House too large for one system?

attofaradJune 13, 2011

Total replacement of gas furnace and ducts, also adding A/C. I personally wouldn't pay for A/C in my area, but it will probably make the house easier to sell when the time comes.

Fairly mild climate, San Jose, California, 95132.

The house is single story L-shaped over crawlspace, about 3400 sq ft. I come up with 93k BTU/hr peak heat loss after I remodel. Heat gain isn't too bad, thanks to decks with overhanging roof; very little sun-glass exposure until a couple of hours before sunset.

Too large for a single system?



Thank you for reporting this comment. Undo

A single 100,000 BTU furnace at a 95% efficiency, or 120,000 BTU furnace at a 80% efficiency would handle the heat load. The problem is getting an even distribution of heating and cooling over a large space. This could be accomplished with zoning. You need to find a good contractor to study your duct system and come up with a proposal.

    Bookmark   June 13, 2011 at 4:41PM
Thank you for reporting this comment. Undo


you would be better served comfortwise with two systems or at least one system with zoning controls.

But something seems amiss with your load calculation which I find highly suspect.

see below average heating and cooling temps for ZC 95132.

Climate averages for: SAN JOSE, CA, 95132
Hot weather Cold weather
Average days per year above 75: 0 Average days per year below 40: 0
Average high temperature: 72.7 Average low temperature: 49.4


    Bookmark   June 14, 2011 at 8:11AM
Thank you for reporting this comment. Undo

I can tell you that an ancient 150kBTU/hr ~80% furnace barely keeps up on a breezy 30F degree night, which corresponds pretty well with my pre-remodel heat loss calculation of about 130kBTU/hr. I presently have 700+ square feet of single pane glass, no floor insulation, not sealed, leaky ducts etc. The improvement to 93k is from replacing some glass, better sealed ducts, and floor insulation, even though I'm adding 200 square feet. Maybe improve infiltration a bit more, but I don't think the load is going down to 75k.

It gets to slightly under freezing here, and will have a few days 96+; It has hit 106F here, but most years will not exceed 96-98, and will only hit the 90's a few days.

So far, I have one pro who suggests a single system with zones, another pro quoting either zoned single or two separate systems, and my general contractor quoting two separate systems (no A/C quoted in his case, since I wasn't looking for it earlier). None of them are inclined to do load calculations.


    Bookmark   June 14, 2011 at 3:19PM
Thank you for reporting this comment. Undo

I decided to go with one system, Carrier Infinity with zoning, 58MVC 120k BTU furnace, and a 24ANB7 5T 2-stage A/C. My thinking is the getting the multi-stage heat and cool lets me be slightly oversized for heat, to quickly warm the house when I arrive home, but still be quiet with longer run cycles for more even temperatures. The duct/vent/return plan looks good.

When this system runs on low or medium heat, will the low air velocity at the vents make for poor "throw" and distribution?

    Bookmark   October 17, 2011 at 8:42PM
Thank you for reporting this comment. Undo


You are being oversized on the furnace.

At least drop down to the 100 K size, still has a 5T rated blower.

Sorry, but oversizing is never good.

Ductwork insulation should be minimum R-8.

How many zones will new system have?


    Bookmark   October 18, 2011 at 2:33AM
Thank you for reporting this comment. Undo

The 100k unit puts out about 91kBTU/hr at my elevation in a horizontal position. So slightly less, but pretty much at calculated design load of 93k. Might be okay, and I could insulate some floor (over vented crawlspace) to tip the balance if it doesn't cut it on a cold/windy night.

I haven't totally decided on number of zones; either 2 or three. If two zones, it will be master/laundry/bath/half-bath/den (about 1100 sq ft), and living/kitchen/mud/bath/half-bath/3-bedrooms (about 2300 sq ft). If three zones, the 3BR's and half bath will be separated from the rest of the living area -- that zone would have a vent dumping some flow into the living area zone since it is a bit less load.

    Bookmark   October 18, 2011 at 3:37AM
Thank you for reporting this comment. Undo


You need another load calc performed. Something is not correct with the one you are going by-definitely heating and possibly cooling.

Get it in writing and compare to yours.


    Bookmark   October 18, 2011 at 8:27AM
Thank you for reporting this comment. Undo


Thanks for the feedback. What makes you think that the load calculation is significantly off?

I have pretty good correlation between heat load calculation and performance before my remodel, but maybe my assumption on the efficiency of my old furnace is too high. I'm assuming that it puts out 80% once it is running for a while (not AFUE). It is a 150k BTU input, so I've been using 120k output.

Do you think that the old furnace would be putting out significantly less than 80%? If so, that would mean pre-remodel correlation is off.

I put in an extra ACH of 0.5/hr pre-remodel, as a way to account for duct leakage. Much of that leakage isn't totally lost, in that it heats the crawlspace under my uninsulated floors.

