Home HVAC Advice Needed

Joined
Aug 5, 2002
Messages
21,417
Location
Silicon Valley
My old house had a 2-ton AC with a 48K BTU furnace.

A 100K BTU furnace is quite large and would work with even a 5-ton unit, even if it only has a 3-speed PSC motor.

A 100K BTU furnace is also specified in this climate, which gets colder than where you live, for even 3000 sq ft houses.

A 600K furnace? Are you heating a warehouse or something?
Sorry it was a typo. I was thinking of a 40-60k furnace and a 3.5 ton AC match.
 
Joined
Nov 11, 2018
Messages
5,648
Location
Great Lakes
There are way too many variables to make a direct comparison.

Your house could be better insulated than his.

Your house may have more south-facing windows than his.

Your house may not be as drafty as his.

He may have an electric water heater and use more hot water than you.

And so on.
Still, a direct comparison on my own house showed it would cost more to switch to a heat pump than to keep my 95% furnace.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
Back to the drawing board.

I discussed (summarized) my findings to the wife. She suggested adding a mini-split to my upstairs office and guest room to supplement the deficiencies of the central a/c system in those areas. This way, I no longer have to find a solution to the ductwork issues. The tricky part is the power situation; my main and sub panels are full.

So, any thoughts on the various mini-split options? Mitsubishi is the priciest. Rheem has their own line, but they’re reboxed Fujitsu units.

For the central a/c itself, we’re going to go with the single-stage Rheem RA16/R95T combo and the contractor will throw in the air scoop at n/c.
 
Joined
Dec 23, 2006
Messages
10,333
Location
Canuck - moved to —> California —> Texas —> ???
@The Critic I would seriously look into having two zone system with two thermostats and dampers installed, like it was suggested by skyactiv. I had that in my house in CA and in the current house. They still have a single furnace/airhandler and condenser outside, but the two zone controller directs the air either to upstairs, downstairs or both. It works really well.
Ductwork sizing is not relevant because it is still the same size system and dampers can be set up so that they do not divert 100% to one zone or the other.

For example, in summer I mainly use the upstairs thermostat and let the cool air drop downstairs. Winter time it's the opposite since warm air rises. This way I have very even temps between upstairs and downstairs. For best results, your attic insulation needs to be done properly.

As far as your ductwork, I highly doubt it is undersized. It is a standard line from the installers that have no idea how to calculate it or what is a manual J for that matter. Ductwork needs to be smaller the further away you get from the air handler to keep the airflow up, otherwise they will barely blow any air. Also, slow flowing air has more time to exchange heat, so either get warmer during summer, or colder during winter.
If you replace the furnace/airhandler with one that has similar flow, there will not be an issue.

I would suggest you get model numbers from you furnace and the outside unit to find out what size they are, because the contractors may try to up size your equipment and if that happens, you may find your ductwork inadequate, loud and not seeing any improvement in cooling/heating performance.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
So, I learned about the Cool Calc software and performed my own Manual-J calculation to the best of my abilities.

Thoughts?

Manual J.JPG

Based on this information, I should have a 3 ton A/C and a 60k btu furnance. Even if we assume that some of the defaults are a bit off, 3.5 ton a/c and 70k btu furnance.

So far, all of the installers are quoting me a 4 ton A/C and a 85k btu furnace. Am I way oversized or do I need more "cushion" since these calculations may be a bit off?
 
Joined
Apr 28, 2020
Messages
802
Location
North Dakota
So, I learned about the Cool Calc software and performed my own Manual-J calculation to the best of my abilities.

Thoughts?

View attachment 107317
Based on this information, I should have a 3 ton A/C and a 60k btu furnance. Even if we assume that some of the defaults are a bit off, 3.5 ton a/c and 70k btu furnance.

