I recently had an Aprilaire 800 Steam Humidifier installed in my basement so I would have a full house humidifier. Before installation, I was told by my Heating and Cooling installer that the steam humidifier was going to be very quiet, but it turned out that the default installation was extremely loud.
Once it got installed, I realized that the steam hose pipe did not have the proper upward pitch going into the duct work, nor was the hose properly supported, so it caused water to pool up in the hose and cause a strange thumping sound (kinda like clothes being tossed around in a dryer). To fix this, the installers had to come back and lower the Aprilaire 800 humidifier on the wall, as it was too high up, which didn't give the steam hose proper pitch (2" drop for every 1 foot of hose).
So after the steam hose was fixed, I was still left with a water hammer noise and water fill noise, everytime the humidifier would fill up with water. Basically, every time the water would start filling up the canister in the Aprilaire 800 humidifier, it would be very loud while filling up. Then when the fill valve closed shut, you could hear a loud THUNK throughout the basement and first floor. I lessened the noise initially, by turning back the water shut off valve going to the humidifier so it was almost closed (this lessened the fast flow of water), but it did not solve the loud THUNK sound when the humidifier would stop the water.
I realized I needed a Water Hammer Arrestor to help get rid of the loud THUNK noise. So after reading some reviews of other's who have the Aprilaire 800 and experienced the same problem, I purchased this Water Hammer Arrestor for around $20 here: Supply Giant IS-BB-36DPNQ Compression Tee Hammer Arrestor, AA 1/4 in. OD COMP x 1/4 in. OD Femail Compression Copper
It was super easy to install, it took about 5 minutes and I just needed a crescent wrench to install, (so just about anyone can do this themselves). The water hammer arrestor gets installed right where the 1/4" copper water line screws onto the humidifier (in my case at the bottom of the Aprilaire 800).
Here's the install process:
1) Turn off the Aprilaire 800 and let it fully drain.
2) Next turn off the main Water shut off to your house, (and if you have shut off valve going to the Aprilaire 800 Humidifer, turn that off as well). I turned on a faucet in the house, to drain any excess water out of the lines, just in case as well.
3) Begin by unscrewing the Compression Nut that attaches the 1/4" copper line going into the Aprilaire 800, using a crescent wrench (counter clockwise).
4) Next tighten down the Water Hammer Arrestor to the Aprilaire 800 using the "silver" captive nut. (The arrestor will be positioned horizontal, which will work just fine according to manufacturer)
5) If the brass Compression Nut and collar/sleeve is screwed onto the Water Hammer Arrestor, unscrew it. (In my case, the compression nut / 1/4" collar sleeve was just in the box, and NOT screwed onto the arrestor). NOTE: If you don't have a 1/4" copper pipe with a collar / sleeve, you'll need to to put the Compression nut with the collar on the 1/4" copper pipe.
6) Then just insert the 1/4" copper pipe (with the already installed collar/sleeve and brass compression nut still on it), and tighten the brass Compression nut down.
7) Now, Turn back on the main Water shut off valve back on to your house. Open any closed shut off valves going to the Aprilaire. If you drained the water from your lines, you may want to turn them on and let your faucets run for a bit to fill the water lines back up.
8) Lastly, turn the Aprilaire 800 back on, and hopefully the Water Hammer THUNK noise and water fill noise will be eliminated or drastically reduced.
Keep in mind, you may still hear a slight clicking noise when the drain opens up, to drain out water, but it likely won't be heard throughout the house like the Water Hammer noise or Water fill noise.
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If you still hear more noises that you want to eliminate completely, you might think about using Rockwool Safe N' Sound insulation in the ceiling and walls around the Aprilaire 800 humidifier.
Overall, I'm really happy that this Water Hammer Arrestor which has nearly eliminated the loud THUNK sound, and I didn't need to hire a plumber to fix the problem.