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Old 08-28-2020, 02:07 PM   #1
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Replacing furnace, A coil question


Hey guys!

Question on sizing.. I have a Grandaire WSFR100C060A furnace I picked up to replace my aging system. The furnace is 100k input, 95% efficient. This is for a 1600sq foot home.

https://www.questargas.com/ForEmploy...daire_WFSR.pdf

I also picked up, in a separate transaction, a 3 ton Airtemp condensing unit and A coil # M00063706.

Now, reading the manual for the furnace I noticed it has a "cooling capacity" of 6 ton, with a CFM rating of 1520 head / 2145 cool.

So my first question is, would I have any problem using this A coil with this furnace?

I'm 'assuming' I could slow the blower down, since it has 5 speeds, but I just wanted to ask anyway.

I believe it's set up for natural gas, so I need to order the propane conversion kit for it. It's going to replace a 20+ year old Amana command 90 furnace and 3 ton r22 unit that's hobbling along.


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Old 08-28-2020, 02:25 PM   #2
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Re: Replacing furnace, A coil question


That’s an enormous furnace for that size house.
I’d take a step back and calculate how big of a furnace you need before installing or buying anything.
Do you have the ductwork for 3 tons of cooling or 100k of heating?
Is this used equipment?
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Old 08-28-2020, 02:41 PM   #3
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Re: Replacing furnace, A coil question


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Originally Posted by roughneck View Post
Thatís an enormous furnace for that size house.
Iíd take a step back and calculate how big of a furnace you need before installing or buying anything.
Do you have the ductwork for 3 tons of cooling or 100k of heating?
Is this used equipment?
It's only slightly larger than the 90k unit I have now, which runs quite a bit both in summer and winter.

Some background..

I did rip all the walls apart and replace all of the insulation, but 900sq ft of the house has 2x4 construction so R15 in those walls, and the ceiling space only allowed for an R30. The other 700 sq feet has R21 in the walls and r38 in the ceiling.

I have a very large fabricated plenum running the length of the ranch, with multiple 6 or 8" ducts coming off and going to each room.

The 'new' part of the house has sufficient heating and cooling although humidity control is very poor, but my older R22 system just isn't getting the A coil cold enough.

The 'older' part of the house has very poor heating and cooling. The past 10 years I've run a pellet stove in that section of the house to suppliment. While remodeling this summer, I replaced the pellet stove with a gas fireplace so I'm hoping the new insulation and new furnace help.

I also picked up a zone controller, and will be installing dampers with my sheetmetal guy so we can control the 'old' part of the house on a different zone since a lot of the time the 'new part' will be comfortable while people are either freezing or sweating in the old part. Some clever programming of the dampers and I'd think we can get the system to provide more run time to that section of the house.

The 'older' part of the house is on a crawl space with a non-insulated floor. The 'new' part has R30 in the floors. Attempts to insulate the floors on the 'old' part have been done over the past 30 years, but it always seemed to invite rodents. It just always ended up a gross disaster, so I've just accepted heat loss.

To give you an idea of how cold the crawl space gets, There is two sections - the main part which is the majority of the 'old section', and then a 12x12 kitchen at the front of the old section on a separate crawl space. That crawl space would get so cold in the winter that the water pipes would freeze. In a renovation, we moved the water supply to insulated cavities in the walls to stop that from happening. The crawl space is only 12" or so, and I can't get anyone to go in and spray foam the block walls.

Old houses suck.

The AC stuff is new. The furnace is newish, my buddy removed it from a house after 1 season to replace it with a bigger one for the customer.

My buddy is great at sheetmetal, brazing pipes, etc. Not so much at sizing systems.


20 years ago, before the new section of the house was built, the 900ft old section had a steam boiler and radiators. 130k btu boiler, and the house was still cold haha.


