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AC and Furnace Replacement

Posted by novaguy70 (My Page) on
Tue, Jul 31, 12 at 14:40

All,

My A/C stopped cooling and was determined to have a leak (not explicitly identified - the coil was frozen and the unit had to be recharged with 5 lbs of Freon) I am looking at replacing the A/C unit and since the furnace is 16 years old, am also looking at replacing the furnace. Need assistance in determining the replacement.

Current System:
AC - Trane XL1200, 2.5T
Furnace - Weathermaker 9200, 60K BTU, 90% AFEU

I received the following quotes:

Contractor 1:
Trane
AC Unit: 4TTX5030A1000B
Coil: T4TXCB036BC3HCA
Furnace: TTUH2B060A9v3va

Contractor 2:
Carrier
AC Unit: 24ANB630
Coil: CNPVP3017
Furnace 59TN6A060v17-14

Contractor 3:
Bryant
AC Unit: 186BNA030
Coil: CNPVP3621ATA
Furnace: 986TA048080

Question: Contractor 1 and Contractor 2 are recommending 60,000 BTU furnace, while Contractor 3 is recommending 80,000 BTU furnace.

The Manual J computation done by one contractor indicates Heating BTU of 27K and Cooling BTU of about 24K.

Since my master bdrm has 4 registers (bedroom plus sitting area) and no return and since the door is closed during the night, we are also having an additional return installed.

Question: What factors should I consider in the selection of furnace BTU? Is 80K BTU an overkill? What precautions should I take with respect to the duct size and the return size? Will the heating CFM be too high with 80K BTU and create static pressure building in the ducting? The total number of registers in the upper level (including bathrooms) are 9 and 1 return vent.

What other factors should i consider in determining the furnace BTU size? I don't have any data on the duct leakage. It is a 24 year home.

Any assistance/analysis will be appreciated.

PS: The contractor suggesting 80K indicates that the CFM can be toned down if necessary.


Follow-Up Postings:

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RE: AC and Furnace Replacement

Also, this is for the upper level which is about 1660 sq ft. The other AC unit which is 3.5T (XL14i) services the main level and the basement. The furnace I believe is the same though not sure on the BTU rating.


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RE: AC and Furnace Replacement

Another question:

Contractor 1 and Contractor 3 are quoting a 3T coil for a 2.5T condenser unit. Is this ok?

Have to pull the trigger soon and would appreciate some feedback.

thanks,


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RE: AC and Furnace Replacement

How well did your current AC work before it went bad? Did it cool sufficiently? Was it running constantly on hot days?

As to the ManualJ, what were the temperatures they used? (inside/outside temps for both winter and summer)


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RE: AC and Furnace Replacement

Summer: outdoor dry bulb : 91, indoor dry bulb: 75
Winter: outdoor dry bulb : 23, inddor dry bulb: 70

AC worked well except for the master bedroom which is over the attic and felt a little cooler/warmer than the rest of the bedrooms. On hot days, yes it would run constantly.


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RE: AC and Furnace Replacement

I would go with number 2 and over sizing the furnace is a bad idea and completely kills the efficiency of the unit. On/off and short cycling is no good.. the furnaces today and designed to run for longer periods with dc motors.


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RE: AC and Furnace Replacement

You can use a 3 ton coil with a 2.5 ton condenser. Contractor No.2 should also be able to use a 3 ton coil. The price differnce is very small.

The 27K BTU heat load calculation seems low. If it true the 60K furnace is overkill. It will rarely operate in the high stage.


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RE: AC and Furnace Replacement

Is there a benefit to using a 3T coil with a 2.5T condensing unit?

I had the manual J computation done by another company and they came back with similar number: total heating - 21871, total cooling - 29004. Actually, this guy had the orientation slightly different, while the first guy had the correct one (se when facing the main door from the outside - 2nd one had it facing east and hence the higher cooling and lower heating). Both had accurate building size, windows, registers and skylights.

Again manual J was for the upper level only, about 1664 sq ft.

Also, had a concern with the duct size if 80k BTU furnace was used. Is there a way to determine the size of the ducting (registers, return, supply) for a given btu furnace based on the low/high heating cfm?

Thanks,


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RE: AC and Furnace Replacement

Also, one thing I forgot to mention was that the coil and the furnace are in the basement and not in the attic. So the ducts are running from the basement into the attic and then distributed across the various bedrooms in the upper level. There is only one return in the hallway (as indicated previously, planning to add another return in the bedroom).

Not sure if that impacts the cooling tonnage and heating BTUs.

