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Question DetailsAsked on 6/17/2011

Do I need 1 or 2 furnaces for 3,000 sq ft home?

We live in a home in Sausalito CA with 2 old furnaces (1 for up, 1 for downstairs). House is about 2500-3000 sq ft. and has poor insulation, but temperature drops to 35-40 f at low. If I only need one, is there a good mechanism to control upstairs and downstairs separately - ie when downstairs is on, upstairs usually off, or different temperatures to both. What would be the best configuration and technology available for that? havent been impressed with the zone idea, because as its been explained to me, too much heat blowing through to single zone - dont really save - and hard on the system. What are the best options available today for a forced air heating system to control heat to different areas of the house? Cost a factor, but more importantly looking for the ideal solution. I DONT want to under-buy - even with these two old systems, the house has been chilly last few years, and I DONT want to heat the entire house when the heat is on. Most friends I have talked to have one furnace, but end up heating the entire home when its on.

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6 Answers


There are various products on the market to increase your home's energy conservation. Unfortunately, any product that will perform to your needs will cost you on the initial installation. I would not make cost a factor. If you do you may not be happy with the outcome. To quote an old adage, you (will) get what you pay for! Zoning the duct-work is an easy and least expensive option. It works fine [when installed correctly]! The advantage is the ability to control two separate zones. The disadvantage is the initial setup will be done in the heating or cooling season. The structural (or house) airflow is different in the summer than it is in the winter season. If you design the system for the cooling season you'll lose efficiency in the heating season and vice versa. You can, as you suggested, install two separate systems to give you more direct control of the zones but the initial installation cost is high. Note: It is extremely critical the two aforementioned options have the duct-work tested and balanced for proper airflow. This is not an option if you want your system to perform at its rated capacity and highest efficiency. Another option is to set your central heating and/or air-conditioner system on a conservative setting and utilize a multi-zone Mitsubishi system to assist with conditioning the space. My gut tells me your dealing with high energy bills and that's why you’re looking at alterative products. Please take this last suggestion to heart. If that is in fact what you’re dealing with I suggest keeping your single zone setup and install a 90%+ unit with a high efficient motor. Also have the duct system tested and balanced by a certified NCI duct balancing company. Industry testing revealed the average home loses 43% of its rated capacity due to crappy ductwork (check out my YouTube videos). I can say with authority, after performing T&B I've seen an increase in my customers energy savings between 25% and 35% annually. Whatever you decide I suggest you get Factory Authorized Contractors to discuss these options in person with you.


Answered 9 years ago by Stans HVAC


Get an HVAC contractor who is specially energy trained. They may be Building Performance Institute certified.

This type of contractor will use the "Energy Star Quality Installation Guidelines." This will include calculations of how much of a system you will need, exactly. The old rules of thumb are no longer valid ways of sizing systems. The contractor has to measure the house and calculate the sizing based on what is calle Manual J 8th edition.

This will include evalution of the duct system to ensure it will deliver the heat / cool it needs to, and make repairs or replace. Ducts can be 30% of your heat cool energy loss. So they are very important.

Also a specially trained HVAC contractor will understand your home a lot better, and can evaluate the duct system.

Dont forget about comfort. This is why we spend money on energy. Specially energy trained HVAC contractors are in a better position to make determinations on how to improve your comfort and health.

I have routinely installed one smaller system in the place of two larger systems, and my customers are rave about the energy savings and comfort increases. I dont ignore their ducts in the procees. That is very important when you do this kind of thing.

Source: See:

Answered 9 years ago by chris


Unfortunately sizing heat/air is not as simple as how many square feet one has to heat and cool. The VOLUME is as, if not more important. A home with cathedral ceilings needs more capacity than one with flat ceilings. A home with lots of glass will need more than one with very few windows. How much insulation do you have, does the house "leak" (infiltration). What is the orientation (long east-west or long north-south).

An architect, or engineer (design professional) can help you determine your requirements. It is a "math game" that only someone with the appropriate training can do. And, in many places, there is a legal requirement that the person doing the calculations has a state license.

While some HVAC contractors have the staff and ability to engineer your system for you, it may not be the best route to go. Much like asking the fox to guard the hen house. Having an independent design professional evaluate and determine a solution separates the profit motive from the final solution. A design professional is YOUR advocate during construction. They are paid a fee to work for you. They receive no benefit from a more, or less expensive solution.

As it was pointed out in a previous post - you get what you pay for. If you take free advice don't expect much.

