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Are Heat Loss/Gain Calculations Important?

Nov. 14, 2018
I learned, the hard way, that I needed to do my own heat loss/gain calculations decades ago.

“Ask Dave, he’ll know the answer.” A friend and competitor was having a problem with a five-zone radiant heating installation. He explained that if any three zones called for heating, everything worked as he had promised his customer it would. If four or five zones called for heating at the same time, the boiler couldn’t get past 130ºF and the customer was not happy or warm. I said the boiler is undersized, which brought forth a cry from the supply house office corner “It can’t be.” Why not I asked. “Because I sized it.” Said the supply house in-house guru of Btus. That led to my asking a series of questions: did you use a heat loss/gain program? “Yes, Manual-J.” What kind of exterior walls? “2’ thick stone.” What R-value did you assign to the walls? “R-19.” What kind of windows? “Single pane hand-made glass. You know, the wavy kind of glass.” Sash? “Wood.” R-value? “R-2.” Basement conditioned and is the floor insulated? “No and no.” Attic insulated? “Yes, R-30.” Were you able to verify that? “No need to, the owner told me that.” I turned to my friend and said you’re going to need a bigger boiler! I asked the corner in-house guru of Btus where he got the R-values for the walls, windows, and doors? Someone told me those were the values I should use.

I’ve had the “privilege” of being subpoenaed to testify in several jury trials regarding heat loss/gain calculations that either were not done or were horribly flawed. A subpoena means you have no choice – if you fail to appear you can be arrested. In one case, the 93-year-old owner had offered to pay the installer to hire someone, anyone, to fix the problems and she would pay for the work. During a deposition, when asked why in the world he had not taken her up on the offer, he said he figured she would die before he would have to fix his mistakes. I was hired to determine why her master bedroom and master bathroom radiant heating system could not get the room temps above 60ºF on colder days. The radiant floor product came with a chart the designer/installer could have used to determine exactly how many Btus per square foot were going to be produced at any given water temperature being circulated through the loops. His RHVAC Manual-J calculation and mine were a virtual match on the heat loss side, but he had never applied the affected rooms heat loss to the manufacturer’s chart. Had he done so, it would have been crystal clear the master suite’s rooms fell short and the resolution (radiant walls, ceilings, or flat panel radiators sized for the delivered water temperature) would have been simple and far less expensive than paying lawyers. The jury awarded the owner more than a quarter-million dollars!

In another case, a “contractor” removed two high efficiency direct vent 85,000-Btuh propane furnaces, two four-ton central air conditioning units, an 80-gallon power-vented gas-fired water heater and installed two 2.5-ton air-to-water heat pumps. Each heat pump was connected to an air handler with hydronic coil. He had used an online heat loss/gain calculator that carried a disclaimer stating it should not be used if you needed accurate calculations. The supply house questioned his calculations and he was adamant his heat loss/gain calculations were spot-on and that was documented via emails. I was hired by the manufacturer to do a complete heat loss/gain and used the Elite Software RHVAC Manual-J program ( The manufacturer offered to provide the homeowner with new properly sized equipment at no charge. As it turned out, the four-ton AC units had been the correct size and so were the 85,000-Btuh furnaces. In addition to the heat pumps being critically undersized, the attic contained a virtual nightmare of far-too-long runs of flex duct. The flex duct was severely compromised by being pinched off as it snaked around and over the wood framing. To add insult to injury, these two undersized heat pump chiller units were connected to two 40-gallon indirect water heaters that never could achieve the targeted 120ºF storage temp due to both heat pumps constantly struggling to heat or cool the home. The contractor was long gone and had declared bankruptcy. As it turned out, he had been banned by another state from ever doing any more work within their borders.

As the old saying goes: liars figure, but figures don’t lie.

The homeowner decided to sue the manufacturer of the heat pumps rather than accept new, properly sized heat pumps at no charge. The wholesaler was off the hook because the contractor had assumed all responsibility for sizing the equipment and, as you might expect, the manufacturer bore no responsibility and the homeowner lost the case.

I learned, the hard way, that I needed to do my own heat loss/gain calculations decades ago. I relied on a wholesaler to size HVAC equipment that turned out to be undersized and had to replace the equipment – at my own expense – a lesson learned in the school of hard knocks. Back then there were no computers, so we had to do our Manual-J calculations by hand. A single home’s calculations could easily take hours to complete. Today, via computer, tablet, or smart phone, you can knock out a whole home’s heat loss/gain calculation in less than an hour. Time well spent.

I substituted for a professor at Thaddeus Stevens College of Technology and worked with his first-year HVAC class for several days. We discussed how we professionals size boilers and HVAC equipment. Here’s what I told them. For boilers, we use the doorway method: we measure the door width and then get a boiler that is the closest to that width. That or we use the curb method and for heating we hold up our hand vertically at arm’s length to see how many fingers it takes to cover the home and that’s how many sections the boiler needs to have. For AC we hold our hand horizontally and the number of fingers it takes to cover the house equals the tons of cooling required. One student raised his hand to ask: “What if the curb is close or far away from the home?” Before I could answer, a few started to catch on that I was pulling their collective legs! We then opened up my RHVAC program and did a heat loss/gain calculation for the classroom. Then we used the copies I gave them of the Burnham Heating Helper book to determine if the hot water radiators in their classroom were under- or over-sized.

I recently visited a home that is not heating or cooling properly. They had an RHVAC heat loss/gain calculation that was done by a local supply house that, at first glance, looked okay. On closer inspection, it had so many mistakes that it could not be used to analyze the home’s issues. I met with the wholesaler’s individual who ran the calculations and asked him if he had any training on using the RHVAC program. He told me he was self-taught. I encouraged him to seek out training and get a certification in case he ever winds up in court. In PA there is a four-year statute of liability limitation on PHVAC work, but a 10-year statute of liability limitation for designers. RHVAC reports are only as good as the detailed input utilized: garbage in = garbage out. As I’ve seen first-hand, everyone directly involved in the design and installation of RHVAC systems can be held responsible when those systems do not perform properly. If you are going to accept heat loss/gain calculations from an outside source, find out who will be responsible if the calculations are wrong. In all probability, you will still be the one on the liability hook.

Speaking of being ultimately responsible, what happens when you know you were right, but the building is wrong? We were faced with that exact situation after a new addition was completed and our newly installed HVAC system was not able to properly heat the new offices. It didn’t help matters that the owner and his wife were, shall we say, exceptionally difficult characters. Not only could we see daylight at the base of the new exterior walls, a lit match would be blown out when held near the wall’s base on windy days. Nothing we could say, or show via our RHVAC report, was acceptable. “You aren’t getting so much as a thin dime until you fix this.” Fortunately, I’d helped a local engineering firm on several radiant heating designs and they sent a certified mechanical engineer to the job site. He reviewed my RHVAC report, duct sizing, and provided me with a report backing up our position. The builder denied responsibility, the owner refused to pay, so we sued and won 100% judgment in court. As the old saying goes: liars figure, but figures don’t lie.  

There are other online and downloadable heat loss/gain programs you can utilize but check with the AHJ (Authority Having Jurisdiction) to ensure they accept whatever heat loss/gain program you will be using.

Dave Yates material both in print and online is protected by Copyright 2017. Any reuse of this material (print or electronic) must have the express written permission of Dave Yates and CONTRACTOR magazine. Please contact via email at [email protected].

About the Author

Dave Yates

Dave Yates material in print and on Contractor’s Website is protected by Copyright 2017. Any reuse of this material (print or electronic) must first have the expressed written permission of Dave Yates and Contractor magazine.

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