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If I have a 1800 square foot home 8' ceilings. If I have a 4…

Customer Question
If I have a...

If I have a 1800 square foot home 8' ceilings. If I have a 4 ton heat pack unit on the roof what should I have coming out of the registers CFM's with regards ***** ***** and AC

Contractor's Assistant: Did you double-check the thermostat's settings? And what about the filters?

The filters are new and the thermo is new Honeywell digital.

Contractor's Assistant: Do you plan on doing the work yourself?

Its already when the heat kicks on it seems to throw out room temperature air for about 10 minutes.

Contractor's Assistant: Anything else we should know to help you best?

We have two registers supply lines we added another one when we placed the new unit on the roof, 16x20 and 20x30"

Submitted: 6 months ago.Category: HVAC
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Answered in 30 minutes by:
3/19/2018
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,878
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello, air flow requirements vary moderately with the design of the equipment, and vary further with the climate... beyond that air flow varies in some multiple speed blower systems with how loaded with dirt (restricted) the air filter is.

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To put some perspective on it, in cooling mode air flow requirements range between 350 and 500 cfm per 12,000 btu. hr of cooling capacity.... and 30 to 40% less air flow in the same unit for gas fired furnaces...the air flow being a little higher for high efficiency condensing furnaces.

The specifications for the equipment will call out the required minimum air flows for that equipment.... it was never perfect. Until lately with the variable speed blowers and two stage or variable gas flow gas valves... driven by a micro computer (IC chip) in the furnaces printed circuit board... that senses air temperature and humidity differentials as they change and adjusts the air flow accordingly... so that no fixed measurement means very much.

If your system produces room temperature air for 10 minutes when first going into heat mode, then the time delay circuit in the furnaces printed circuit board is bad, Or the thermostat is wired to run the blower at high speed as soon as heating is called for, I suspect the latter.

.

Take close up, exceedingly well focused photo's of the wiring diagram glued inside the furnace/air handler door, and the printed circuit board mounted on the blower, , that shows where the thermostat wires connect, and a photo of the thermostat sub base after you pry the thermostat from it, and the brand and model number of the thermostat, and I will mark them up with trouble shooting instructions.

Use the 'add files' link next to your send button to get the photo(s) to me.

.

We can go from there with training, coaching and diagnosis.... without a time limit, In depth with as much detail as needed .

.

When we finally get the situation solved, please select the 'finish and rate your expert link' near the top of your page and click on it.

.
My goal is to give you the best experience possible!
I hope I have earned a 5 star rating! Please remember to rate my service by selecting the 5 stars at the top of the screen before you leave today.

If you need more assistance, please use the reply box and let me know.

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Customer reply replied 6 months ago
Honeywell Pro TH3110D
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Thanks, click here for the manual on the thermostat you have

Ialso need to see how the thermostat is actually wired as mentioned earlier, that thermostat you have can be wired in many differnt ways, in some ways it will run the blower whenever the heat is called for and that will produce the cold drafts you are mentioning.

I still need the photo of the furnace wiring diagram or at least the furnace brand and model number from a sticker inside the same furnace compartment as the gas valve, that will tell me if you have a variable speed furnace or not, and from that I can pull the manual on that furnace that will have the air flow specifications.

That however is not relevant to your furnace blowing cold air for 10 minutes prior to the heat coming on, for that I need the wiring diagram asked for and you will need a multi meter and some considerable skill to perform the diagnostic steps I define.

-------------

If you do not have any of these skill, tell me which direction you are from the center of the nearest town or city and I will get you a short list of well rated service outfits that will arrive and correct the wiring errors,

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Customer reply replied 6 months ago
On the 4 ton gas pack unit which is what we have, and based on the 1800 Sq. foot home 3 bedroom etc. can you give me what should be a typical CFM reading coming from the registers on heat and AC?
HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Hello again, yes, generally 1200 cfm under a call for 100% heat output, and less if you have a variable speed blower and the thermostat is set at 70F with room only at 68F, then in that case the air flow will drop to around 900 CFM.... thats IF you have a variable capacity heating system.

Standard design velocity for registers is in 500 feet per minute under low heat output conditions and 750 fpm under a call for high heat.

.

A bit about myself, I am 77 years old, semi retired, earlier in my career I owned my own HVAC company and built it to 27 staff, we did projects in the $50,000 to $200,000 range at the high end. then in 1989 I sold the company and began consulting nationally, for fortune 500 companies, on projects generally between 5 and 100 million dollars, I became known for my ability to turn failed projects around and to that end was called to assist the US dept of Energy with problems they were having in our nations nuclear weapons labs... in 1994 I testified before the US congress on related issues for 2 hours. I still consult privately and answer questions for Just Answer

click here to get an idea of a typical project, this was 400 tons of minus 60F blast freezer equipment aboard the Hawaiian princess, I did all of the engineering, drawings, piping and electrical controls.

.

My remarks regarding variable air flows that will be seen with your equipment, lacking model numbers, are about as accurate as possible... air flow however is not the problem in your case, its the time delay controls that are supposed to keep the blower OFF until the burners warm the furnace up.

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HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Its time to rate my input for the day, that gets me paid, locate the "finish and rate your expert" link near the top of the page and click on it. I will appreciate that very much.

I am in every day from 9 am to 11 pm west coast USA time to do any follow up needed including coaching on a full diagnosis of the cause, you will need a multi meter and some skill with it, plan on 2 to 4 hours, there is no additional charge.

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HVAC Technician: Phil, Mechanical Engineer replied 6 months ago

Some background information, all furnaces of any type feature adjustable air flows in a range of plus or minus 30% from a mean, thats a lot, this is selectable by the installing contractor via dip siwtches in variable air volume systems and by choosing with speed among 3 or 4 that the blower motor is wired for.... then even with those selected and set, air flow will vary with how the air ducts have been sized... smaller ducts (very often too small) will restrict air flow.

In order to be complete, cooling air flows are 50 to 70% greater than heating air flows where gas fired furnaces are concerned, and only slightly less for heat pump applications.

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