Thanks again,


    Bookmark   October 18, 2011 at 10:33AM
Thank you for reporting this comment. Undo


Who provided you your load calc both heating and cooling?

What outside winter temp low was used?

What outside summer temp high was used?

What indoor thermostat setting for both seasons was used?

Who supplied the details of your home's specs?

You have a copy of the findings on the software's letterhead?

Take a look at the climate averages that I provided.

I would still maintain that you don't need more than 100 K MVC model for heating. Even cooling is suspect.

Has quoting dealer performed a load calc?


    Bookmark   October 18, 2011 at 10:48AM
Thank you for reporting this comment. Undo


I did the calculations. There was also one done as part of my remodel title 24 plan approval, but it was ridiculously inaccurate, pretty much there to satisfy the legal requirement. HVAC-calc also had flaws, so I just tabulated the losses using an Excel spreadsheet.
Unless my old furnace was putting out substantially less than 80%, correlation between the calculations and the observed performance was quite good pre-remodel.

I don't know where you got your data with "average days above 75: 0" and "average days below 40: 0 ". Even if it means average over 24 hours, the 75:0 is wrong. It gets below 40F every winter, usually just barely hits freezing a few nights, and has hit 26F at my house. I've had outdoor pipes freeze on several occasions. It doesn't get above 90F for more than a couple of weeks, and usually maxes at at about 96F. It has been 106F at my house one time, and over 100F twenty times or more.

I used outdoor design temperatures of 29F and 95F, with indoor at 70F and 75F. Maybe a bit overkill on the low end, unless it happens to be windy on that cold night, but not much. I agree that 5T may be a bit more than needed (certainly for me, but ask my wife during a hot flash and get a different answer), but my calculation showed 4.8T, and I didn't put in anything at all for solar heat gain through windows. Again, insulating my R-2.5 floors over the crawlspace would drop that quite a bit (about a ton).

Of the 4 dealers I talked to, none do load calculations. These range from a 2 man shop to 30 man shops. When I asked, one said that he had someone I could pay to do it. I think Title 24 legally requires it, but none do it, and the inspectors must not flag it.

Do you think my old furnace could have been putting out much less than 80%?

    Bookmark   October 18, 2011 at 1:37PM
Thank you for reporting this comment. Undo

From Title 24: "The outdoor design temperatures for heating shall be no lower than the Winter Median of Extremes column. "

Winter Median of Extremes for San Jose is 29F, the temperature I used for my heat load calculations. I'm uphill a bit, so maybe 2 degrees cooler where I live (when there is a breeze -- when it is dead calm we get an inversion layer at night).

    Bookmark   October 18, 2011 at 2:24PM
Thank you for reporting this comment. Undo


I have expressed my reservations.

My last suggestion is that as a condition of the purchase, Carrier dealer performs and provides you a written copy of the load calc in writing on the software letterhead.

Dealer plans on using Infinity zoning controls (which are the best on the residential Mkt) and you have qualified him as experienced in this area of HVAC and ductwork design by checking on his references for jobs like this?


    Bookmark   October 18, 2011 at 3:36PM
Thank you for reporting this comment. Undo


Thanks again for your input. Yes, I've checked out the installer, and I've reviewed his ductwork design. Infinity controls, Carrier dampers. He would be happy to go down in size to 100k input/91k output, but that isn't based on any calculation on his part.

If I were doing 1-speed, I'd get something with approximately 92k output (a bit less than my calculated load), rather than the ~109k that the MVC-120 produces, so that I would have it more quiet and longer cycles. I could always replace more glass and reduce loss another 5700 BTU if I didn't quite have the capacity.

    Bookmark   October 18, 2011 at 4:39PM
Sign Up to comment
More Discussions
Thoughts on proposed HVAC system?
To start: my house is two-level (main plus upper)...
Maytag/Nordyne/Nortek HVAC
Wanted to share my HVAC experience. Hopefully save...
Who do I call about a whole-house fan?
I'd like a whole house fan. This might be a DIY project,...
Moving house AC unit
Hopefully this all makes sense. In our potential kitchen/house...
Jennifer Weinman
Moving floor vent to toekick area
Hello. We are renovating our kitchen as well as baths...
Sponsored Products
Conrad Contemporary Thermostatic Shower With Hand Shower
Signature Hardware
Batter Pro
$10.99 | zulily
Calhoun Leather Corner Sectional (3 piece) - Brighton Black Black
Joybird Furniture
Home Decorators Area Rug: Normandie Rust 9' 6" x 13' 9"
Home Depot
Echo Steel Aquamarine Nickel Tech Lighting MonoRail Pendant
Euro Style Lighting
LED Peyton Brown Tech Track Pendant for Lightolier Track Systems
Euro Style Lighting
Persian Design Super Kazak 2'x6' Sangria Hand Knotted Wool Floral Area Rug H5130
BH Sun Inc
European Cottage Media Chest - 007-23-11
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™