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Question DetailsAsked on 2/16/2014

How do you determine the right size A/C and furnace for your home? Should I have home energy audit done first?

I have a 3700 Sq ft home built in 1968. It currently has two zones. The second zone just controls the master bedroom and one other small bedroom. This zone is via a heat pump. It is old and inefficient. The Aux heat comes on at 64.

The main furnace is natural gas and the other A/C runs rest of the house except an addition with cathedral ceilings, which has a split unit and a gas fireplace we put in last year( Mendota). It's a cap cod home.

The main furnace and A/c are at least 15 years old. Made by Carrier.

SHould I get a home energy audit done first before I have a contractor come out?

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2 Answers


An energy audit or evaluation by a qualified remodeler may not be a bad idea regardless.

Any efficiency improvements done to the home and envelope will lessen the necessity of size out of the system.

The HVAC contractor will run the manual J calculations in order to size the system properly.

Answered 6 years ago by WoWHomeSolutions


1) At 15 years your A/C is probably nearing its functional life, but furnaces commonly run 25-30 years or more before needing a major overhaul, and even then can commonly be overhauled to like-new performance, though of course not as efficient as a modern unit, and may not be as easily integrated with a new A/C system.

2) IF you are considering a total home energy upgrade, then yes - you should get an energy audit. My recommnedation - by an auditor certified by the Building Performance Institute (BPI). Do NOT believe someone who tells you they are EPA certified - the EPA certification is a travesty - so minimal I passed it with perfect score in about 5 minutes with zero study, after not having been involved in HVAC design to any extent for about 10 years before I took it.

3) An energy audit will tell you where your primary losses are and where insulation can be beneficial. If you are planning general insulation improvements (which is almost always more energy efficient that HVAC system modifications), I always recommend getting weatherstripping brought up to snuff FIRST (to eliminate major air loss/inflow locations that are easily fixed, and which can overwhelm the overall picture in an audit. Sort of like leaving a window open during the audit blower door test - will throw the whole picture out of whack if you have significant air movement around doors and windows and vents. Weatherstripping is also where you commonly get your greatest bang for the buck - easy, inexpensive way to knock out your largest losses of conditioned air, which commonly are more significant than energy losses/gains through the insulated surfaces (walls, top floor ceiling, bottom floor slab or floor.

4) Then get the energy audit, which will tell you where additional energy loss locations are and where insulation changes might be most effective. To get more than a general picture, you really need a full thermal IR scan with a DVD or thumb drive you keep (in a computer readable formatfor your computer) to let you review the insulation and air leaks afterward and with insulation contractors. Get insulation fixed to the extent you have budget and inclination, because changes there will affect the size HVAC unit you need. Taking care of any really bad insulation issues can save you money not only in future energy bills, but also in HVAC system capital cost because a better insulated home needs a smaller system.

5) Then you are ready to evaluate your system requirements - and with the hodge-podge you have, you certainly want to have an expert in energy system design and analysis. Unfortunately, while there is a simple EPA required certification of technicians regarding A/C system servicing, this does not cover system design at all and is really keyed to greenhouse gas prevention in recovery of refrigerant gases. There is no one certification in the HVAC design field - there are a number of certifying programs, some independent, some trade association, some manufacturer based; so you have to select a contractor based on reputation and what he says in talking to you about how he goes about an energy evaluation before recommending a particular system.

6) Now the technical side - there are 4 primary Manuals from ACCA (Air Conditioning Contractors of America) which should be used to design a residential HVAC system. In simplified terms, in the order they should be performed:

Manual J - determines the heating and cooling "loads" or "demand" each room and zone presents

Manual S - determines the sizing (capacity) of the HVAC furnace and air conditioner to meet those loads

Manual T - determines the airflow required in each room, and vents/grillage area

Manual D - determines the sizing of the ducting to carry the required airflow to each room, and in combination with the Manual T results the split of airflow between rooms within a zone, and balancing of airflow between zones coming off the system

7) Only after these have been done can an actual system capacity and duct sizing be settled on, leading to a range of possible furnaces and A/C's that will suit that demand. The same calculations can be run on the existing systems to see how compatible they are with the loads in the various locations - with your system, unless they were carefully designed, it is quite likely they are fighting with each other. I have seen add-on systems in larger houses where one unit is running heat while another is fighting it with A/C because they were not designed to be compatible, which generally can only be done reasonably if you either allow a wide difference between the temperatures where heat and A/C kick in (typically 10 degrees at least), or isolate each system form the other with doors with air seals under and around them, like you would with a fire door.

8) If a potential contractor cannot rattle of each of these analysis and design manuals from memory and explain to you why they are important, then he is probably an installer, not a trained HVAC system design technician. Commonly, only the senior person in a normal sized residential HVAC company will have this training, so he will design the system and assist you with selecting the correct furnace and A/C system, this his installation crew will do the actual install. And no,it is not mandatory to have the designer dothe install - sometimes you end up with one contractor being paid to do the design calacs (which you need to make sure become yours contractually if you are paying for them), but in bidding the system you end up with another contractor for whatever reason. Of course, if you go with another contractor, then you have to be suyre it is in WRITING that he agrees with the suitability of the calcs, including any modifications he makes to them.

Answered 6 years ago by LCD

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