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Old 10-23-2011, 11:41 PM   #16
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Repair or Replace 13 yr old Furnace with Cracked Heat Exchanger


I don't know of any free programs nor do I have a link but I know these other guys know of some and have links. You can try a site search to find it as it's been posted a few times.

You have to buy the brogram and input all data.

If you really want to study up on it you can Youtube search "Heat Load Analysis" and find some excellent lectures.

Some of are very lengthy as in an hour each and in four parts but well worth it.

These guys will be here tomorrow to post some links.

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Old 10-23-2011, 11:46 PM   #17
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There is a $50 load calc provided on another site.Someone should have that site handy for you.
I guess I'm miss reading your answer.Are you saying that as long as you can't see bad duct sizing,you want to live with it???
A maqnuel "J" is either a hand written exersize or a computer program using paradiems set forth by HVAC engineers.It is accepted in every state requiring load calculations for almost all homes.Anything else is wild eye guess
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Old 10-24-2011, 12:03 AM   #18
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I don't necessarily want to live with poorly sized duct work but am wondering how one goes about performing a Manual D. I'm not familar with how that is done and curious if it would make any difference if the ducts cannot be seen (e.g. is a manual D only used for designing?). Is it typical practice to request a Manual D during a furnace replacement?

For the manual J, it sounds like it should be standard practice for reputable companies to provide a handwritten or computer print out of the calculations. I'll check for one of those on-line programs you mention and also call the furnace dealers to see if they can provide something in writing.

Thanks again for all the helpful feedback.
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Old 10-24-2011, 03:17 AM   #19
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With different sized systems, producing different cfm, you'll be NEEDING new duct work. A Manual D is related to the Manual J by way of the J sizing the sytsem for a home which produces said certain cfm and the D allotting the full capacity of cfm as a total in a breakdown to each room via the duct.

Make sense? I think I lost myself on that one!

http://www.ahrinet.org/App_Content/a...alculation.pdf
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Old 10-24-2011, 04:31 AM   #20
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Quote:
Originally Posted by Thor73 View Post
Thank you all for the feedback! To follow up on a few of the threads, no company provided a written load calc but after talking to all three, it sounds like it is oversized. My current unit is 75k BTU. The Lennox dealer did not provide any load calc information when asked and specified a 70k BTU unit. The Trane dealer talked through his hand written calculations and specified a 60k BTU unit. The Bryant dealer said his experience tells him that the unit may be oversized but he specified a 75k BTU unit. The only dealer to do a static pressure test was the Trane dealer and he indicated that the test indicated that there is a pressure differential indicating the A-coil may be dirty.

Based on the differing feedback, I am discouraged by the quality of each company except the Trane dealer who seems better than the rest. However, I'm not sure that is a good thing.

Sounds like the most important thing to do is get the sizing right. Couple questions about sizing it correctly:

1) Should a manual J be more official than "experience" and/or hand calcs?

Yes, a load calc is more accrate then an off the cuff "by experience guess". Load calcs were done by hand before computers and software were around.

2) Is there an easy "Do it yourself" J calc or free on-line calculator?

Yes.
http://hvaccomputer.com/talkref.asp

3) Does a manual D make sense to do on an existing house where I have no exposed ductwork except the 6x8 furnace room?

Thanks for the continued feedback!
If the install manual is there for your old furnace. proper static pressure check used in conjunction with the fan data in the manual will tell you/them if your duct work is sized adequately for a new smaller furnace. Or if it is still under sized for a new smaller furnace9and how under sized it is for the current furnace.

Depending on how under sized it is, it may only need a minor alteration. Or none at all for a smaller furnace.
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Old 10-24-2011, 07:54 AM   #21
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Repair or Replace 13 yr old Furnace with Cracked Heat Exchanger


If your not changing the ductwork or the AC coil there is no need to do a manual J. You started this thread asking about a heat exchanger or a new furnace. If you want a new configuration then ask the service techs about a new system and not the cost of a new furnace vs. a heat exchanger. I would start by asking the rep. what he suggest for correcting this problem and then weigh my options from there.
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Old 10-24-2011, 11:16 AM   #22
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Quote:
Originally Posted by Patrick Eubanks View Post
If your not changing the ductwork or the AC coil there is no need to do a manual J.
Sure there is. Why put a 60, or 70,000 BTU furnace in when a 40,000 may still be over sized. but the duct work is better suited to a 40,000 then a 70,000.

