tankless troubles

miatafrank

Last summer I installed a new Bradford Infinity tankless water heater for a customer here just outside of Philly. In December they called to say the water heater often made a low rumbling sound and when it did so, they saw its jacket flexing in and out. Long story short, Bradford gave me a new water heater because they had no parts available. The new unit does the same thing. They sent out a tech who could not figure out the problem exactly, but his combustion analysis showed the heater being starved for gas (propane). The propane people have replaced their regulator twice and claim there is no better regulator that can be used. They also ran a temporary gas line above ground from the propane tank to the house, just to make sure the underground fuel line was okay. The rumbling and shaking still occurred. I even changed the 10 feet or so of inside fuel line to the W.H. and made it 2 sizes larger - just in case the the unit was modulating faster than the regulator could. Did not help at all. All the gas pressures remain steady and are within range, and the pipe sizing has been double checked. Venting is direct vent with about 10 feet of length. The water heater gives no error codes ever. There is a reference though, in the trouble shooting guide, about a "low pitched noise with vibration" that would be caused by a "combustion flame with air in excess or lack of gas (combustion adjustment is lean)". Their solution is to make sure the gas type and set up are the same (done), check gas supply pressure (done), and to check CO2 and adjust the air (done). The air and fuel ratio adjustments can be done automatically (the W.H. goes through a 7 step adjustment by itself and has been tried numerous times). If that does not help it can also be adjusted manually. The manual adjustment never goes smoothly. When prompted to press the + or - buttons we can never achieve a "middle of the bar" power setting. Bradford claims they have had zero problems elsewhere and are considering providing us with a competitor's model to switch it out! My customer's have a very efficient geo-thermal heating system and do not want to go back to a tank type water heater. The W.H. does its rumbling and shaking only when it is firing at a high rate and especially when the flow from the faucets is then adjusted. Any thoughts will be greatly appreciated.

Larry Weingarten

Hello @miatafrank , Your clients might not want to hear it, but condensing tank-type heaters are basically as efficient as condensing tankless and far simpler. If the current unit can't be made to behave, that might be a good way to go. Bradford White has condensing, tank-type heaters

Yours, Larry

Peter_26

You have any pictures of the install? Sounds like something is wrong with the air intake side.

PerryHolzman

Not so far out possible engineering explanation.

This may be a case of the natural vibration frequency of the combustion chamber or the jacket being excited by the combustion process at high firing rates.

Replacing the unit with one of the same exact design would not change it. Adding appropriate mass (weights) or stiffeners to the affected parts would make it go away.

Many new industrial and power plants have low rumble vibration problems. A company comes in with testing equipment to find what structure part or large component is causing the issue - and weights or stiffeners are added. Problem solved.

In theory, a component manufacture should be testing their equipment for this in all the various mounting or support modes and eliminating it in the R&D Testing Stage. However, that's often not done completely (or properly) for many consumer items.

Is this installed and supported exactly as recommended in the installation manual?

You can also try gluing or "pop riveting" some stiffeners to the jacket and see if that solves the problem (and hopefully you can do that inside so it does not look real ugly). If its the combustion chamber as the source then this is the wrong piece of equipment.

Otherwise, you need a completely different design unit if this is the problem (and it's a very plausible problem).

Hope this helps,

Perry

Zman

If you could post the results of the gas pressure test when the unit is high, low and off that would help. The combustion analysis results on high and low fire would also help. Is the gas line csst? What size and how long to the regulator?
Does anything make the noise change? Partially covering the intake, throttling the gas valve, changing the gas pressure or combustion?
It sounds like a weird harmonics issue, it is going to take some creative troubleshooting.

pecmsg

Did this tech perform these tests with all other appliances operating?

miatafrank

Thank you all for your comments. Unfortunately I don't know how to upload my cell phone pictures to show them. The installation is textbook perfect and the unit is hung exactly as instructed. I won't try to modify the jacket - the vibration is incredibly strong. The jacket moves in and out a lot. I believe the combustion analysis results showed the CO2 to be 9.6 max. The tech said that 9.7 to 10.4 was the correct range. He also said that the vibration occurs when the CO2 level drops. The incoming gas pressure at the unit is 12.5 inches water column and drops to 10.65 inches when the unit is firing. I will try throttling the gas valve and partially covering the air intake. The house propane tank sits 25 feet from the house and has a first stage regulator on it that drops the pressure to 10 psi. From there, a half inch copper line runs underground to the house and up to the second stage regulator above ground just outside the basement wall. This regulator is a Rego brand and it reduces the pressure to about 12.5 inches water column. The water heater requires 8 inches minimum to 13 inches maximum static pressure when operating on propane. I ran 3/4 iron pipe from this regulator to the water heater but since replaced all of that portion inside with 1 1/4 pipe. I agree Larry, a condensing tank type water heater may be the best solution. I must find one with a lifetime or at least a long warranty on the tank. I am coming to believe that somehow this water heater just does not have what it takes to do the job right. Incidentally, this Bradford water heater is actually a re-badged Bosch unit - Greentherm 9000 I believe.

miatafrank

This is also the only gas appliance of the house.

miatafrank

The second stage regulator has a 1/2 inlet and a 3/4 outlet. The propane people tell me it is good for 800,000 btus/hour.

miatafrank

The air intake and exhaust piping are 2 inch pvc. The run up about a foot from the top of the water heater then turn with elbows, run 3 feet more then turn with 45's and run 5 feet to the outside. They are parallel to one another and outside I have the well far apart - much more than the minimum distance. I actually tried to get some System 1738 or polypropelene pipe to vent the unit, because I know that pvc is not ideal. None of the major plumbing suppliers in my area had anything to offer but pvc.

DZoro

I have seen issues with a "new" delivery of propane. Had the tank been filled just prior to the issues starting? A tank that has been filled to the max 85% can sometimes cause some liquid into the lines, and mess up a gas valve. The % fill gauges are not always very accurate, and can also depend on the angle that they are viewed. I suggest to my customers that have high efficient boilers to only have the tank filled to 80%.
Just some thoughts
D

miatafrank

Thanks D, I've been getting an education on propane. 99.9% of my customers have natural gas from the utility and I can't help but think that the propane delivery is causing this problem.

mattmia2

Could it be that the inlet and exhaust piping is effectively too short? Is there a minimum spec in the manual? Is the regulator that is part of the gas valve and the inducer relying on a certain amount of resistance in the piping to get the right mixture?

HTP makes some stainless high efficiency tank type water heaters. Check the water chemistry since they need relatively low sodium for the warranty to be valid.

miatafrank

Thanks for your thoughts mattmia2. The venting I've installed falls right in the middle of the min and max developed lengths. Interestingly, the instruction manual says that the unit's control unit calculates the length of the air inlet and exhaust pipes and adjusts the fan speed accordingly. Seems very high tech to me. If we decide to go with a high efficiency tank type rerplacement, I will check out the HTP.