So far, all of the installers are quoting me a 4 ton A/C and a 85k btu furnace. Am I way oversized or do I need more "cushion" since these calculations may be a bit off?
I'm going to assume my house is insulated better given where I'm located, but even then I'm at 2400 sq. ft. and when it's -20F my 60k BTU furnace has no problem maintaining temp on stage 1, which means output is just under 50k BTUs. So, on the heat side it does sound like 85k BTU is oversized for sure given your climate, unless your heat loss is insane.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
I'm going to assume my house is insulated better given where I'm located, but even then I'm at 2400 sq. ft. and when it's -20F my 60k BTU furnace has no problem maintaining temp on stage 1, which means output is just under 50k BTUs. So, on the heat side it does sound like 85k BTU is oversized for sure given your climate, unless your heat loss is insane.
I took some time and measured all of my windows. For comparison, I ran the numbers thru the loadcalc site. I would like to think that the loadcalc site is more "accurate" since it required more inputs.

289054486_1937332253080367_8322113413476168171_n.jpg

291749799_324088413145198_3445040594766153959_n.jpg


This calculator says I need about 3.58 ton for a/c (vs 3.1 with the other calculator), so the 4-ton a/c unit is probably correct.

For the furnace, the loadcalc says I need 61,223 (vs. 56,414 with the other calculator). These numbers are pretty close and suggest that the 85K btu furnace is probably oversized. However, Rheem offers their 70K BTU furnace with either a 1/2 HP blower or a 1HP blower (which is the same blower used in the 85K model).
 
Joined
Dec 23, 2006
Messages
10,333
Location
Canuck - moved to —> California —> Texas —> ???
Sweet, way to grab the bull by the horns!

Thanks for sharing the website, last time I did the calcs manually, it's the only reason I still keep my Ashrae HVAC book. This site is a time saver.

The loadcalc is more accurate for sure because it takes in more data, like the orientation of the walls, shaded areas, number of appliances etc. The more data is used, the more accurate the final calculation will be.
 

dishdude

$100 Site Donor 2023
Joined
Nov 14, 2008
Messages
14,259
Location
Phoenix
I like the mini-split system for simplicity and as a backup. If the main system breaks in the middle of summer, you at least have some cooled areas until you can get service.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
Upon further review, it looks like I made a few errors on the loadcalc/Manual-J. Special thanks to an astute member on the diychatroom site for noticing : 1) crawl space type (should be vented, not open), 2) duct work location and R value, 3) tightness of construction.

1657561948485.jpg

1657561977426.jpg


Problem is, does anyone make a 50 or 60k furnace with a blower that is large enough for a 3.5 or 4 ton condensing unit? I think you usually need 375-400 cfm/ton for cooling and the Rheem 60 and 70K furnaces max out at 1200 cfm.
 

JHZR2

Staff member
Joined
Dec 14, 2002
Messages
49,491
Location
New Jersey
“ I think you usually need 375-400 cfm/ton for cooling and the Rheem 60 and 70K furnaces max out at 1200 cfm.”

I guess it depends upon how “need” is defined.

If you have a single stage AC and the airflow is too slow it will probably cause it to short cycle. If a multi stage it should just scale back except in the worst cases. CFM is also going to be based upon pressure drop in your ductwork, so a fan nameplate isn’t necessarily the real value.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
“ I think you usually need 375-400 cfm/ton for cooling and the Rheem 60 and 70K furnaces max out at 1200 cfm.”

I guess it depends upon how “need” is defined.

If you have a single stage AC and the airflow is too slow it will probably cause it to short cycle. If a multi stage it should just scale back except in the worst cases. CFM is also going to be based upon pressure drop in your ductwork, so a fan nameplate isn’t necessarily the real value.
True - if your ductwork is undersized for a 4-ton system, you will never get 4-tons worth of airflow anyway...

Like anything else, I guess you can spend many hours trying to make this perfect on paper, but the real world is a bit different.
 
Joined
Aug 5, 2002
Messages
21,417
Location
Silicon Valley
I know South Bay is a bit more moderate than say Walnut Creek, and my house is 94 instead of 97 and only about 1750 sqft volume / 1450 sqft space, but I think based on my observation I can give you a bit of an opinion of your manual J calculation:

1) My typical max temp outdoor is up to 84F, worst case 91F, and my 3 ton is running 50% if my outdoor temp is 80F and I set my thermostat to 76F. If you think your house in Walnut Creek will suffice with 3.5ton you are going to be expecting it to "sometimes" not cooling to the precision you set it to, and you may have to settle for it being a few deg higher because you are cutting it too close. It is good for efficiency most of the time, it may be exactly what you want if you are using both a minisplit and a central in the worst case. 3 ton for sure is too small for you and you will need at least 3.5 ton, and I would probably do 4 ton if I don't have duct issue. Oh, I have a north facing roof, and if you have a south facing one expect to use more AC than that too.

