Advice on new furnace and AC unit

Reply

  #1  
Old 03-28-15, 06:57 PM
K
Member
Thread Starter
Join Date: Mar 2015
Posts: 10
Received 0 Votes on 0 Posts
Advice on new furnace and AC unit

I was consulted by a project manager of Central Cooling and Heating up here in the northeast (MA), and he told me to get the following for my roughly 1900-2000 sq foot home. Do you think the below items are appropriate given the size of my home? Since I am a new homeowner, I would like any and all advice. Apparently, the units I have now are quite old, from 1992-1995 and too big for my house (existing furnace has 6 burners)...

Furnace - Carrier 80K BTU Modulating Gas Furnace
AC - Carrier 3 Ton 17 SEER 2-Stage condenser with 410a refrigerant

I have a proposal from him and he said he will do a heat loss manual J calculation before proceeding. He will also help with the rebate forms from my energy companies.

Thanks
 
Sponsored Links
  #2  
Old 03-28-15, 08:16 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 14,254
Received 25 Votes on 23 Posts
You can do your own manual J & I suggest you do so.
 
  #3  
Old 03-29-15, 07:00 PM
K
Member
Thread Starter
Join Date: Mar 2015
Posts: 10
Received 0 Votes on 0 Posts
Do you think 13k for the above items and installation is an appropriate price? There will be some rebates I can get from the energy company, so the prices should go down to around 10k ish. But the 13k ish price is what I will be paying upfront...
 
  #4  
Old 03-29-15, 07:16 PM
user 10's Avatar
Member
Join Date: Mar 2004
Location: NA
Posts: 2,068
Received 44 Votes on 38 Posts
That's a lot of money. I would personally go for less expensive, simpler equipment. After the warranty, you'll be paying through the nose for very expensive motors and circuit boards. (carrier parts are known to be expensive)

Furnaces are sold based on input BTUs, but it's the output BTUs that count. If your current furnace is a 100-110 btu mid efficiency, an 80k 95%+ model is a direct replacement capacity wise.

The 80k furnace quoted is large for a 2000 sq ft house unless poorly insulated or in an extremely cold climate. (...or a bungalow)

You might be able to get away with 60k. A load calculation must be done if you have oversized equipment now.

The a/c i'm not sure about, but it may be on the large side unless you get above 90-95F frequently.

Oversizing reduces the benefit of a modulating furnace. You can get similar performance from a 60k two-stage furnace at a lower installed cost.


Two stage goes down to 66%, modulating goes down to 40%:

That's 32k for the 80k on low and 40k for the 60k 2-stage. You won't miss the 8k btu difference and would still get long cycles on low with the smaller 2-stage.

In order to take advantage of the modulating feature and get long cycles on low, you need a very expensive (think $400+) carrier proprietary t-stat. Otherwise, it will run on low only for a short time and switch to high to quickly finish the cycle. The 2-stage can be used to it's potential with a basic multi-stage honeywell.




High SEER 2-stage a/c is also of questionable value. Two stage condensers have a circuit board and sometimes a very expensive variable speed fan motor.

Also if you only need 2 or 2.5 tons of cooling, the 3 ton will function like a smaller single stage condenser and never hit high.

You can get up to 15 or sometimes 16 seer with a simpler, cheaper a/c. Two-speed may give better humidity control, but you can get good dehumidification with single speed if properly sized. (...you can also get a humidistat connected to the furnace control board that will slow down the fan when the humidity exceeds a certain threshold.
 

Last edited by user 10; 03-29-15 at 09:56 PM.
  #5  
Old 03-30-15, 08:12 AM
K
Member
Thread Starter
Join Date: Mar 2015
Posts: 10
Received 0 Votes on 0 Posts
Thank you for the input.

Like I said, I am new to all this, and I will take your comments to heart and try to learn from them to better my own furnace/AC experience. I contacted my contractor to make sure a heat calc will be done, so I know what size equipment we should in theory be getting.

I forgot the price (10-11k after discounts) also includes making the return vent/duct bigger since the contractor said currently it's not supporting proper air flow and/or it may be too small.

The house is kinda poorly insulated admittedly I think. I had to use some plastic to cover some windows this winter and my exterior is the really durable cement asbestos shingles. There is nothing wrong with the shingles, but I have a feeling the insulation behind them may not be as good as that behind the vinyl siding. I live in a suburban town out of Boston, MA and the houses here are quite old, so I am not surprised.

In my head, I guess one advantage of a modulating furnace is that it avoids oversizing issues to a certain extent, since the flame/gas input would be adjusted accordingly. You are right though that this may lead to more expensive repairs/complications down the road. The contract does say there is a lifetime warranty with the heat exchanger, but the other parts/labor only has like a 1-2 year warranty.

Thanks
 
  #6  
Old 03-30-15, 12:41 PM
user 10's Avatar
Member
Join Date: Mar 2004
Location: NA
Posts: 2,068
Received 44 Votes on 38 Posts
Are the walls framed or double brick?

sizing could be right if double brick or uninsulated frame.

with load calcs, garbage in = garbage out. r-values and infiltration have to be known.

it's good practice to reduce heat loss if possible before changing the furnace.
 
  #7  
Old 03-30-15, 01:15 PM
K
Member
Thread Starter
Join Date: Mar 2015
Posts: 10
Received 0 Votes on 0 Posts
The walls are framed to my knowledge.

I just talked to the contractor and he said the equipment is pretty standard for the residential homes here in suburban MA, my house was built in the 1900...

If money were not an issue, I could in theory replace the one or two windows that are more drafty.
 
  #8  
Old 03-30-15, 05:03 PM
user 10's Avatar
Member
Join Date: Mar 2004
Location: NA
Posts: 2,068
Received 44 Votes on 38 Posts
There is absolutely no standard, especially in old homes that have been partially updated.

Every home is different and every home needs a heat loss/gain calculation.

Insulation needs to be checked in the attic and exterior walls. Air leakage needs to be measured with a blower door test.

Empty exterior framed walls can be dense packed with cellulose. Attic insulation can be topped up if you have less than R25-30 and air leaks around light fixtures, outlets, chimnies, etc sealed before replacing the furnace.

Replacement windows don't save enough to justify the cost unless single pane.

At the end of the day, the building envelope is more important than the furnace you select. When it comes to comfort and gas use, a home with insulated walls and a correctly sized builder's furnace will out perform a poorly insulated house and a fancy furnace.

When it comes to furnace selection, proper sizing and setup take precedence over everything else.

---------------
It's cheaper and smarter to get a load calc and energy audit than to spend hundreds extra on a fancy modulating furnace just to compensate for incorrect sizing.
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: