I am NADCA Certified Air Systems Cleaning Specialist. Over the years I have learned on simple answer to this questions: there air duct cleaners and there are pretenders. There will be no benefit at all if you use on the very cheap pretender because they will not really clean anything. On the other hand if you call a qualified air duct cleaner with a NADCA Certification you will, like 99% of my customers, report improvement with your respiratory problems, you will see less dust and the job will pay for itself with energy savings and repair prevention.
HRV/ERV Ventilators: These whole-house ventilators exchange air. They are ideal for homes with what is called a “tight envelope,” homes where house wrap has been applied and airgaps in the attic, around windows, doors and outlets have been sealed. Air in these homes becomes stale and polluted, so exchanging the air is important to IAQ. Heat recovery ventilators (HRV) are designed for cold climates. Energy recovery ventilators (ERV) are best for warm climates.  Your cost for an installed HRV or ERV will range from about $2,000 to more than $5,000 based on its capacity and features.
Here in Frisco, TX…just had a tech come out to clean vents. I used a 30 bucks groupon for “complete vent cleaning”. He told me I have 3 AC units and groupon covers only 1. I told him to do the unit upstairs where the kids sleep. He decided on his own to do the downstairs unit. I yelled at him why he is not doing the unit I told him to do! He said they are all connected. I will clean two for you for the price of one… Oh really?
In addition, the black dust was found in most of the heat registers. He could rub it off on his fingers but did nothing but spray compressed air on it which didn’t make it disappear since it wasn’t loose. Also, in the register in the basement he described the black dust as being oily. The reality of that situation is that I had the AC on until the morning of the service and minutes after he snaked the hoses in my back door it started raining. The basement register had condensation from the humidity all over it. It was not oily but wet. He asked for an old towel and wiped it out. Never saw anything so black – because I never had to clean soot. That is what I have in my ducts. Soot from years of the old coal furnace.
Ventilating or ventilation (the V in HVAC) is the process of exchanging or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, carbon dioxide, and other gases. Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air.

Pro Tips: Before you buy one of the learning thermostats available, you should beware of their shortcomings. First, many require a c-wire to work properly. A common problem is that the old thermostat might not use a c-wire, and this makes it confusing when swapping wires to the new thermostat. Secondly, if you change your thermostat setting often, you might become frustrated with a nest thermostat or other learning thermostat. It will be constantly trying to learn patterns in your random setting changes, and will therefore change your home’s temperature when you don’t want it to. If you have a fairly consistent schedule or don’t mind overriding the unit either manually or via the app on your way home or when leaving, then you won’t experience this issue.
HVAC is an important part of residential structures such as single family homes, apartment buildings, hotels and senior living facilities, medium to large industrial and office buildings such as skyscrapers and hospitals, vehicles such as cars, trains, airplanes, ships and submarines, and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors.

These factors are included in a "Manual J" calculation. Contractors who make these calculations before recommending a size can take a couple of hours collecting the information and making the calculation. If your contractor doesn't do it, there are services that will do it for around $99. If you're feeling particularly on the ball, there are also free online calculators.

CIBSE publishes several guides to HVAC design relevant to the UK market, and also the Republic of Ireland, Australia, New Zealand and Hong Kong. These guides include various recommended design criteria and standards, some of which are cited within the UK building regulations, and therefore form a legislative requirement for major building services works. The main guides are:
My Carrier furnace occasionally (one-two times a day) does not start. It shows error code 15 ("BLOWER MOTOR LOCKOUT -This status code indicates the blower failed to reach 250 RPM or the blower failed to communicate to the variable speed furnace control within 30 seconds after being turned ON in two successive heating cycles."). However, when I turn off and on the furnace power switch it starts and runs fine for a while. Sometimes it runs fine all day, but does not start only once in the evening or in the night. It is 13 years old I was thinking to replace whole thing if repair cost is too high. But I am not ready to pay $10,000. 
I have lived in Calgary for quite some time now. When I first moved here I couldn’t stand the humidity levels. Its like a desert come winter. When I first moved into my new home, the house inspector insisted I have my furnace and ducts cleaned. I was very sceptical about duct cleaning to begin with. The company I hired came on time and were very respectable from the start. Before mentioning anything they asked me what my concerns were and what issues I was having. They were more concerned about addressing my issues then bringing anything non related up. I ended up going with this company and getting a humidifier installed a couple days later after the cleaning. Its been almost 4 years now that I have been here. The humidifier is working great and the difference is incredible. They took me around explained step by step what they were doing and the reasons they were doing it. With all those other companies out there, I am glad I found them at this site.
If the condenser coils are clogged, the compressor can overheat and shut down. You'll experience intermittent periods of minimal cooling, followed by no cooling. Even if you're “sure” the condenser coils are clean, clean them again. Turn off the power. Flip the air conditioning service and furnace circuit breakers in your main electrical panel to the “Off” position. Next, turn off the power switch right at the furnace or air handler. Then yank the disconnect block (Photo 1) and clean the condenser coils (Photo 2). If the air conditioning service still doesn't work properly after you've cleaned the condenser coils, installed a new filter and opened all the supply vents, proceed with the following repairs. Your AC won't work fully unless you do this.
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