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Old 08-28-2020, 02:53 PM   #4
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Re: Replacing furnace, A coil question


Take a step back. A big step.
You now have a well insulated house and your increasing equipment size. And you already have humidity problems.
Run a manual J. And use manual D to design your ductwork. Multiple 6” and 8” ducts don’t necessarily add up to very much airflow.
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Old 08-28-2020, 06:04 PM   #5
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Re: Replacing furnace, A coil question


You may only need 40 to 60k of heat and 1.5 to 2 tons of cooling.

Yours ducts may not be large enough for what you want now and zoning can really compound problems.

if your duct system was done properly, in most cases zoning should be necessary provided you're okay with a small temp difference between floors.
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Old 08-28-2020, 08:57 PM   #6
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Re: Replacing furnace, A coil question


Quote:
Originally Posted by user_12345a View Post
You may only need 40 to 60k of heat and 1.5 to 2 tons of cooling.

Yours ducts may not be large enough for what you want now and zoning can really compound problems.

if your duct system was done properly, in most cases zoning should be necessary provided you're okay with a small temp difference between floors.
Hello,

So this is an 84 foot long ranch, so just one floor.

When we did the calcs PRIOR to re-insulating the old section of the house we came up with 39k cooling and 79k heating.

Now with that said, I did all new insulation in the old section this spring, and my cooling was still terrible in that part of the house. But, my old R22 system seems to run for 18 hours out of the day (on hot days), the A coil doesn't really get that cold, etc.

When we re-did the calc for the new windows, better wall insulation, etc we came up with 34k cooling and 75k heating.

Furnace is connected to a plenum that's basically 20x20, which then connects to the main duct which is 16x8. That duct runs a significant portion of the house in each direction.

Them like I mentioned previously, 6" or 8" round ducts are run to each room, depending on size, with 2 going to each room except for the bathrooms and laundry room.

Furnace is actually 70kbtu input, 63k btu output.


So my thought was to install the new furnace directly under the duct, so the duct connects to the plenum and has equal size in each direction - see the problem right now is it effectively has a choke point (see below picture). My thoughts are if I ran a section of 16x8 duct on either side of the plenum I'd effectively have one whole duct for one side of the house and one for the other, rather than it sharing a single outlet from the furnace. (if that makes sense)

I'm open to suggestions. I could keep the furnace I have and just fix the duct work + replace the R22 unit (which I am positive is on its way out, plus the outside coil is severely rotting away). But the furnace is 20 years old.

Thank you much!
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Old 08-28-2020, 09:06 PM   #7
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Re: Replacing furnace, A coil question


Those ducts are undersized. You need ~1200 CFM for a 3 ton unit. That transition looks like a choke point.
You can’t base equipment size off an old unit with a rotten condenser.
You really need a room by room manual J with a manual D to design the duct system.
How did you decide what size duct to run to each room?
What is your design CFM at what static?
These are things you need to be able to answer when designing a system.
What calculation did you use for determining size?
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Old 08-28-2020, 09:12 PM   #8
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Re: Replacing furnace, A coil question


I find it difficult to believe reinsulating walls only dropped heat loss by 4k btu/hr.

The load calc numbers just don't look right - too high based on the limited info. Garbage in = garbage out.

It's easy to over-estimate air leakage as well as mess up the other inputs - double dip on heat loss. (ex - put in floor over unheated area as well as uninsulated foundation)

What did you use for the load calculation? Software, website, spreadsheet?

Can you post a breakdown of the inputs?


*My previous most of worded incorrectly, meant in most cases zoning should not be required.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 08-28-2020 at 09:21 PM.
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Old 08-28-2020, 09:25 PM   #9
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Re: Replacing furnace, A coil question


Quote:
Originally Posted by roughneck View Post
Those ducts are undersized. You need ~1200 CFM for a 3 ton unit. That transition looks like a choke point.
You canít base equipment size off an old unit with a rotten condenser.
You really need a room by room manual J with a manual D to design the duct system.
How did you decide what size duct to run to each room?
What is your design CFM at what static?
These are things you need to be able to answer when designing a system.
What calculation did you use for determining size?
Hello,

So I used some HVAC software many years ago. It took all sorts of inputs, room sizes, window types, insulation, wall height, ceiling insulation, foundation depth, etc etc.

