HVAC seeking to boost air flow

[BrowserGuy]

Trying to remedy a rube goldberg duct work system that results in, ..., less than optimal air flow to the far end of a run. What I can determine by eye, tape, and feeling around up inside the duct looks like this:

• GMPN080-4 furnace/AC, plenum & duct work originally serving 1288 sq ft

• All original branches used top take offs & 6" flex pipe - all work fine

• Two long runs into garage/workshop area were added later - 576 sq ft

• Each run to 3 4x12 registers spaced above windows & along outside walls

The concerns I have with one particular run are all the twists and turns and I know thats killing the air flow. I make it about 65 feet but I know each change in direction makes it seem like more. How much more I don't know...

• Top take off to 18 ft of 7" flex pipe between floor joists

• Then to a metal 90 and 5 ft of 7" flex pipe across the bottom of the joists

• Then to a metal 7" round to 7" oval straight boot

• Then to 2 vertical 45s 7" ovals to get back up inside the joists

• Then to a 90 and 10 feet up inside the wall cavity with 7" oval

• Then to another 90 to get horizontal again

• Then to a 7" oval to 7" straight boot

• Then 24 ft (or so) straight 7" metal pipe with 2 of the registers

• Then a 90 to 8 more feet of straight pipe with 1 of the registers

I can't see any other way to have done the ducts given the constraints of joists / walls / register locations. I can see 3 different things to try:

1) get rid of the flex duct and go all metal (reduce resistance ?) 2) install an in duct aux fan in basement and/or out in work shop 3) boost the main fan speed

#1 appears easiest/cheapest thing to try but will it help ?

#2 might be exactly what is needed but I really have no idea.

#3 is beyond me. I have no manual & no idea how to check it but I've read that this unit has 3 selectable speeds. I'd really hate to pay a pro $100 or whatever to come out and flip a dip switch. If anybody has the book on this unit...

Any advice from a pro/handyman/DIYer welcome.

[udarrell]

Are dampers in the other duct runs? If not, dampers should be installed so you can balance the airflow.

If you can use high-speed fan, it will increase the pressure through the long narrow ducts, providing you can damper balance the duct system.

You have a real mess there.

udarrell Darrell

BrowserGuy wrote:

[Joseph Meehan]

I am sorry, but there are not easy fixes and without a proper total evaluation and correction, you will only be adding to the Rube Goldberg system.

To start with you need to do a Manual "D" to get all the data you need to figure all this stuff out. It does the math to get it right.

I also suggest that you will need a experienced professional. This is an art form that needs the years of experience of finding ways of addressing those construction situations that seem to defy doing the job right. Any good professional will do the Manual "D" and I would not be surprised if they wanted to do other "Manuals" to assure your HVAC equipment was up to the task.

[udarrell]

Right on Joe!

[SQLit]

Ok you want to do this yourself, I would not; but that is an different story.

Start with the fan. You have an model number, hopefully there is an manufacture. Locate an wiring diagram FIRST. Failing that locate an wiring document for the motor and go backwards.

NOTE this is will effect the run time of the unit. Faster fan speed = lower out going temp in heat mode, longer run time.

Personally I would call someone to do this. What are you going to do when you short out the motor and you have no heat at all?

[Gary R. Lloyd]

Nope.

Faster fan speed = Higher heat output = Shorter run time

Gary R. Lloyd CMS HVACR Troubleshooting Books/Software

[PrecisionMachinisT]

To start with its likely the manual "D" was *already* done, and now with this clusterfu*k glommed onto it's just costing him in money and lack of overall comfort.

Probably would be best to just tear out that hacked in ducting, and go back to using *that* system as it was designed.....

Set up a complete separate system to heat / cool the garage and workshop on an as-needed basis, this unless perhaps he spends more time in the garage than he does in the house--and even in that case by having a separately controlled system he would have the option of controlling / adjusting both systems to heat / cool only the space he is currently using.

[m Ransley]

Gary , you said " faster fan = higher output = shorter run time " . nope, faster fan without more btu cannot equal higher output. It will be Lower at supplys because more air is moving over the same heat source. It moves the air around more but btu is a btu, or for efficiency you would need and be specified high fan.

[Greg O]

This part about the garage scares me, totally against code in most areas to heat your garage with the house's furnace. Now if this is remodeled space that you can no longer get a car into, then no problem.

If it is still a garage, rip out all the duct work that attaches to the house's furance, and add a second furance to heat just the garage. The two systems need to be completely independant of each other, and no connection what so ever for air to flow from the garage to the home. Greg

[Dan]

You could legally have a heat run into the garage, but you couldn't have a return out of it. That means it wouldn't work properly anyway. The best idea is a seperate system, pay attention to the elevation requirements and the UL listing of the unit you choose to use.

