Hvac Calculations Pdf
Umbelina Baublitz
This second edition has been expanded to include new and revised sections and provides practical, easy to follow methodologies for a range of calculations used in the design and sizing of heating, ventilating and air conditioning. The guidance is primarily based on data and...
In reality, however, you know that very specific formulas govern good HVAC work, informing the decisions technicians make out in the field. Of course, not every HVAC technicians knows these formulas by heart, and some may not fully understand how they work.
As you can see, a small imbalance in voltage can lead to an increase in temperature of over 26%. Ensure that your technicians stay on the lookout for this issue when examining overheating compressors.
While not meant to function as a comprehensive list, the formulas selected and listed above will be of great assistance to your technicians in their typical, day-to-day work. Encourage your employees to print this out to use as a cheat sheet, or merely direct them to this resource to study in their downtime.
If your team utilizes our HVAC software solution Smart Service, you can store some (or all) of the most important formulas or calculations in a custom form. This will allow technicians to easily reference the calculations via their mobile device. You can also store previous calculations for a given customer or piece of equipment so that your company can reference it during a future service call (businesses that offer preventative maintenance contracts will find this especially useful).
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We usually use Spaces to run UK HVAC calculations in schedules using custom formulas based on fresh air rate per person (litres per second per person) and air change rate volume (metres cubed per second for the total volume). Whichever value is greater is the flow rate we need to provide via the ducts.
We have an issue with large spaces in that they need to be sub-divided into perimeter zones of 27m and internal zones between 50-70m so that the calculation for each part of the space, which is provided by a specific fan coil unit, is separate.
We have been looking at diving up an open plan office using Space Boundaries, Hatch, Mass and hand-calculations but none of this is really what we want because we want to use Schedule automation in Revit and each method has different drawbacks.
We are using shared parameters for all of our data as we would like the information to be scheduled and tagged. I've tried out the Heating & Cooling Loads tool and it's quite inaccurate. We are producing the information to send to a thermal modelling specialist for analysis.
On fetishists and homeowners
Nice series of articles, Martin. Sounds like you need to create a simple app to size HVAC systems in high performance homes. Why not do it on our smart phones, they're more powerful than the computers on the space shuttle anyway.
I agree on your take on the fetishists who insist on taking everything to extremes instead of just making do with +/-99% accuracy.
Finally, on your comment about homeowners messing things up, that is certainly the truth and reminds me of something I heard John Straube say: "You can't idiot proof anything, the idiots are too smart."
Response to Armando Cobo
Armando,
I agree -- my blogs on load calculations do not represent everything there is to say on HVAC design. However, I have already covered some of the topics you mention in previous articles. See, for example:
"HVAC commissioning errors almost always result in needless increases in energy costs. Many HVAC specialists can share horror stories about commissioning errors, running the gamut from irritating to outrageous (for example, air-source heat pumps with electric resistance elements that operate for most of the winter)."
"A mechanical system also requires careful commissioning to ensure optimal operation. During commissioning, mechanical equipment is checked for proper operation, distribution systems are balanced, and controls are all checked. All too often, careless mistakes in installation and setup result in poor system performance. Without a commissioning process, these mistakes might not be caught for years. Commissioning is important in both commercial buildings and homes. Clear, thorough documentation is critical to ensure proper ongoing maintenance and operation of systems of any size."
"Every HVAC system needs to be commissioned. Commissioning includes measuring the airflow at the coil, and the airflow at each register in every room, to verify that the delivered cfm matches the design cfm."
Response to Robert Bean
Robert,
The high bar you set -- only engineers who thoroughly understand and are able to defend the mathematical calculations made by their Manual J software should be allowed to design residential heating and cooling systems -- will certainly guarantee work for a limited number of mechanical engineers. The group of engineers who can leap above your high bar is surprisingly small, as many energy efficiency experts will attest.
Engineers enjoy mystification and complicated algorithms. Your premise -- that only mathematicians and software geeks should be allowed to specify furnaces, boilers, and air conditioners -- serves no one but a limited number of expensive professionals.
In the systems I get involved in you have to be a licensed professional or a certified designer through the Canadian Hydronics Council (CHC), or the Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI).
Why run a heat loss for a temp below equipments operating range?
I am having a conceptual challenge with sizing mini-splits for homes here in Portland Maine. If a mini-split is specified to work at temps of 5 deg or above and we have a back-up source for colder days (nights more likely) shouldn't we run a heat loss for the equipments lowest operating temp?
The whole point of a Manual 'J' load is to properly size equipment for design conditions. Naturally one would have to oversize the equipment, since being a little short is not generally acceptable. How much you are over-size HVAC equipment is what separates the educated from the experienced. If you happen to have both, you are my kind of designer.
It most certainly matters more than before, since modulating burners are for the most part, not intended to replace proper sizing, rather make a properly sized gas-fired appliance more efficient in less-than-design conditions. Every modulating furnace or boiler has a minimum fire rate, so that loads presented below this minimum will cause the appliance to short-cycle (death to efficiency, no matter what the AFUE). There is at this time, only one boiler, with a range and fire rate, that would serve most residential applications, given radiation was sized properly using, you guessed it, a room-by-room Manual 'J' heat load.
Finally, my experience designing HVAC systems (using Manual 'J' based software) all over N. America, runs contrary to your opinion that tighter and better insulated homes somehow make " heating and cooling distribution systems become less important". I have never had a client relay this sentiment to me. Our experience does reflect your cavalier (frankly insulting) statements in that the average HVAC installer thinks that close enough is good enough. As the loads get lower with better building practice, the equipment should get smaller. Unfortunately, we regularly see equipment (boilers in particular) at twice the output of the load and a few with "minimum output" exceeding design conditions of the home!
As we are often called in on HVAC systems that do not or have never worked properly we take Manual 'J' very seriously and find that we can do a better job correcting other designers mistakes but that they were in fact mistakes in sizing equipment.
With the lack of HVAC design - of any kind - in the residential building market, I think it is irresponsible to suggest that it shouldn't or needn't be done. I attach a sample of a proper heat load. If you can do this in your head, well...
If you have read all five of my articles on load calculations, and you consider them carefully, you will find that I am a strong proponent of load calculations. I have been performing such calculations since 1976.
You wrote, "Our experience does reflect your cavalier (frankly insulting) statements in that the average HVAC installer thinks that close enough is good enough." Your statement is confusing. Perhaps you meant the opposite of what your wrote -- namely that your experience does NOT reflect my statements?
Whatever you intended to write, my conclusions about residential HVAC contractors have been confirmed by every academic and technical research study into the issue. The vast majority of residential HVAC contractors never perform a Manual J calculation, and a customer who asks an HVAC contractor to perform one is (in many areas of the country) often met by a blank stare.
Concerning the question of whether point-source heating works in a compact house with a tight, well-insulated envelope: believe it or not, it does. I have been living in such a house since 1981. If you read about Carter Scott's homes in Massachusetts, you may become a believer: Just Two Minisplits Heat and Cool the Whole House.
Perhaps it is you whom meant the opposite of what you stated. I have read your earlier installments (with much interest, thank you) and obviously agree with the vast majority of your various assertions. But I would argue that these two "myths" are in fact, not.
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