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Case Studies – When Bad Manual J’s Happen to Good Houses...


One of the most prevalent mistakes in designing a geothermal system for a given house is the lack of design. All good designs start with a Manual J calculation in conjunction with several other tests. The disturbing fact about the Manual J is that it, when done incorrectly, can have a devastating result on the actual load and the comfort level of the home. This is extremely evident in old home designs. Why? The Manual J is only as good as the information that is plugged into the software.

Older homes present special circumstances that must be dealt with for the design to have a happy outcome. Some of these are: high infiltration rates, poor insulation values, period windows, and so on. Geo in these homes can be done well when the Manual J is executed properly.

Let’s start with my personal home as a prime example. A Manual J was done by two independent contractors who both came up with the same design, 2 tons upstairs and 3 tons downstairs in my circa 1900 farmhouse with some envelope improvements. I felt fairly secure with both their evaluations and the Manual J calculations, so I went with their recommended designs. What I did not take into account was that both of my contractors had limited or no old house experience.

What we found out that first winter is what is called the balance point for a system. We achieved the balance point when the outside air was a chilly 35 degrees with a steady wind of 3-5 mph. When your system reaches its balance point it can no longer provide the cooling or heating that the house (load) is calling for and discomfort is soon to follow.

I went back through all the design numbers with both contractors and found that they both had made the same mistake using two different software packages that resulted in the same poor results. When the software asked for the homes infiltration rate both contractors checked off average. The choices were: tight, average, and poor. I immediately keyed on the fact that from my experience doing repairs to our home that nothing in regards to insulation or infiltration is average, and further yet, it is generally poor. In the help section for the software it gave descriptions of the categories.

Tight = spray foam, average = new construction, and poor was everything else with the note that a blower door test should be done to evaluate the exact amount of “poor”. When the blower door test was performed and the “poor” box was checked for the infiltration rate for my home’s design, it made a whopping 1.5 ton difference for the downstairs and a 1/2 ton for the second floor!! I was not willing to cover a 1.5 ton disparity in the design with stage 3/electric heat, so what to do?

The best money spent is on building envelope improvements, but that does not alleviate the problem now or the discomfort in the home prior to making these improvements. It is all about the design. In some older homes building envelope improvements may not be allowed or wanted by the homeowners or historical committees.

This is a shining example of how bad Manual J’s can happen to good houses, leaving them with poor design issues. Remember “garbage in yields garbage out”.







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