boiler nounting board reverberating

condenseit

Hi all,
New installation. Unfortunately I was bullied into an oversized wall mount combi boiler. It is a 151,000 BTU gas hot water boiler, and I have a 1,600 sf home in zone 1. Thanks to this forum I learned (albeit too late) that I only need 60 BTU/sf on the high end. The reason for the oversize was my "DHW needs". So instead of heating needs being the priority, he said I had to be able to handle the once in a blue moon event where I have just: a) started the dishwasher, b) started a load of white laundry in hot water, c) was doing dished in the sink, and d) was also taking a shower, I now have a boiler that can do that. Convenience over function. How American. I wish I had stuck to my request for the 100,000 or 131,000 BTU furnace to be put in. Not only is the 151 too big, it is also significantly louder (dB) than the 131 or 100.

Being this is the way it is, I am searching for advice on how to make what I have as quiet as possible. The boiler is mounted to a 4x8 sheet of plywood, The plywood is attached to six 2x4 studs. The studs are screwed into six individual floor joists. The studs do not touch the floor, so the whole caboodle is hung from the joists (including pumps and piping). I feel the vibrations from the boiler are being amplified by the plywood, and transmitted into the house structure, like a subwoofer. The noise is not steady but cycles and speeds up like a bad thrash-band base-line wuuub wuub wub bub bub bub thrummm. Because the boiler is oversized it starts up and shuts down more frequently, and goes through this harmonic pattern repeatedly. I may have been able to cope with the noise if it were steady. I did finally get ODR installed, so the boiler is doing less short cycling, and running at low-fire mode more often.
What can I do to quiet the thing down?
Attach some sound deadening insulation to the plywood? What is that and where do I get it?
Ideally (aside from a miracle: getting the correct sized and quieter boiler) I may bite the bullet and have the boiler moved. Right now the plywood board is in the cellar directly under my living room couch, where I like to sit and relax. The plywood is mounted such that it attaches to half a dozen joists, so it puts that vibration energy into many locations of the floor and walls. I want to have the boiler moved from the side wall to the back wall of the house (so it will be turned 90 degrees), and attached parallel to the joists (actually one single joist). This one joist is the last one before the foundation wall, and only 9' long. Combined with the single short joist span, and the further away location, and some sound deadening material, I can again nap on my couch (if the opportunity arises), which is much more likely in winter than turning on all the hot DHW taps simultaneously. Thank you for your attention, it feels better talking about it, but I want to do something(s) about it.Hindsight really is 20/20

Jamie Hall

I'm not sure I could come up with a better way to amplify and transmit the sound of that boiler... I'll bet the plywood is half or three quarters inch, too.

Oh well.

First thing I'd do -- and it probably won't be that hard -- is to mount the boiler on vibration absorbing mounts. Take whatever screws or bolts are holding the boiler out (one by one!! though you may have to loosen others) and place a piece of soft rubber or neoprene (cut from the kind of floor mats sold for workers to stand on) between the boiler mount and the board, then put a washer of the same stuff over the boiler mount opening, then put the bolt or screw back in (use a metal washer under the bolt head to protect the rubber). Do the same thing for any pumps which are bolted or screwed to the board.

Then -- and this may be harder -- figure out a way to support the whole mess from the basement floor -- which I presume is a slab? -- rather than from the floor joists. You may need a strut or struts at the top to prop it against the basement wall -- I'd have to see the arrangement -- as well, but disconnect it completely from the floor joists.

Another thing you can do but which is harder is to make the mounting board itself a sandwich. A layer of half inch or three quarter inch plywood screwed to the support studs, Then a layer of rubber. Then another layer of plywood screwed at the edges only through the rubber to the studs, then the mounts as above -- but those only go through the top layer of ply, not all the way through the backing layer (may have to cut holes in the backing layer to assure no contact between the mounting hardware and the backing layer).

condenseit

Thanks! After having written this post I got brave and called my contractor. He said "I've never had this problem" yada yada yada. BUT he will come and fix the noise.
We will likely detach the boiler form the joists, and have it supported by the slab floor with a strut to keep it from tipping over. Maybe a damper to one or two joists. I like the idea of a few rubber isolators between boiler and plywood.
Haha, you really couldn't have planned a better noise transmission system? I have the best of something!

hot_rod

Most boilers have a way to derate the output, either ramp delay of locking it into a lower output. Some have both functions. Check your manual.

Actually the very best efficiency would be an oversized boiler, running low turndown and condensing temperatures.

You present a lot of HX surface to a small flame and more condensing to pull energy from.

condenseit

what a great chart Thank you hot_rod.
I am feeling a bit better about the size of my boiler.
My contractor also told me his calculations put me over the 131, so I needed the 151 in his view.