Dan

[Gary R. Lloyd]

If you slow the air flow, for example plugged air filter, the stack temp rises because less heat is being transferred from the combustion side to the air side. Lower air flow = lower output = longer run time

Increasing the airflow has the opposite effect.

The temperature will be lower, but the heat output will be higher. Heat takes into account both temperature and volume. The temperature is lower, but the volume is higher.

Gary R. Lloyd CMS HVACR Troubleshooting Books/Software

[TURTLE]

This is Turtle.

#1 Don't fool with the trashy Duct fans.

#2 Boost the fan speed to see.

#3 Do increase the duct sizes to make the move to area that are not getting it.

If you would like to get a run on the sizing of the ducts to increase them to get the area to the ones that are not getting it. i can size them for you from the duct sizes stated by you.

You can increase the fan speed if you get with me and i may be able to explain it to you to run it on black or high. I know the GMPN's series for I install then. If you e-mail me do include the serial number with the model number here. It is really nothing to change the speed on them.

Get with me if you want to do #2 or #3.

TURTLE

[m Ransley]

Gary , yes a plugged filter reduces flow, but if what you were saying was true the Low factory fan speed would reduce AFUE , higher fan speed would increase AFUE. And that simply is not the case, Increasing fan will not improve efficiency rather it will lower it by more electrical consumption.

[Gary R. Lloyd]

Increasing air flow yields diminishing returns. There is a point where the small diminished increase in output is outweighed by the increase in electrical costs.

Everything is balances and trade-offs.

Gary R. Lloyd CMS HVACR Troubleshooting Books/Software

[m Ransley]

I see, higher blower speed = more KWH cost, a trade off.

[Gary R. Lloyd]

Exactly so.

The other way to increase heat transfer thus output, without increasing air flow, is to increase heat exchanger surface area, and that's what higher efficiency systems do. This strategy also yields diminishing returns.

The old 'more-is-better' mentality has no place in HVACR. Like the three bears' porridge, not too hot, not too cold, but just right.

Balances and trade-offs.

Gary R. Lloyd CMS HVACR Troubleshooting Books/Software

[m Ransley]

But an interesting thing is Carirers 96.7 % Infinity is only one unit , the smallest 47000? btu unit, the others in that Infinity line are in the 94 range, but if you look at the weight of the 96.7 unit you see it is 20 or so lbs heavier than the next larger unit. Obviously more sq ft in the exchanger. A marketing ploy yes, can furnaces be made 2-3% more efficient, yes. So why aren`t they, competition and lack of Gov incentives and mandates. The VS DC motors boost AFUE by 1 or so % if I follow this right.

[TURTLE]

This is Turtle.

When you have a 96%AFUE furnace now and about 5% of the public is buying them and the other 95% of the public is buying 80%AFUE furnaces , Well you can leed a horse to water but you can't make him drink.

You can get the Government to start telling the manufactor what seer and AFUE rating they will start offering and really just telling them what equipment to make , but then you will come to a point where the manufactoring company will say this. Well Mr. Government you have control here and I'll tell you what. You start making these furnaces and we are going to make other things. Can you picture what a gas furnace would cost with the Government making and designing it. Does the words " A $1,200.00 Toilet seat ring a bell anywhere here" ?

TURTLE

[m Ransley]

Turtle refrigerators are 75% more efficient now because of regulations, so it works. Competition keeps prices down. Polution equipment on cars works to, from regulations. And regulations keep food safe, go to Mexico where seafood is kept on a uncooled table in hot sun, and produce is watered with sewage and meat is sold unrefrigerated, yes ive seen it and got seafood poisoning there. Dont take so much for granted we have it good here and outlawing 80% units would be a good idea, competition would bring down prices just like it has done with electronics.

[Gary R. Lloyd]

Protecting health is one thing. Dictating efficiency is another. People will save a dollar if they can. No law needed. But they tend to look at the payback. The environmentalists don't give a damn about payback.

Imagine you are cleaning a room. You can get most of the dirt wth a broom and a mop, at a very reasonable cost. Doing it with a toothbrush and magnifying glass will get more of it, but the price goes up considerably. Environmentalists would have you cleaning your room with a microscope, and the cost would be enormous, but they aren't paying for it, you are.

Can there be a furnace that comes close to 100%? Maybe, but that last few percentage points is going to cost more than all of the other percentage points put together. The environmentalsts don't care, because it is your money not theirs. The end user should make the decision, not the government.

BTW, government stifles competition. I agree that competition is a very good thing. Get the government out of the way and you will have more of it.

_______________________________

Liberals are thieves and dictators, unlike conservatives who are dictators and thieves.