I need to buy a new HVAC system for my 1930s home. I currently have forced air duct system. My gas furnace was installed in 1985 and is rusting out, and I'm not sure about the air conditioner.


I had a local big name HVAC company come out to give me an estimate. His sizing calculation consisted of going through my home and counting the registers and returns (not every room in my home has returns). He didn't take any measurements of the rooms ... which seemed odd to me.


Is there a certain kind of professional that I can hire to inspect my home and design a heating and cooling system ? I think I'd like this person to be different from the person that will be selling the system to me.

Questions I would like this person to answer:

  • What is the correct size system for my home?
  • What is appropriate type of system for my home: heat pump, gas furnace, etc ?
  • Are my existing ducts adequate ?
  • What should I consider when choosing one system over an other?
  • 2
    On something I am not really familiar with I usually get 3 estimates from different places. I often have best results with the small shop closest to my home. Commented May 24, 2015 at 19:28

2 Answers 2


In theory this is what HVAC professionals are supposed to do. In practice, most of them rely on antiquated rules of thumb to size equipment and use existing ductwork, even if it has major problems. A certified energy rater is who you're looking for and who will be able to definitively tell you what you need to know, but you can learn a lot yourself to be a more informed consumer.

The correct size of equipment for your home is discovered by doing a Manual J load calculation. You can do an 85% as good approximation yourself in 15 minutes here: http://www.loadcalc.net

For a 1930s house, unless major air sealing, insulation, and window replacement work has taken place, the load may be as high as 1 ton of AC per 500 square feet and 30 heating BTUs per square foot. But those would be worst-case scenario numbers; run them yourself with that web tool and get a ballpark estimate of your house's loads.

The most appropriate type of equipment for your house depends on your climate, your budget, and the fuels available. If you live in a mostly-cold climate and have natural gas, then a gas furnace makes the most sense. If you live in a mixed hot-and-cold climate but it doesn't get too cold, then a conventional heat pump may be a good bet. If it gets both very hot and very cold, then AC + a gas furnace may make the most sense if you have gas; if you don't, then a Japanese ducted mini-split or conventional heat pump is a better choice.

As for your existing ducts, it's impossible to say whether they're adequate or not without knowing anything about them. But they may be inadequate if they are in the attic, or if they are in a vented crawlspace, or if any rooms get much hotter or colder and other ones, or if a lot of air leaks out of them.

Another thing to keep in mind is that you're not really hiring someone to design your home HVAC system; your home already has an HVAC system (well, the "H" and "AC" parts, at least). If every part of it is unsalvageably wrong, expect to pay mucho dinero to completely replace everything. New equipment and new ductwork and all the demolition and drywall repair entailed by such a job will probably cost you $15k or more... possibly much more if you live in a high-labor-cost area. Consider how much you actually need to replace and how much you can simply live with, especially considering that many comfort-related deficiencies in a house's heating and cooling infrastructure can be ameliorated by improving its insulation and air sealing. There are many such opportunities in an old house, including the extremely low-hanging fruit of air-sealing the basement, weatherstripping any old windows, and adding more attic floor insulation.

In general, it's a good idea to do all this work before replacing your equipment, since if you buy correctly-sized units now, they will be too big after you perform the work. You can mitigate this to a certain extent by purchasing "modulating," "variable speed," or "two-stage" units, as they are capable of reducing their output at low-load conditions. In theory you won't have to use your furnace for another 4+ months at least, so you have some time to tackle those sorts of projects before having to replace it, which will allow you to purchase a smaller one and save money, or a better one for the same price.


Although the installing contractor should perform load calculations and make recommendations based off your existing conditions (like iLikeDirt stated), a mechanical engineer can also provide the services you seek, but will likely charge more than if the contractor were to include it in his bid.

  • Should, but sadly never do.
    – iLikeDirt
    Commented May 26, 2015 at 3:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.