2) You probably won't have 27F all winter, I don't think that's how cold Walnut Creek usually get to. In South Bay I am seeing 45F in the winter sometimes but usually 50F, I think you are probably ok with 60k BTU, even if it means you may have to turn on a few computer to supplement the warming in the winter. At night when you sleep, assuming you are upstair, you should be "warmer" if you are undersized.

3) You mentioned you don't want to have a mini-split as supplement, but this may actually be allowing you downsize your central to 3 ton and you can add a 1.5-2 ton mini-split or even window unit upstair to supplement the overall system. Sure it looks ugly but this is probably the most energy efficient way to make the system perfect in size, and if you use window unit as a supplement, you don't even need to worry about multi stage furnace, AC, duct size, and you can just go with 3 ton 60k BTU and call it a day, and only turn on the window unit on those 90F days, with not much labor cost.


My other opinions:

1) Back in the 26c/kwh days I know people yanking out heat pump for gas furnace because kwh is 5x more expensive than therm for the same energy. Even if heat pump is 2x as efficient it won't matter, you are using way more than you will save.

2) 80% furnace here all the way, unless your house is build with 90+ you will spend a lot on the flue work remodeling and you will have to deal with a lot of cost venting them sideway, condensation, leak, etc. Not worth it if you are paying someone to do it (labor is the biggest cost you will never get back ever). Since it is a 97 house, it is not build with 90+, it is not worth it.

3) Buy something that you can buy parts and replace if things go wrong. I don't like having to call someone for a service just to get parts (York? Lennox?) but someone I can buy parts off internet and install myself would be better. Parts are cheap, even if you are paying $500 every 5 years for something stupid, but each tech visit out of warranty would cost you at least $400 here. This is what you can control and keep things simple and stupid.

4) I don't like inverter stuff, or variable stuff. So far I am not impressed with residential grade inverter stuff in fridge, in ECM blower, in lowish cost (still expensive like 5k) AC compressor. Same as reason 3 I think most in the industry spend too much money on sales and installation but not enough on the inverter reliability and motor quality. I think this is one area we need more improvement and I may change my opinion in 5 years, but so far I'd be conservative and cautious to avoid headaches.
 

The Critic

Thread starter
Joined
Aug 30, 2004
Messages
28,639
Location
CA
I know South Bay is a bit more moderate than say Walnut Creek, and my house is 94 instead of 97 and only about 1750 sqft volume / 1450 sqft space, but I think based on my observation I can give you a bit of an opinion of your manual J calculation:

1) My typical max temp outdoor is up to 84F, worst case 91F, and my 3 ton is running 50% if my outdoor temp is 80F and I set my thermostat to 76F. If you think your house in Walnut Creek will suffice with 3.5ton you are going to be expecting it to "sometimes" not cooling to the precision you set it to, and you may have to settle for it being a few deg higher because you are cutting it too close. It is good for efficiency most of the time, it may be exactly what you want if you are using both a minisplit and a central in the worst case. 3 ton for sure is too small for you and you will need at least 3.5 ton, and I would probably do 4 ton if I don't have duct issue. Oh, I have a north facing roof, and if you have a south facing one expect to use more AC than that too.

2) You probably won't have 27F all winter, I don't think that's how cold Walnut Creek usually get to. In South Bay I am seeing 45F in the winter sometimes but usually 50F, I think you are probably ok with 60k BTU, even if it means you may have to turn on a few computer to supplement the warming in the winter. At night when you sleep, assuming you are upstair, you should be "warmer" if you are undersized.

