Need a tie breaker
I need a little advice on which way to proceed with a new system.
Im looking to have both the furnace and central A/C replaced in my house. The system is 18 years old and has worked fine but the A/C is starting to make me nervous. It seems to run for longer periods at a time and the electric bill is reflecting this.
My only complaint with the old system is I think it could dehumidify better on moderate days. The house is 2850 sq ft, with about 1350 of that being finished walkout basement. On days when the furnace is not running or the A/C is not on for extended periods of time the air feels clammy. Also, I think a two stage furnace would be nice for better winter comfort.
Ive had three contractors submit bids for the work. Ill list the specifics below:
Dwelling: 1500 sq ft Main floor, 1350 sq ft walkout basement. About 100ft of wall below grade. Well insulated and newer windows. Eight foot ceilings.
Trane 92% 80K BTU Furnace Single-Stage
Trane XE1000 2.5 Ton AC Single-Stage
Contractor #1: A little concerned that the AC would not run on the high stage much.
Heating: 80K, 96% 2-stage, variable speed.
Cooling: 3 Ton, 2-stage
Contractor #2: Concerned that on the hottest days, the unit would not keep up, but would dehumidify very nicely.
Heating: 85K, 95% 2-stage
Cooling: 2 Ton, 2-stage
No Load Calc. Based on lengthy conversation and performance of old system recommended the following:
Heating: 80K, 95% 2-stage
Cooling: 2.5 Ton single stage.
I appreciate any input you experts care to offer. Also, I'm an electrical engineer with a passion for home automation and AV systems. I'm happy to help any of you that I can with my expertise.
Lord....your home has a 70,000 BTU heat loss????? Wow, since it sounds like it's about a 1500 sq. ft home with most of basement finished. I'm assuming someone has actually done a manual J to confirm this? Short cycling a 95% furnace due to oversizing will result in a very poor efficiency and short life. What are the manual J figures on heat loss and gain?
OK, I'll need to talk these guys and see what the manual J figures are, assuming that's how they figured it. I think the BTU's that I listed were the required BTU's that the equipment would need to be capable of. But really, my biggest question is related to cooling and the fact that the current 2.5 TON seems to be sufficiently sized. And if I were to go 2-stage they are telling me that you either round up to 3 TON or round down to 2 TON.
Originally Posted by wahoo
Two TON seems a little light, but three TON might not ever operate in the high stage and would be a waste of money.
I'd have (or simply do) a manual J heat loss/gain computation, and then start from there. If you want better dehumidification, the LAST thing you want is to oversize the cooling unit. Maybe look into the two ton two stage with optional dehumidifier! But I'd get some idea of what you really need first as the older 2 1 /2 ton unit may have been too large and shutting off too much. You won't dehumidify if the unit is shutting off in hot high humidity weather.
A bigger or 2 stage A/C won't help this. Best bet is a whole house dehumidifier, especially since you have the large basement which is 40% or so of your total sq ft. This area has little sensible heat gain adding to the A/C needs but does at a lot of latent. Personally I'd go with a good single stage A/C with a good latent capacity and a good dehumidifier. Can be tied in to the duct system or just dry the basement which will lower the humidity in the entire house. Many are ventilating to bring in and dry outside air freshening the place.
On days when the furnace is not running or the A/C is not on for extended periods of time the air feels clammy.
I'm with the others questioning the 80K 95% furnace. The insulated basement will not add that much to the whole house heat needs. Of course if you are in a very northern, cold climate, you might need all that heat. Don't think I saw anywhere your location.
Northern Indiana, lakefront with a lot of windows facing the north.
a trane 3 ton 20i would do it 1.5 ton in first stage 3 ton in second stage
We really need change now
I'm wiht others. 60k and 2 tons should be plenty since most windows face north. The lake will also temper summertimes temps, but can raise humidty, especially at night. A 2 tosn 2 stage might help that clmminess, but as mentoned, a ventilating dehumidifier will work best... and with a whole house dehumidifier, it shifts load off the AC so a 2 ton will easily hold the load in a house that size.
Thanks for the all replies. I did talk to my contractors about their calculations and come to find out, I'm a little disappointed. Contractor #1 who claims to be Trane's highest volume and most awarded dealer in the Northern Indiana area, simply used data from a website that utilizes Google Earth.
Contractor #2 couldn't tell me the name of the program he used, actually his daughter who works in the office does the load calcs. But at this point, I'll give him credit for having something.
I ended up buying the residential version of Don Sleeth's HVAC-calc and here is what it is telling me.
Outside Temp: 100 degrees
Inside Temp: 75 degrees
Heat Gain: 25,262
So, based on this info, is a 2 ton sufficient?
Actually, here is the report.
Looking over the load calcs, the latent load is lower than I would have expected. ASHRAE suggest 500 btus of latent load per occupant. A home your size, should have a fresh air change in 4-5 hours. This is an additional 3,000 btus of latent load. Also consider that the latent loads are independent of the sensible cooling loads. That is the reason that the home is damp when there is low/no cooling loads. Without a significant cooling load, even a two speed a/c is unable to remove 2,000-4,000 btus of latent cooling load. With a naturally cool basement space, expect the home to be clammy at best. After a couple months of +65%RH, basements smell like basements and dust mites grow in bedding and favorite furniture.
To maintain <50%RH throughout the home and get adequate fresh air change, adding a whole house to a good basic a/c with keep the home at <50%RH and fresh air throughout. Now for the commercial, check out the Ultra-Aire whole house dehu, a sponsor of the site. There are other brands also. A 3 pint per hour dehu is minimum for your application.
Keep us posted.
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
Here's a good example of gigo (garbage on garbage out) summer design temp I'd NOT 100f. Only parts of Texas and az hold that distinction. Out only 87 for Valpo with a fairly humid 74f wet bulb. I Bet 100 had only happened 4 or 5 days since 1940 and your record is like 105. In comparison, out hot 100 or higher here i think 3 times last summer and in places like Texas, it a normal occurance mid summer
Originally Posted by lakebilly
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