Main Supply Too Small? Consequences?

[muswell_hillbilly]

I'm in the process of replacing my HVAC system, and would sure appreciate some guidance on a couple of issues.

My existing system features a 100K in/92K out Heil furnace with 4 ton blower. A/C is 2.5 ton Heil 5000. Furnace is factory-wired for 1585 cfm cooling/1425 cfm heating (@0.5 ESP). Main trunk is only 18.5" x 7.5", which after reading this forum, seems too small??? Total trunk length is about 50 feet, with two reductions along the way - last 32 feet are 14" x 7". Total of 11 6" round branches. So my questions are:

1. How mismatched is the current blower to the ductwork and A/C unit? What are consequences of mismatch?

2. Assuming new equipment is properly matched to existing ductwork, is there any capacity left for additional branches?

[mark beiser]

I doubt you are pushing the 1585 cooling CFM, or even the 1425 heating CFM through that duct system. If the blower is actually set up for that, whatever the airflow actually is would be very loud due to its velocity.

[pstu]

As I understand the classic A/C airflow is 400 cfm/ton, sometimes up or down 50 depending on design goals. You should expect about 1000 cfm for cooling. Hopefully someone who better knows heating, will offer an estimate about what is reasonable for your furnace output.

ACCA Manual D is supposed to contain all the answers you need for duct design, but it's not an easy book to read without first learning some early stuff -- which I haven't learned yet. But one of the key points they state is that hard pipe ducts (not flex) should be designed for air speed of 900 feet/min, with a maximum limit maybe 1000 (going from memory).

Your 18.5 by 7.5 main trunk has 139 square inches, which is 0.964 sqft in cross section. That would allow up to 864 cfm airflow and still respect that 900 feet/min recommendation. If indeed your cooling airflow is the greater one, and you use 400 cfm/ton, then you are a little over the recommendation but not extremely so.

Your 11 6-inch round ducts total 2.16 square feet cross section, so it appears your main trunk is much more of a limiting factor than the branches. Even if they are flex duct, which is limited by Manual D to a 600 ft/min recommendation, it would be OK as far as I know.

If indeed you were pushing 1585 cfm airflow, then I calculate you would be up to 1645 ft/min which is extremely high. At modest too-high speeds the result is excessive duct noise. I dunno what to expect at this high a speed.

I am a homeowner and decidedly an amateur with this. Most of my learning is so that I can understand what is happening with my own system and know what needs fixing. Absolutely if some of the better experts contribute something that contradicts me, you should listen to them and discount what I say.

Hope this helps -- P.Student

[dash]

Our "student" has summed it up very well.You really have a very undersized duct system,however with a 4 ton drive on high speed ,you may get enoygh air for cooling(1000 cfm),but the heating is likely not near enough to prevent damage.

Test is yourself,what the temperature in and out of the furnace,after 15 minutes of run time?


The correct temperature rise is likely on the furnace door.That's about all you should do,then call a Pro.

[muswell_hillbilly]

Thanks for the detailed inputs -

mark beiser: The airflow rates are from furnace technical manual. Blower motor has 4 speed taps: HI, MED HI, MED LO, LO. Factory wiring is cooling to HI, heating to MED HI. This is how I found it, so factory settings were unchanged. As for noise, I don't have anything to compare it to.

P.Student: Thanks for the analysis. This helps it make a bit more sense. Without instrumentation, can't confirm the speeds, so I suppose noise is best indicator of excess speed? Again, I don't have any point of reference...

dash - By damage, do you mean excessive delta T damaging furnace? Furnace lists delta T 40 - 70 F. I'll take your advice and measure it. Also advice to use pro for changes. Not a DIYer (at least not for HVAC!), just a home owner trying to understand my situation to better interpret recommendations from contractors.

I do appreciate the input...

[muswell_hillbilly]

dash - ran the temp rise test, and, as might have been expected, it measured 90 F, exceeding specification.

So, is excess rise due to insufficient airflow across the heat exchanger, due to limited capacity of ductwork?