Originally I had a "spider" design that I interpreted after reading the D manual. (this is like 12+ years ago). It worked well, but with all sorts of round ducts running everywhere it was difficult to do any work in the crawl space.

Soooo I came up with the idea of running a long main duct, furnace in the center, and coming off that to each room. (Loosely based on what I read in the D manual). This figures in 16x8 duct based on 700 CFM to one side of the house, and 700 cfm to the other. (Furnace max was 1540 CFM so I figured it would work ok)

I asked my buddy to do the plenum to main duct transition since he's really good with sheet metal, however there was a miscommunication about what I wanted vs what he did. My intention was that he would connect BOTH sides of the house to the plenum indvidually, but he instead ran the main duct, then ran another 16x8 to it from the pleum, which tells me it's a 700cfm choke point. That's what I get for leaving him alone for the day.

The return has always been a single return in the middle of the house. It's 22x12.

My solution, temporarily, was to lower the blower speed when cooling. One would think that would cause the A-coil to freeze up, but it didn't because something is wrong with the condensing unit.

The new furnace has a CFM range of 1225-2145, so I figured if I 'fix' the plenum to main duct connection it still falls within that range, however I do understand that oversizing the heating BTU could be problematic so I am looking for advice.

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Old 08-28-2020, 09:34 PM   #10
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Re: Replacing furnace, A coil question


Quote:
Originally Posted by user_12345a View Post
I find it difficult to believe reinsulating walls only dropped heat loss by 4k btu/hr.

The load calc numbers just don't look right - too high based on the limited info. Garbage in = garbage out.

It's easy to over-estimate air leakage as well as mess up the other inputs - double dip on heat loss. (ex - put in floor over unheated area as well as uninsulated foundation)

What did you use for the load calculation? Software, website, spreadsheet?

Can you post a breakdown of the inputs?


*My previous most of worded incorrectly, meant in most cases zoning should not be required.

I understand and agree, mistakes on inputs could have causes gross errors in calculations. Unfortunately, I don't have access to the software anymore and have not in years.

I've looked at websites that have similar inputs but they also ask for a lot of additional information I don't know how to answer.

I know the construction, rough R values of each wall, dimensions of walls and which direction they face. I've spent hours with my IR probe checking various parts of the house figuring out the thermal envelope - I can say that a lot of my loss is from non-insulated floors over the crawl space, and probably also the source of humidity?

The break down is this:

1600 sq feet
84 foot long ranch
8' interior walls.
R21 on two 40x8 outside walls, and one 17x8 outside wall. R15 on all remaining walls.
R38 over 700sq of ceiling. R30 over the remaining 900sq.
R30 floor insulation on 700sq, over a basement with 7 foot concrete walls.
No insulation on 900sq floor over crawlspace and block foundation walls (barely frost protection).

Windows 15 windows in total, all double pane argon (2010 or newer), with about a total of 70 sq feet of glazing.

Entry door is a double plane glass 80x36.

-- Joe
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Old 08-28-2020, 09:41 PM   #11
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Re: Replacing furnace, A coil question


Quote:
The new furnace has a CFM range of 1225-2145, so I figured if I 'fix' the plenum to main duct connection it still falls within that range,
It doesn't work this way.

The range is only what the blower is capable of over a wide static pressure range, not airflow required.

3 ton cooling - typically 1200 cfm
95 000 btu output at 55F temp rise, middle of the range = 1600 cfm
(1600 cfm is too much airflow for a small house)

Now, that furnace can technically run as high as 70F rise without tripping limit, but really shouldn't if you value lifespan and efficiency. 1257 cfm minimum.

Blower speeds get set to deliver proper airflow in each mode.

Needless to say, the 100k furnace is totally inappropriate for this house -> it will short cycle, be noisy and drafty.

