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OT - question for HVAC folks

webbch on Fri July 11, 2008 11:15 AM User is offlineView users profile

I know this is a little off topic, but I've noticed we do seem to have some folks that do HVAC type work on the board (air conditioning-specific). I was just wondering if MVAC gives one a good jumping off point (and tools) for HVAC, or if it's a whole nuther ball of wax. It would seem the recovery machine and gauges and many of the methods of leak detection would be appropriate in HVAC.

First up, I am a DIY, not a pro, but I've acquired the tools for doing complete system service on my vehicles A/C systems (recovery machine, tanks, manfiold gauges, micron gauge, nitrogen, etc.) and thus far I enjoy doing the work (Yes, having the tools does not automatically make one proficient at using them, LOL). I'm pretty mechanically inclined and don't mind tackling large projects.

I realize I have a lot of research to do yet and I'm afraid this is an overly broad topic, but I was contemplating installing A/C in my 25 year old home as well, probably using the existing vents (and possibly eliminating the swamp cooler). All the forums I've been to about residential A/C seems to be exclusively for professionals, who are very anti-DIY, as if we're encroaching on their business or something. Granted I can still learn a lot there, but asking questions about sizing the system will get one quite a lashing (I have a copy of the Manual J for sizing systems, just haven't sat down and done the calculations for my house yet).

Given that this forum is DIY friendly and there are some HVAC folks here, I was wondering what kind of major differences you have to watch out for when transitioning from MVAC to HVAC. Alternatively, if anyone knows of a reasonably DIY-friendly residential A/C forum, a link would be appreciated. Thanks.


Dougflas on Fri July 11, 2008 12:26 PM User is offline

As a HVAC contractor, I can tell you that residential work is another ball game. If the system is sized incorrectly, you can get a great case of mold. If the airflow is wrong, you will shorten the life of the equipment. Charging residential systems is critical for longevity of the system. If the ductwork is too small, static pressure will be off the ricther scale. If ductwork is oversized, not enough velocity will result and you can end up with ducts that condensate. If the airflow is not designed correctly, you'll have hot rooms and cold rooms. You must size systems according to the sensible and latent requirements and then select equipment that can handle that requirement.

The reason why DIYER's are not welcomed on some forums is because of liability issues. Some homes have gas fueled furnaces that if not properly set up, death can result.

webbch on Sat July 12, 2008 1:01 PM User is offlineView users profile

Thanks for your input. I plan on spending the time to design the system up front long before buying any units. Is there a particular reference source you'd recommend - there's a lot of books out there on HVAC systems. I'm not a heat transfer expert, but I do have a Mechanical Engineering background, so heat transfer and airflow concepts are not entirely foreign to me. I expect the planning phase of this to take far longer than the installation (and I'm not really quick on installation ;-) ) Is there any practical design guides that you would recommend? Thanks.


BTW, how is charging different from MVAC systems? From what I've read, it seems that pulling vacuum levels down to the 100-300 micron range are pretty routine given the copper lines. Also, I expect you wouldn't have an exact charge spec like you do with motor vehicles, because each installation will have different length of lines, which would affect refrigerant capacity of the overall system.

Dougflas on Sat July 12, 2008 9:25 PM User is offline

Not only does the length of the copper lines determine the charge, so does the airflow in the duct system.

You should go to and get manuals J, S, D, T. The recommendation for manual J is the unabridged. Also purchase "Bob's House" as it'll take you thru the design process.

webbch on Tue July 15, 2008 11:57 PM User is offlineView users profile

Awesome!! That's exactly the kind of information I was looking for! I'll be ordering those design manuals and slowly put together a reasonable system for my house.


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