3) You mentioned you don't want to have a mini-split as supplement, but this may actually be allowing you downsize your central to 3 ton and you can add a 1.5-2 ton mini-split or even window unit upstair to supplement the overall system. Sure it looks ugly but this is probably the most energy efficient way to make the system perfect in size, and if you use window unit as a supplement, you don't even need to worry about multi stage furnace, AC, duct size, and you can just go with 3 ton 60k BTU and call it a day, and only turn on the window unit on those 90F days, with not much labor cost.


My other opinions:

1) Back in the 26c/kwh days I know people yanking out heat pump for gas furnace because kwh is 5x more expensive than therm for the same energy. Even if heat pump is 2x as efficient it won't matter, you are using way more than you will save.

2) 80% furnace here all the way, unless your house is build with 90+ you will spend a lot on the flue work remodeling and you will have to deal with a lot of cost venting them sideway, condensation, leak, etc. Not worth it if you are paying someone to do it (labor is the biggest cost you will never get back ever). Since it is a 97 house, it is not build with 90+, it is not worth it.

3) Buy something that you can buy parts and replace if things go wrong. I don't like having to call someone for a service just to get parts (York? Lennox?) but someone I can buy parts off internet and install myself would be better. Parts are cheap, even if you are paying $500 every 5 years for something stupid, but each tech visit out of warranty would cost you at least $400 here. This is what you can control and keep things simple and stupid.

4) I don't like inverter stuff, or variable stuff. So far I am not impressed with residential grade inverter stuff in fridge, in ECM blower, in lowish cost (still expensive like 5k) AC compressor. Same as reason 3 I think most in the industry spend too much money on sales and installation but not enough on the inverter reliability and motor quality. I think this is one area we need more improvement and I may change my opinion in 5 years, but so far I'd be conservative and cautious to avoid headaches.
1) My max outdoor temp is probably 105F, not 97F. It's been over 100F quite a bit during the summers.

2) We see 35-40F somewhat regularly, especially up here in the hills.

3) The mini-split being installed (in my office) is for usage when the rest of the house does not require a/c, or if I desire a lower temperature.

4) The cost to add the condensate drain for a 95% furnace is peanuts.
 
Joined
Aug 5, 2002
Messages
21,417
Location
Silicon Valley
1) My max outdoor temp is probably 105F, not 97F. It's been over 100F quite a bit during the summers.

2) We see 35-40F somewhat regularly, especially up here in the hills.

3) The mini-split being installed (in my office) is for usage when the rest of the house does not require a/c, or if I desire a lower temperature.

4) The cost to add the condensate drain for a 95% furnace is peanuts.
3) What size is your mini split? If you have enough amperage to run both the central and mini split together, you can technically have them be a combined 4.5 tons.

1) If you are going to install a mini split in your office, then might as well just use single stage AC.

2) If you are going to use a smaller central AC, then you don't need to worry about the big AC small furnace problem, just get like a 3 ton central and a 1.5 ton mini split so you can use it to do a variable 1.5, 3, 4.5 system at max efficiency for example.

4) Then I remove my objection to 95%, I think you should decide whether it is worth the risk (reliability) and reward (gas saving) of going 95%.
 
Joined
May 7, 2004
Messages
13,157
Location
Nokesville, VA
Upon further review, it looks like I made a few errors on the loadcalc/Manual-J. Special thanks to an astute member on the diychatroom site for noticing : 1) crawl space type (should be vented, not open), 2) duct work location and R value, 3) tightness of construction.

View attachment 107647 Problem is, does anyone make a 50 or 60k furnace with a blower that is large enough for a 3.5 or 4 ton condensing unit? I think you usually need 375-400 cfm/ton for cooling and the Rheem 60 and 70K furnaces max out at 1200 cfm.

Yes, Trane does. Probably others do too as well.

These two are 60K input, 48K output BTU, 80% AFUE:

S8X1B060M4PSAA
S8X2B060M4PSAA

The second one is a 2-stage model. They both have 3/4HP fan motors. (Probably ANY furnace with a 3/4HP motor will do the CFM you need for a 4 ton AC).

The fan performance data says that on tap 9 it will do 1743 CFM with an external static pressure of 0.9WC.

Here's a link to the data for that line of furnaces:

 
Top