I would not use it just to save a buck.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 08-28-2020 at 09:57 PM.
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Old 08-28-2020, 09:50 PM   #12
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Re: Replacing furnace, A coil question


Quote:
No insulation on 900sq floor over crawlspace and block foundation walls (barely frost protection).
This needs to be fixed! At a minimum, insulate that floor!
The heat loss is probably through the roof especially if the crawlspace is ventilated.
It's probably really contributing to difficulty keeping the old part of the house comfortable.

Really, it's far better to air seal, insulate the crawlspace walls, put a vapor barrier down on floor if it's soil.

Insulating a foundation is far more effective than a floor. (ideally with rigid insulation, not batts) Also, ducts should really be in the insulated/conditioned space.

The vents to outside get closed/sealed if applicable. In the summer, you run a dehumidifier if required.

For winter, get some heat in there to keep surfaces warm and condensation free - one or two 6" ducts plus some return.

Effectively, crawl gets converted into a mini-basement.

-----------
It sounds like you know enough about the house to do a load calculation properly.

What you don't know shouldn't be difficult to figure out.

https://www.loadcalc.net/ is simple.
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Old 08-28-2020, 09:58 PM   #13
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Re: Replacing furnace, A coil question


Quote:
Originally Posted by user_12345a View Post
It doesn't work this way.

The range is only what the blower is capable of over a wide static pressure range, not airflow required.

3 ton cooling - typically 1200 cfm
95 000 btu output at 55F temp rise, middle of the range = 1600 cfm
(1600 cfm is too much airflow for a small house)

Now, that furnace can technically run as high as 70F rise without tripping limit, but really shouldn't if you value lifespan and efficiency. 1257 cfm.

Blower speeds get set to deliver proper airflow in each mode.

Needless to say, the 100k furnace is totally inappropriate for this house -> it will short cycle, be noisy and drafty.

I would not use it just to save a buck.
Thank you, I appreciate the info. So perhaps I should just focus on re-working the duct a little, and replacing the cooling part if my existing furnace otherwise works properly and is sized ok.

The insulation is a huge issue.. The kitchen is the worst, basically 12" to soil with no room to do anything. I kinda wanted to pay someone to just spray foam the foundation walls but nobody is interested.

The rest of the crawl space is also very very difficult. Previous owners have tried installing insulation batts but rodents end up making homes out of them, the staples and such got ripped. Just a disaster.

A lot of these ranch's in NH were originally seasonal and hunting buildings. My house was originally built in 1945 as a 1 room 900sq 'camp'. It's been through lots of remodels, but nothing significant was ever done with the crawl space..

the dirt is uneven and random rocks/boulders were left in place, so even 'capping' it with concrete wasn't an option when I built the addition..

-- joe
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Old 08-28-2020, 10:04 PM   #14
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Re: Replacing furnace, A coil question


3 ton a/c may not be the right size.

NH is a northern state with mild summers, right?

I imagine where it gets it gets to over 100f, the 3 ton may be necessary.

For the northeast where 90F is a hot day and it rarely hits 95, 2 ton could very well be correct.

Do not base on sizing on performance of what you have - especially with that duct bottleneck.

The heat exchanger of your current furnace really needs to be inspected before making a decision as it may be cracked. 20 years is normal lifespan.



Quote:
the dirt is uneven and random rocks/boulders were left in place, so even 'capping' it with concrete wasn't an option when I built the addition..
What about putting a vapour barrier over the soil to stop all the moisture from coming up?

That's absolutely necessary if you insulate the walls and close the vents. Otherwise - off the table.

Quote:
Previous owners have tried installing insulation batts but rodents end up making homes out of them, the staples and such got ripped. Just a disaster.
How's the insulation in the basement portion? I see nothing on the foundation.

Batts are not good in a crawlspace.

If spraying the foundation walls or putting rigid boards isn't practical, what about spraying in between the floor joists? Doesn't encapsulate the crawlspace, but at least greatly reduces floor heat loss.
It is a nasty job either way.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 08-28-2020 at 10:14 PM.
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Old 08-28-2020, 10:15 PM   #15
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Re: Replacing furnace, A coil question


The main duct you have run from each end of the house, is it a consistent size or did you reduce the duct size for lower air volume?
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