A 70,000 95% should have 1231 CFM to have a 50F temp rise, while a 40,000 95% only needs 703 CFM for a 50F temp rise.
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Old 10-24-2011, 11:46 AM   #23
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Agree with this except my point is that correcting the sizing of a system needs to start with the AC. Sizing a system for heat is a moot point when you are handcuffed by a certain AC coil. 740 cfm is not going to be enough for a 3-ton AC coil. The furnace should be selected for its blower capability with the minimal gas that is available. A 3-Ton blower may only have 72,000 or 90,000 BTU available. I am a strong supporter of load calcs. but also believe that if your not going to adjust the entire system to the findings, your wasting your time.
I dont disagree with your point and on a heating olny system this is absolutely true. Maybee Im assuming too much in assuming AC is installed as well. Sometimes I have a tendacy to assume too much.
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Old 10-24-2011, 11:52 AM   #24
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Quote:
Originally Posted by Patrick Eubanks View Post
Agree with this except my point is that correcting the sizing of a system needs to start with the AC. Sizing a system for heat is a moot point when you are handcuffed by a certain AC coil. 740 cfm is not going to be enough for a 3-ton AC coil. The furnace should be selected for its blower capability with the minimal gas that is available. A 3-Ton blower may only have 72,000 or 90,000 BTU available. I am a strong supporter of load calcs. but also believe that if your not going to adjust the entire system to the findings, your wasting your time.
I dont disagree with your point and on a heating olny system this is absolutely true. Maybee Im assuming too much in assuming AC is installed as well. Sometimes I have a tendacy to assume too much.
Most 40,000 BTU furnaces have a 3 ton drive. So as long as the OP's A/C, if they have it, isn't more then 3 tons, it makes no difference weather they have A/C or not.

In some areas of Texas and some other western states, its common to have a 100, or 120,000 BTU furnace in a home that only needs 60,000 BTUs of heat, but needs 5 tons of A/C. I don't believe that is the case with the OP though.
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Old 10-24-2011, 12:22 PM   #25
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I dont think we are disagreeing on anything except the necessity of a load calc. I would select the minimal BTU available on 3-Ton blower if its a 3-ton ac. In my case, the minimum BTU available is 54,000. In severe cold climates 54,000 BTU might not get it done but Im betting the contractor instlling the unit has looked into this.
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Old 10-24-2011, 12:34 PM   #26
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I find that most contractors will just put in a furnace that they are sure won't give them a call that it can't maintain set point. Even if it means its 50% larger then needed.

It will be interesting to see if the OP does a load calc and comes back and post the results. Along with weather or not they have A/C and what size it is.
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Old 10-24-2011, 05:14 PM   #27
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Some more information and and update:

- My home is in MN and I have a 3 ton AC.

- I contacted all three companies about getting the load calcs and only one refused. Unfortunately, it is the Trane rep who had been the most helpful so far. He said he will share it if I wind up going his way. Not sure why he is holding this tight to the vest when it could lose him a job.

- Once everything comes in, I will share for comment.

Thank you for the continued feedback!
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Old 10-24-2011, 06:09 PM   #28
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So contractors don't want their work(load calc) being used by other companies, so that those companies can charge less because he did all the load calc work.
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Old 11-06-2011, 04:28 PM   #29
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Hi all -

I have some more information to run by the group for feedback.

A) I received two load calculations that are very similar and indicate a requirement of 45,739 BTU. This was then multiplied by a safety factor of 1.1 in each case to around 50,313 BTU. Both conpanies indicated that their 96% 60k BTU furnace would be the correct size.

They both checked the duct work and were satisfied with the size and quantity of delivery and return air vents. Neither said that they would do a manaul D calculation.

B) I did ask about the size of the furnace seeing that the blower compartment is 16" and my existing unit is 20" and what their plan was during installation to ensure the air flow was adequate. They responded differently:
1) build the unit up on a box so that it has a 20" opening that matches my current filter size and return air duct.
2) fit a reducer on the air filter to funnel the air into the smaller opening. They would also adjust the return air duct leading into the filter to ensure the air flow was appropriate and the static pressure was appropriate.
Any one way better than the other?

C) They offered two different thermostats. One a Braeburn 5000 and the other a Honeywell 800. Any thoughts?

D) Last but not least, the labor warranty was different. One was 2 years and the other was 5 years.

Thanks for the feedback!
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Old 11-06-2011, 05:03 PM   #30
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I prefer Honeywell thermostats.

Either way for the return will work. Its which one you feel more comfortable with.

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