Concerns on installing undersized equipment
Thank you in advance for reading my post. I am new to the site and need some advice as I am in the middle of replacing my heating and cooling system. I live in the Boston area and have a relatively small 2-story house (1,300 sq feet) plus a basement.
My current system is a Carrier 10 SEER 3 ton R-22 condensor and an Armstrong 80K BTU oil fired furnace (in basement). I am looking to switch over to gas. My conractor did a manual J-sizing of the house and is recommending a 2 ton American Standard Allegiance 16 SEER two stage air conditioner and a American Standard 3 ton 60,000 BTU 96.7% Ultra High Efficiency 2-stage furnace.
I have a couple of concerns I was hoping to get some advice on:
(1) The equipment seems much smaller than what I had. I know that my equipment may have been oversized but what risks do I run if the new equipment is undersized? I don't have a seperate unit it the attic like most new houses do to cool the second floor and I use to have a 10+ degree temperature differential between the first floor and second floor during the summer. I had someone come out and look at the system and I was told that my duct work was "unbalanced". The HVAC technician closed some vents on the first floor and that seemed to greatly help in cooling down the second floor. American Standard makes a 2.5 ton condensor as well but it is single stage compressor and only 15 SEER. I wanted to consider intalling this one to make sure the house would proprerly cool down on the second floor (as you may be able to tell I am afraid of moving down a whole ton based on my prior experience above).
(2) I also heard that the system should be properly matched (condensor, coil and furnance). Does that mean if I have a 2 or 2.5 ton condensor that the coil and furnance both need to be the same tonnage? The current proposed configuration would have a 2 ton condensor but a 3 ton furance.
(3) I would like to move up to a 80K BTU 95% American Standard 2-stage furnace as we are going to add a register in the basement to assist in heating it. The furnace produces 49.5K BTU's on the first stage which I would think would make it run on the first stage more often if it is slightly oversized which I would think would be good. What drawback is there if any if I slightly "oversize" the furnance according the J-Sizing?
I know I posted alot of information here but I need some assistance. Any help would be greatly appreciated.
If the contractor did a proper Man J and sized accordingly it is still over sized. Man J has built in fudge factors that over size by 10 -25% depending on who you talk to. I would be more concerned that the duct imbalance be corrected. We have taken out a lot of equipment that wouldn't keep up , replaced it with the same size or smaller and it worked just fine once the duct was fixed.
The duct issues need to be corrected and improvements to the building envelope should be considered, then the load calculations equipment selection can proceed.
If you are comfortable that the structure is as "tight" as you can make it, just be clear with the contractor that the basement is going to be heated it was included in his manual j calculations and considered when the equipment selection was made.
Did I say the duct issues need to be corrected? It will not matter what size or efficiency the new system is - if the air does not move properly through the house you will not be able to balance it properly.
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Fixing the ductwork will have a much better payback than going with a 16SEER A/C unit.
Did you get a quote for a variable speed furnace, or is it just a 2-stage? If you're fighting temp differences between the floors, a varialbe speed blower motor running all the time will help alleviate some of that temp delta. Also, I'm not sure you should be terribly concerned over a high seer AC in your neck of the woods. Unless Boston is hotter than I realize, a more efficient air conditioner won't run long enough during the year to pay for itself, so it would have to be a comfort decision (i.e. you want to get to 2-staging that requires a min of 16 SEER). I think your money would be better spent either getting your ducts fixed or installing a zone system so the 2nd floor can get the cooling it needs and you can be more comfortable year round (and actually save money by not undercooling the first floor in order to try to "force" more cold air upstairs). Personal opinion - 2-stage variable speed furnace, 13 SEER single stage AC, Zone system. I think you would be happier overall and cost difference wouldn't be too much if you step down from the 16SEER your condsidering currently
The furnace is in fact a variable speed 2-stage furnace.
Originally Posted by frumper15
How do I correct the duct work? Is there a calculation or measurement the contractor can do? The duct work is rigid sheet metal already framed into the house. Do I have to rip out the duct work and related finished areas of my house or can some of the ducts be closed or boxed off to re-direct air flow?
Proper procedure would be to do a room you room load calc, which tells the contractor how air each room needs. From there manual dampers can be installed in the individual heat runs and set to control the amount of air that goes to each room. Some finished areas my need to be opened up so that damper can be installed, it all depends on duct layout and basement finish.
3 tons sounds B I G for a 1300 sq ft house in MA. 2 ton sounds more in line as does 60K 95%. You find that that rarely goes off of low except in bitter weather.
I agree with 54, doing other stuff to the house like the right ductwork & insulation will pay back so much faster than the XL16i.
My cousin is building a new home in Michigan that the HVAC guy quoted a 16i. I pointed out that likely the unit will be outta there before he breaks even on the huge amount that costs over the XR13 which got 14.00 SEER with the XV95.
Yep, in Norwest Indiana/South Chicago where we're at, the higher SEER AC units simply won't pay for themselves in cooling before they die. It has to be a comfort decision to get 2 stages. Now, if they're on bottle gas and could use a heatpump, maybe then we're talking about the economics of payback. I would suspect Boston and Michigan are similar cases.
Oversizing the AC unit will make the "two story" effect even worse. Short run cycles will amplify the difference between the two floors simply because the unit is not running, so heat rises to the upper floor. Longer run cycle length (smaller unit), should greatly improve this, and if you're concerned just discuss your concerns with the contractor. Make him put his promises in writing if he is sure it'll perform as you requested!! Otherwise ask a bunch of the previous questions as it sounds like your old system was over sized. I'd also recommend the variable speed blower if it's available as you can get really long run times for less electricity use!
Some of you temp difference between the first and second floor was caused by the over sized system. Letting the contractor install the proper sized equipment as he suggest will help improve your comfort on both floors.
You reaped the woes of an over sized system, time to enjoy the benefits of a properly sized system.
Also in Massachusetts, the AS 15 SEER condenser, when paired with 96% 2 stage VS furnace is capable of 16+ SEER and also qualifies for the rebate from Masssave.com... This would save you some $ up front and you would reap the benefits of the rebate! Of course, proper sizing is key. Ask to see the load calculation.