After reading this forum, it seems the required airflow for nominal temp rise (55 F) is calculated as: 93K btuh/1.1/55 = 1537 cfm. The MED HI blower setting for heat is 1425 cfm. The HI setting is 1585, so I'll try this and see if temp rise is in spec. Of course, we've already determined that capacity of trunk is well below this (assuming nominal velocity of 900 fpm).

1st question is: If the main trunk is not sized to accept output of blower, is the velocity in the duct simply increased, or is there also some secondary effect that impacts temp rise?

2nd question is: What is the impact of excess temp rise on unit?

[dash]

Originally posted by muswell_hillbilly
dash - ran the temp rise test, and, as might have been expected, it measured 90 F, exceeding specification.

So, is excess rise due to insufficient airflow across the heat exchanger, due to limited capacity of ductwork?

After reading this forum, it seems the required airflow for nominal temp rise (55 F) is calculated as: 93K btuh/1.1/55 = 1537 cfm. The MED HI blower setting for heat is 1425 cfm. The HI setting is 1585, so I'll try this and see if temp rise is in spec. Of course, we've already determined that capacity of trunk is well below this (assuming nominal velocity of 900 fpm).

1st question is: If the main trunk is not sized to accept output of blower, is the velocity in the duct simply increased, or is there also some secondary effect that impacts temp rise?

2nd question is: What is the impact of excess temp rise on unit?

1. Velocity is increased,but when the fan reachs it's Static limit ,the cfm's are decreased,and the temp rise ,is above the spec.,as yours is.

2.Cracked heat exchanger,which is very dangerous,in fact I have it tested ASAP.

The motor spec,ie speed=cfms,is based on a certain Static Pressure,your static is above that Static Pressure.

[muswell_hillbilly]

Thanks for the info, dash. I'll definitely have it checked out, just to be safe.

We're replacing the furnace anyway, so I guess it doesn't really matter, but I'm curious how this mismatch could be resolved it were weren't replacing it?

[dash]

If you put in the same size furnace ,it will or is very likely to have the same problem.

The mismatch is,an undersized duct system,for the size of your furnace.

It probably can deliver the cfms needed for your 2.5 ton A/C.

[Black Adder]

Just wondering what you are replacing your old furnace with? According to your specs for the Heil furnace that is a 92% efficient furnace. What is your reason for changing the furnace? How old is the unit? If it is operating well and in good shape I don't think I would replace it.

[muswell_hillbilly]

The "plan" is to replace with another high efficiency unit, either XR90 or XV90, 80K.

We've gone back & forth about whether to replace it now or later. We need additional duct capacity to service new living space, and are either going to add a secondary trunk, or move the furnace to the center of the existing trunk and split it into two. Given the current layout, it's simpler to just move it, since the routing for another trunk would be complicated.

The Heil unit is from 1986. We're haven't had, and are not having any particular problems with it, other than now realizing it's not sized properly. Since we're going to be tearing into the system anyway, we thought it might be the appropriate time to just replace it. Haven't decided for sure, but we're leaning that way.

[hvacservice]

A blocked return or too small of a return would increase the temp. rise. I assume you checked for a plugged filter.
possible just overfired because of gas pressure. Not everyone does a good job of setting up furnaces on start-up. Not even in 1986. What is your altitude? Also check blower fins for build-up. If you have had poor filtration over the years these could be plugged and would limit blower output. Are all eleven registers open?

Need to know actual system S.P. before you can determine anything about ductwork.

[Edited by hvacservice on 04-03-2005 at 07:24 PM]

[muswell_hillbilly]

Can confirm filter is O.K. - Space Gard 2200, just changed filter.

Raw capacity of return is >= supply, but access to it may be a problem? Large "common" return services 3 bedrooms + bath (i.e. no returns in these rooms, common return in hallway). 2 other returns service balance of load - 1 in kitchen, 1 in living room. I noticed a large (15" x 7") return register cut into side of return trunk right at furnace, at entry point to filter. Is this common?

Milwaukee is at about 650 ft above sea level. Regarding blower condition, it appears very clean, perhaps due to Space Gard filter? All 11 branches have in-line dampers, all wide open, although furniture positioning may be restricting free airflow is some.

