sizes. Do a heat loss (use the Slant Fin OR MANUAL j
Then you can objectively sit down and design a system that will work.

Smith19

Sounds like checking the returns is the first step. I would like to find a lower cost way to seal and insulate the supply and return trunks in the ceiling - I think putting individual dampers (one per register branch) is the only way to zone the system.

Ironman

Let may say right off that zoning a single stage fossil fuel furnace or heat pump has severe limitations. There is a required temperature rise listed on the furnace. This is proportionate to its firing rate or btu input. To maintain that, a certain amount of air flow in cfm is needed. If you start choking the airflow because damper(s) are closing, the furnace will overheat and shut off on high limit. Prolonged cycles of that will cause the heat exchanger to fail from metal fatigue.

I'm not saying that SOME MINIMAL zoning is not possible, just extensive zoning that severely limits the air flow.

The best thing that you could do is have a COMPETENT HVAC contractor, who knows proper duct design, come out and evaluated your system. Check the contractor locator on here to see if there's someone near you.

If not, maybe @Mark Eatherton could recommend someone.

But please, don't make a decision based on an assumption. You'll surely regret it.

Smith19

> @Ironman said:
"Let may say right off that zoning a single stage fossil fuel furnace or heat pump has severe limitations. There is a required temperature rise listed on the furnace. This is proportionate to its firing rate or btu input. To maintain that, a certain amount of air flow in cfm is needed. If you start choking the airflow because damper(s) are closing, the furnace will overheat and shut off on high limit. Prolonged cycles of that will cause the heat exchanger to fail from metal fatigue.

I'm not saying that SOME MINIMAL zoning is not possible, just extensive zoning that severely limits the air flow.
The best thing that you could do is have a COMPETENT HVAC contractor, who knows proper duct design, come out and evaluated your system. Check the contractor locator on here to see if there's someone near you.

If not, maybe @Mark Eatherton could recommend someone.

But please, don't make a decision based on an assumption. You'll surely regret it."



I think the best solution here is to install the bypass damper between the supply and return plenums. It's spring loaded or barometrically controlled and allows excess pressure from the supply plenum to spill into the return in the event that only a few zones are active.

Smith19

Did some exploring today and found some interesting things on the "old" system. (original part of the house). The intake registers nearly don't pull any air in at all, save for the one that is bang in front of the blower itself. That one pulls a ton. As is customary for spec built tract housing in the 1970's, the intakes are comprised of butchered up joist bays and studs.

The supply ducting is slightly better but pretty leaky. Branches are taken off of a 8"x20" trunk and run between the joists to register boxes in the ceiling. These "boxes" are used instead of boots and have strange oval-shaped adapters where the branch is attached. There are no screws or anything holding all this together, and major gaps were visible. Lots of air leakage into the walls and ceiling. I know this because I "scoped" the ductwork with my phone on a selfie stick. . Got some good photos of the innards of my system which I will share tomorrow.

I would like to seal this up because it would appear that there are no major obstructions in the system. Don't know how this can happen without a major drywall job though.

HVACNUT

Looks like either way, you'll be spending $$ to fix your problem. That 3rd. trunk duct doesn't do anything?
You really need a reputable HVAC contractor to assess the situation. One option I would consider if you really want zoning, (but I wouldn't do more than 2) is upgrade to a mod/con furnace and an Arzel zoning system. Neumatic dampers would be installed at each branch duct, with the air hoses running back to the control board. Order the barometric relief damper and SAS sensor. It will sense the load and adjust the firing rate and required CFM accordingly.
Are there return registers in each room? I'm interested to know if you blank off the main return grill near the blower, is there enough return air to match the design CFM.

Smith19

> @HVACNUT said:
> Looks like either way, you'll be spending $$ to fix your problem. That 3rd. trunk duct doesn't do anything?
> You really need a reputable HVAC contractor to assess the situation. One option I would consider if you really want zoning, (but I wouldn't do more than 2) is upgrade to a mod/con furnace and an Arzel zoning system. Neumatic dampers would be installed at each branch duct, with the air hoses running back to the control board. Order the barometric relief damper and SAS sensor. It will sense the load and adjust the firing rate and required CFM accordingly.
> Are there return registers in each room? I'm interested to know if you blank off the main return grill near the blower, is there enough return air to match the design CFM.

I like the idea of blanking out the return grill in the basement to see if I can balance out the system. By pneumatic dampers do you mean vacuum controlled or would I need a crazy pneumatic DA control system like one finds in a school? Definitely don't want a noisy air compressor cycling next to my furnace.

Henry

You could use Belimo electric drives. A properly balanced system does not need zones in a house unless over 3500 sq feet. In my own house, I have one floor grill that I open or close depending on th season. It is the one infront of the front door. In summer it is closed and balances the cooling to the seconf floor. In winter, it is open and reduces by just the right amount of flow upstairs to get even heat.

HVACNUT

> @Smith19 said:

> I like the idea of blanking out the return grill in the basement to see if I can balance out the system. By pneumatic dampers do you mean vacuum controlled or would I need a crazy pneumatic DA control system like one finds in a school? Definitely don't want a noisy air compressor cycling next to my furnace.
>
>>The Arzel board is self contained. The compressor is about the size of a D battery. Very quiet. The hoses connect in series so there's only one hose per zone going back to the board.
The company I work for has been installing Arzel systems for years in mansions in the Hamptons, because God forbid they can't temper 30 rooms seperately.
Google Arzel Air Boss for more info.