Thursday, 4 February 2021

return sizing to prevent thud noises ?

good day everyone,

we just got a 6 ton geothermal system installed in my home (main level 3,000 sqft + basement another 650sqft) and unfortunately we have loud thud/banging noises coming from the return chamber few seconds after the system starts and several seconds after it stops. Originally i suspected that the zone controllers are slumming the ducts, but on a more careful inspection, it was evident that the noises came from high pressure difference in the return chamber. To confirm that, when I take the filter out completely, the noise indeed disappears.

I tried to replace the filter into a lower MERV rating (the installer said we can go up to MERV of 12 and I dropped down to 8), but it didn't help. I also tried installing a high flow electronic filter, but it didn't resolve the problem. I didn't try to measure the static pressure, if there are any guidelines as for how to do so, I am happy to follow.

I read in the forum (many thanks!) that the problem may arise due to an undersized return inlet (mine is 14 x 30 = 420 sqr inches). I found in the web a rule of thumb equation indicating that 144 sqr inches are needed per ton, which means that my return is less than half of the recommended size (6 x 144 = 864 sqr inches), yet when I contacted the installer, he claimed that this recommendation is only valid for commercial settings (is that true?), and that he can tweak the air flow down to prevent the problem. On the geothermal heat pump installation manual, they recommend (in a somewhat non-binding way, to have for that model with that tonnage capacity return dimensions of 32 x 26 = 820 sqr inches (which is again about twice of the one installed).

my concern is that reduction of the air flow will result in hot and cold spots in the house, so I would prefer to have the system return enlarged (e.g. add another 14x30 inlet maybe) than tweaking the system. my understanding is that beyond the annoyance the loud thuds introduce the longevity and performance of the system may be impaired by leaving the system in its current state.

any suggestions are more than welcome

thanks


source https://hvac-talk.com/vbb/showthread.php?2226327-return-sizing-to-prevent-thud-noises&goto=newpost

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