Answered 9 years ago by Belles Architecture


I agree to a point that having a third party design your new system can be beneficial, especially when you’re dealing with a questionable contractor. It is not quite the same as “asking the fox to guard the hen house,” when dealing with reputable contractors. That is an unfair statement to place on the whole industry. Most reputable HVAC contractors have taken the steps to get certified through Air-Conditioning Contractors of America (ACCA). This is the association that writes the codes for sizing, designing and installing HVAC equipment. Our Federal and local State codes have either adopted or are modeled after ACCA’s standards. In today’s market there are computer programs designed to assist the hvac contractor (and architect) in performing building load calculations faster than the handwritten long form. These programs require the end user to populate the windows with information. If the end user/contractor isn’t familiar with ACCA’s data on equipment sizing they will usually not perform this critical but necessary step! It doesn’t hurt to ask him for verification of his calculations. It’s not a question of having another party perform this task; it’s a question of finding a qualified contractor who’s capable of meeting/exceeding these standards. As with any normal purchase you make, decide what it is that you want prior to meeting your contractor. Figure out what your idea budget will be for the installation; ask about state, local, industry and manufacturer’s certifications. Do not forget to verify his commercial insurance and workers compensation coverage. Taking these extra steps will usually weed out the unqualified/questionable contractors.


Answered 9 years ago by Stans HVAC


The SF of your home is not the determiningfactor of the size unit(s) you need. Other factors should be considered,such as how many doors and windows are in the home and are the windows doublepane, how insulated is your home, what direction does your home face, is theduct work leaking. Reputable AC Companies can provide free in-homeestimates and take a look at your home as a whole to determine what size unitor units would work best for your home and your personal needs.

Answered 9 years ago by BayAreaAC


Sounds like when your house was built, it either did not have a furnace at all so it was cheaper to avoid tearing out a lot of ceilings and walls to run duct from one central furnace, or the initial one was undersized so they added an upstairs one. For your size home, one would normally use only one furnace, especially in a "warm" climate like yours without true winters. Two furnace homes are rare in modern construction except in larger than about 7000SF homes, ones with really spread out wings that would involve excessive duct lengths, or duplexes and condos where each unit provides (and pays for) their own heat/cooling.

I get the distinct impression someone may have led you astray on zones - sounds like you are talking about ductless or mini-split systems, where you have individual units in certain rooms rather than one combined central HVAC system. This can be done, but is not the traditional "zoned" ducted system.

In a typical central HVAC system, you still have zones - in older systems (up to 1960's) each room had inlet dampers or adjustable grills coming off a large central duct run and each was actually a "zone", though regulated by your personal sense of feel as to whether it needed more or less airflow in that room, and controlled by adjusting the dampers - still a decent approach for mild climates without severe heating or cooling seasons. Then they transitioned, mostly in the 60's to 70's, to multiple duct runs to different areas of a house (though older multi-story houses in the east had those 2000 years ago), still with one thermostat but with manually set control dampers or plates that changed the relative amount of airflow each zone got, though all were still on or off at the same time. Then, in the 1980's though not highly adopted till the 2000's energy conservation push, did true electrically-controlled multiple duct zones off a central system came into common use in residences, though still in probably a minority of forced air systems in residences as compared to commercial construction where this is standard.

A "zone" usually means a floor or group of rooms that are expected to see similar heating and cooling loads and are controlled as a group by one thermostat. Typically there will be 2-4 zones in a residential zoned system for a 2000SF range house - at least one downstairs and one upstairs and one in the basement (if applicable), then sometimes a separate one for master bedroom suite or conditioned attic or workshop/garage or in-law apartment or such. Each "zone" receives air based on its own thermostat demand, and the airflow is regulated by electrically controlled "dampers" or flapper plates that regulate how much air (if any) a particular zone is receiving at any time. With a large home, if only one zone calls for heat or air at a particular time, it can result in fast heating or cooling of that area because it gets all the capacity of the central system, but that is not normally a problem, and with modern furnaces can be controlled by using variable furnace fan speeds or gas flow rates to prevent excessively fast temperature changes.

What is "best" depends largely on your economic situation, which does not sound like it is controlling in your case, and your house type. If you have a ranch or California raised or basement box house with open basement or crawlspace to allow for easy duct installation then, in my opinion, a central air ductedsystem would probably be best and the lowest long-term maintenance for you, but of course a formal assessment by an HVAC expert is needed to evaluate your situation. If you have a house that would require expensive tearing up of walls and ceilings to install conventional ducts, or you have special finishes like copper or hardwood on your lowest floor ceilings, then perhaps a small-diameter high-pressure duct system, or a mini-split (though I don't like those personally) would be best for you.

I would get several HVAC contractors with good reputations to look at your situation and take measurements, calcs, etc but NOT submit bids right off - tell them you want to choose a system first, THEN get bids. That ay you do not get a central HVAC, a mini split, and a Ductless offer from 3 contractors, say, and have to compare apples to oranges. Ask for their recommendations, then after talking with 3 or 4 responsive ones and hearing what they have to say, THEN decide on what system you want, and ask ALL to bid on that configuration. Be sure, with this appraoch, to stipulate in the contract that the contractor is stipulting he agrees the configuration is suitable and appropriate for the situation (after any capacity tweaking to meet his requirements), so he can't come back later and claim that you told him what size heater or ducts to put in and he is not responsible if it does not heat alll areas as it should.

Answered 6 years ago by LCD

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