Thanks,


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RE: AC and Furnace Replacement

Its very rare to find a home that has been ducted perfectly.
Some people us a slightly larger coil to increase the efficiency of the ac. If you go to large then the ac won't run like it was intended to, but 3ton for 2.5 is fine.


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RE: AC and Furnace Replacement

Novaguy

What is your location?

Current furnace is high efficient condensing model that is already vented through an external wall?

You definitely do not need an 80 KBTU furnace. I assume you want a two
stage var speed furnace. Correct?

The correct thermostat matters in all your quotes. Anything about a whole house air filter cabinet? New linesets to be provided? All details that should be covered.

I like quote 1 and 2-all good equipment. I would think your decision should boil down to price, warranty, your confidence in dealer.

Ductwork should have a thorough inspection.

IMO


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RE: AC and Furnace Replacement

Your present furnace is 60K BTU. Did it heat the upper floor adequately? If so, then why are you considering going to a 80K BTU?

I would want a measurement of the static pressure with the current system. If it is good then the main duct work sizing is good. You need 1000 CFM for the AC. This requires a duct with a 14 inch diameter.

I find it unusual the duct work comes up from the basement and then is distributed in the attic. Your contractor needs to carefully inspect the flexible duct work in the attic. This is where most of your losses come from. I am not sure about having a return going directly from the attic to the bedroom. My concern would be cold air falling out of the return during the winter.


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RE: AC and Furnace Replacement

Tiger:
Location is northern VA, western suburb of Wash DC (about 25 miles). Ok, the contractor #1 indicated that he had selected 3T coil to improve the dehumidification. The cooling would have no impact. Correct - I prefer 2 stage variable speed. The current furnace, though high efficiency, is single stage.

As for thermostat - Trane guy has quoted T802, Carrier guy has quoted SYSTXCCVID-01 while the Bryant guy has just indicated "evolution communicating digital programmable thermostat".

Lineset - All have indicated that lineset would be flushed and nitrogen pressure checked for leaks. The Bryant guy has proposed to replace the lineset at no cost if the pressure test fails. All others have quoted ~$1000. The drywalling would be my responsibility. The basement is fully finished so extensive work is required to replace the lineset. All indicated that 95% of the time they just flush and pressure test the lineset rather than replace. Replacement is done only when the tonnage has changed and requires differently sized line set.

Mike,
No, I am not, I had the same concern with the Bryant guy as to why he was proposing 80K BTU; I had concerned both from the fact that I would always operate in the first stage and end up costing me more from a utility perspective. Bigger concern was what if the duct sizing was not adequate for the higher CFM that would result.

When you say duct with a 14 inch diameter, which duct are you referring to? the flex ducts going to the registers or the supply duct? For the return to the bedroom, given the layout of the house, all are planning to tap it from the existing return duct, run it through the attic and then all 3 contractors are planning to set it up in the ceiling of the master bdr away from the registers. Hmm, no one brought up the issue about the cold air falling out of the return - it makes sense though (cold air pushes down rather than up)

Question: What should I be looking for in the quote as far as duct inspection and testing goes? Currently, nothing is listed? What should I ask for inclusion?

Thanks,


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RE: AC and Furnace Replacement

Affordable,

I agree with you. It did not make sense, especially when the Manual J computations are not backing it up and there have been no new additions.

Thanks,


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RE: AC and Furnace Replacement

Tiger,

Missed on a couple of responses:

Yes, furnace is vented externally.
I have a Trane electrostatic air filter for both zones. All quotes are with the reuse of the existing filter and cabinet. I asked Trane guy what the clean effect filter as there is a Trane promotion running for my area. He indicated that it is not completely free (even though Trane claims it to be) and it would be an additional $600 (it seems Trane expects the dealer to pick up a piece of the rebate and does not include installation). BTW, the Trane guy was the lowest - has no padding on other portions of the equipment. He is well reputed both through references as well as Yelp/Angies List.

Thanks,


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RE: AC and Furnace Replacement

Nova

You are in Northern Va?

Pay attention to my suggestions on lineset and thermostats.

I do not like electrostatic air filters. If you are happy though and believe it to be functioning properly and the usual regular PM/cleaning is not an issue, then keep it. I personally prefer the passive pleated media air filter cabinets where changeout is easy and up to one year life.

IMO


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RE: AC and Furnace Replacement

Tiger,

Correct, am in Northern VA.

