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  1. #1
    Join Date
    Oct 2012
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    Residential HVAC Engineering Specifications and Proper Design

    Hey guys, I'm new to the forum here and am an engineer at a contractor that does alot of large commercial cooling / heating and power station work. However, I really dont know much about engineering for in home applications. So I wanted to ask a few questions and was hoping you guys could help me gain some understanding. (Moderators: if this is in the wrong forum, please feel free to move it to the correct forum)

    Ok here are my questions:

    - With ductwork for an AC system, what is the typical static pressure for the fans and would would be the expected pressure drop over the evaporator coil? Would this be any different for a system that is purely forced hot air vs cooling only? What about a combination system, would would be the best way to do this? I am familiar with the standard 20 deg F drop for air going over a cooling coil, but I never investigated the pressure side of the ductwork (I dont do much air flow where I work so I'm addmitedly weak in this area)

    - If you have a system that does heating and cooling, what is the ideal way to distribute the air in the home (installation factors aside.) In many of the newer buildings the best way to go about air distribution is to use Under Floor Air Distributio, with the returns up high. This way you bring the warm air back to the cooling coil as the air seperates in the space. I would think this would work well for cooling but not for heating. For heating the return air should be closer to the floor. What is typically done in this regard and why?

    - One thing we are seeing alot in the commercial side now is natural gas condensing exhaust gas water heaters. These are heaters where the return to the furnace is ducted into the exhaust stream and remains in the stream for a long enough time that the exhaust gas cools to below its dew point and starts condensing. These heaters are designed for this and have to have stainless steel breachings because the condensation of the exhaust gasses actually forms nitric acid. As such you also need to install an acid neutralization kit (which is essentially baking soda) to neutralize the PH of the exhaust gas. I have heard of high efficiency heaters that have a 95% AFUE, is this the same thing? If not what is different?

    - In the commercial HVAC industry, most large chillers are water cooled using a cooling tower that allows cooling at the outdoor air wet bulb temp as opposed to the dry bulb temp. However, many people do make evaporative cooling units that use water sprayed on to a refrigerant coil to cool the air to lower than the dry bulb temperatures. I would like to know why this never caught on in the residential industry. I would think it would be pretty straightforward to design a unit that uses some domestic water to cool the condenser and saves electric consumption at the expense of purchasing water.

    - I have heard that VFDs are making there way into the residential market. I assume that these are drives that will be utilized to run refrigerant compressors slower so that they are able to take advantage of cooler outdoor air temperatures when available. Is this correct? I am also familiar with VFDs being used on ductwork fans. I would imagine in teh residential market, the fans are to small to have any noticeable benefit from using a VFD but I wanted to see if that is the case.

    Thanks in advance for any help.


  2. #2
    Join Date
    Jul 2008
    Rochester NY
    Post Likes
    Hi Rob, you came to the right place. Some of these discussions happen on the public side, and some on the pro side. If you get your post count up (prove you use the site) you can apply for pro membership and search both sides for the answers to those questions.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to, or RESNET, and find an auditor near you.

  3. #3
    Join Date
    Dec 2010
    The Quad-Cities area (midwest).
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    I size the ductwork for the airflow (CFM) needed to a particular area. I tend to oversize to allow for air balancing. The overall CFM design is based on the blower.

    I will place the returns opposite the supplies, i.e; one high, one low. Depends on which floor, i.e; 1st, 2nd or basement.

    I like to use automatic zoning to implove the airflow (as needed) to the area "calling". I have a three zone system in my home.

  4. #4
    Join Date
    Jul 2012
    Western KY
    Post Likes
    Welcome. Alot of questions and you'll find alot of opinions here. Some really sharp minds to pick on these forums. I'll give some answers here but recommend you get your post count up and apply for pro membership. Way way more detail on pro side.

    On the condensing water heater. Yes furnaces basically the same idea. Vent gas run through primary heatechanger then through a coil in the airstream to ring as much heat as possible out of it.

    The swamp cooler thing is big out west in desert climate but not the way you discribed. Water being sprayed on an air cooled condenser coil is a big no no. It's like giving it a crack pipe. Calcium and such will plate onto the coil, quickly, making the air cooled design useless. It can be and is done as a very temporary band aide to get a system online until a motor can be replaced or such. But once you put the water to it the addiction has started. The only rehab is a new coil if it goes too long.

  5. #5
    Join Date
    May 2000
    Indianapolis, IN, USA
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    On the water heaters, only real high efficiency are the tankless. The residential power vent tank type are very inefficient though many call them "90% water heaters" because of the PVC flue. They can vent in PVC because the inducer is pulling so much air out of the surrounding space, often a warm basement.

    Today's evap coils are getting rather restrictive. If you have a 1" pleated filter, you will have some high static readings as well. So today's duct systems tend to be horrid, from undersizing and poor design, to the coils and filters dealers are using. Usually the thought is a .5" ESP system. With a low restriction evap, fiberglass filter and a decent duct system, 30 years ago that might happen. But if you lose .4 or .5 over a clean 1" pleated and you have an evap of .3 to .4, you are really hurting without even adding in the duct system.

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