[Black Adder]

Return air in furnace room is a big no-no. If you have any naturaly vented appliances in the room you could definetly have a negative pressure on them. Check around the drafthood on your water heater and see if there are any signs of back venting. Look for soot around the draft hood or in extreme cases even the plastic control valve will begin to melt. If the unit is in a wide open basement, the return air will probably not present too much of a problem. Rule of thumb in our area is no return air within 10 ft. of the furnace.

[hvacservice]

I assume the grill added at furnace was added because your return system was too small at installation. Is the blower door hard to remove when furnace is operating? Check opening behind return grills.The opening through floor and the grills any or all could be restricting the flow. How large is the return duct? How large are the returns inside the home? How wide is fin spacing?

[professor hvac]

I glanced over the postings about your questions. I really suggest:
1. First have a load calculation on your house to determine the heat gain and heat loss you have!
2. Have a trained contractor design and size your ducts per the loads.
3. Select the equipment per the loads.

People assume that more is better when it comes to air-conditioning. The consequences for selecting too big of equipment in air-conditioning is; Cycling of the equipment. This means that the equipment blasts cold air for short periods but never runs long enough to remove the humidity. This results in higher electric bills and lower comfort.
Also if the air handler is not sized for the evaporator, the air will be traveling too fast across the evaporator to allow the heat transfer from the air to the refrigerant. This would result in warmer air temperatures coming out of the supply ducts. Beside the noise from the wrong size of ducts that you have currently.
Good luck with your system. But I strongly suggest getting some help from a local reputable contractor.

[muswell_hillbilly]

Apologies for the radio silence - busy last few weeks with new baby!

Thanks to everyone for their input. Finally, with 6th contractor, received an analysis consistent with ideas and input from this great site!

First, to answer a couple of specific questions posted:

Black Adder - Thanks for the safety tip. I've seen that one posted here before. No worry in my case - furnace is only gas appliance and in wide open basement space.

hvacservice (et al) - Contractor #6 was only one that acknowledged return size/access problem. Additional return capacity and access will be added.

professor hvac - 2 of 6 did load calcs and actually came up with same result! Better yet, their results closely matched mine produced by HVAC Calc, which is one of the only consistent results of this whole process! Of these 2, 1 suggested adding 9-11 additional branches to my already undersized supply. Of course, he may not have realized it was undersized, since he never measured it! Anyway, that left only 1, which made the decision easy!

So, the final (and $$$) solution is that we're going to rework the ductwork (supply & return) and have it done correctly. Perhaps we're nuts to go to this extra expense, but we'll be in this house for another 15-20 years, and I don't want to be dealing with this problem beyond 2005!

Thanks again for the guidance...

[pstu]

Wow. Six contractors and only the 6th one sounds good based on building science principles. All I can say is, I'm glad it was only six and not more! It seems par for the course that 80% or more of the guys in the business, are not the ones an informed homeowner would really want.

Congratulations, and best of luck -- P.Student

[dash]

Originally posted by perpetual_student
Wow. Six contractors and only the 6th one sounds good based on building science principles. All I can say is, I'm glad it was only six and not more! It seems par for the course that 80% or more of the guys in the business, are not the ones an informed homeowner would really want.

Congratulations, and best of luck -- P.Student

So the "uninformed" homeowners actually want them???LOL!!

[pstu]

I am embarassed to say, but hell yes. I lived in one house for 22 years and never knew the difference between a good pro and a hack. There were multiple occasions when I got pissed off because of poor results, but sincerely I did not know what to look for.

Looking back at my old records, back in the mid-1980's I paid for cellulose insulation in the attic, but said "no" to an offer for radiant barrier because I was unfamiliar with it. And infiltration was something I had only the dimmest understanding of.

I am convinced a person in business could survive indefinitely selling empty promises. Every day I hear radio ads promising big energy savings by replacing one's AC with a new Trane (as if), and energy savings from buying a filter which only has to be replaced once a year. By the time the homeowner learns it doesn't work for him, the check has been cashed and the business person seems to have developed a case of deafness. Sorry to say.

Best wishes -- P.Student