Is there a particular thermostat that you recommend for Trane as well as Carrier? I am leaning towards Trane or Carrier. Supposedly, Trane is offering Comfort Link II as free in this area (didn't pursue it as I am not planning on a 2-stage condenser and Internet access was hardly a feature that I cared for)

On the lineset, yes, I have reviewed your previous post. I am debating - it is kinda expensive. If the pressure test is successful and flushed properly, do you see an issue? What major issues do you forsee with re-use of existing lineset? Have there been instances where not changing lineset have caused issues? All 3 contractors were kinda surprised that I asked for a quote on lineset replacement as a result of transition from R-22 to R-410A. Seems that they don't get that request in this area.

Finally, on the media cabinet, I am not a fan of the electrostatic filter. It came with the house. I asked one of the contractors and he indicated that I could re-use the cabinet and replace the pre-filter and electrostatic cells with a 4 inch filter (to be replaced every 6mos - 1 year)later when I have issues with the current filter. So didn't really pursue it further. Your suggestion is to just rip out the current filter/cabinet and replace it with a newer cabinet/filter since I am having the entire unit installed?

Thanks,


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RE: AC and Furnace Replacement

The 14 inch diameter is the rule of thumb for 1000 CFM. This would be the main duct coming off the furnace. As it splits it will be smaller. I assume you have hard ducts in the basement up to the attic.

Are you sure you need a return in the master bedroom? Do you see any difference by keeping the bedroom door open versus closed? Turn the AC on high, and open the bedroom door an inch or two. If the air pushes the door, then you have an air pressure problem and would benefit from an additional return. If the door doesn't move, then I question what this added return will do for you.


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RE: AC and Furnace Replacement

All,

I had the attic insulation upgraded to R-60 (blown-in) this spring. All the ducts are buried in the insulation. The guy is supposed to have checked for leaks and sealed; though not sure if he really did that.

Thanks,


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RE: AC and Furnace Replacement

Mike,

The supply duct is going up from the evap coil. The coil sits on top of the furnace. I was looking at the specs of the coil/furnace, seems the width is 17 1/2 / 15 5/8 (outer / inner)where the 2 meet. The supply duct running up is rectangular. I don't have the measurements on it but can check. What should it be?

Thanks,


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RE: AC and Furnace Replacement

You want the rectuangular area to be equivalent to a 14 inch diameter which is abut 154 suare inches. A 20 X 8 inch duct would be good, a 18 X 8 would be acceptable.


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RE: AC and Furnace Replacement

for your manualJ temps, this year we have had over 35 days over 95* and a week over 100*. 91 is a bit low.

As to 23* for winter, that seems kind of low for an average temperature.

Having the R60 in the attic may be what is keeping the numbers down. Certainly think that you don't need the bigger furnace, though the idea may be in order to get a bigger blower. Guys here can comment about that.


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RE: AC and Furnace Replacement

All,

I got a last minute quote from a rheem guy that i would like feedback from you guys.

RGRM07EMAES - questioned the 75,000 btu; indicated to get better cooling tonnage for the 3T coil.
RAPM030JEZ
RCFLHM3617CC
Honeywell 6000 programmable thermostat
Lineset flush

How does Rheem compare with the carrier quote. The price differential is about $1500.


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RE: AC and Furnace Replacement

Tiger,

Would HW VP IAQ work with Rheem 2 stage VS furnace?

Also, looked at the specifications, cooling CFMs for both 60k and 75k btus seem to be the same. The only difference is in the hi/lo heating cfms. They are 725/900 versus 775/1080 for 60k and 75k furnaces. Is this a big deal? What should the heating CFMs be? I have 9 registers and 1 return as indicated previously.

Thanks,
Novaguy


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RE: AC and Furnace Replacement

Novaguy

Pure nonsense on the 07 furnace. Drop back to the 06 model which will give you plenty of BTUs and you will run in low stage except for those cold days.

Wrong thermostat quoted. You want a true two stage thermostat that controls staging not the timer on the control board which really makes you operate in high stage practically all the time.

The HW VP IAQ is a true two stage thermostat plus it allows forbthe dehumidify on demand feature which is great.

RAPM is a nice choice AC condenser. Gets to 15 SEER.

IMO


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RE: AC and Furnace Replacement

Tiger,

Thanks for the feedback. Also, how does rheem furnace compare to infinity? Any feature that i would miss out on by going with Rheem? one thing i noticed was that RGRM does not have hot surface ignition; looks like electric ignition. Not sure about the pros and cons.

Finally, any issues with the heating CFMs. Not quite sure how to evaluate the heating CFMs. Carrier Infinity is rated at 855/1075 for lo/hi.

Thanks,
Novaguy


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