Heating Help

flexing indirect

I have an installation with three Smart 120 indirect water heaters (tank within a tank style)  heated by 2 PS250 gas boilers. When i draw hot water from the tanks it appears that the inner tank is contracting causing the hydronic pressure to drop to 0 momentarily and then when the hot water draw stops, the hydronic pressure bounces up beyond the 30lb relief valve setting spilling water on the floor and of course adding fresh water to the system.
This has caused the failure of both boiler's heat exchangers. It is confirmed that there is no breach between the hydronic side to the potable side on the Smart tanks.

Sounds like...

Either

A) The boiler side expansion tank is non functional, and or

B) There is no potable water expansion tank on the potable side of the system, and or

c) The pressure of the potable tank has not been properly adjusted. or

D) All the above.

Pick one (GO FOR D, GO FOR D...)

Tanks expansion and contraction is to be expected to some degree. I've seen (literally) 2,000 gallon storage tanks look like they are breathing due to water pressure fluctuations.

Even with proper expansion tanks in place, some tank movement is to be expected. Too many dynamics working at once to be able to control all of them.

ME

I vote D

and would like to add that those tank have a lot of volume 126 gallons just in the tanks on the boiler side so the expansion tank needs to be sized quite large as compared to coil style indirects. Extrols sizing chart calls for a model 90 just for the tank volume so a couple sizes larger for the system and the piping then.

I've seen this happen

with a 50 gallon Phase 3 connected to a Burnham V85 boiler . Not as drastic as your situation but the pressure would bounce 5 to 10 psi whenever the hot water tap was opened or closed .

At first we thought the tank was compromised . But we watched the system for about 1/2 hour after we told everyone not to open any taps . The pressure stayed steady as a rock .

Like Charlie said , there's alot of heating surface area in these types of tanks . And alot of boiler water . Sounds like larger expansion tanks on both sides are the way to go

flexing tank

I have been using Triangle Tube Tanks for 20 plus years with great success. I have never had to modify my expansion tank (potable or hydronic) sizing to account for such internal influences that aren't thermal in nature. This is hydraulic. 
 We actually tested that theory by adding a few HFT60s on the hydronic side as well as two st 12s on the potable side to no avail.
Trouble with that whole theory is how do you make the calculation to quantify how much expansion to include for the flexing of the tank. Hopefully, the new Lead Engineer from TTube will have the answer when we visit the site again. 
A Coil style indirect would not have this problem.

While you are throwing things at it...

throw a pressure reducing valve at it to reduce the incoming cold water pressure to the hot water heater...

Also, throw a shock arrestor on the outlet of the tank to arrest shock created by fast closing valves, like solenoids on dishwashers and clothes washers. Something like the piston powered one from Sioux Chief.

If the pressure of the potable expansion tanks were too low, they would actually compound the problem. If they were too high, they wouldn't have any effect at all.

ME

So John what did you find?

Did the engineer come up with a solution for you?

flexing tank

We are installing some slow opening and closing actuators on the pneumatic valves that fill the flush tank. We shall see...
I suspect that this will cure a large percent of our problem. Its a lot less expensive than replacing the tanks with coil style models.

engineer

They dont have a clue. They keep making suggestions that one would not require if the inner tank did not flex. They are coming out.

flexing tank

I had a very similar problem.I had two boilers and two 120 gallon indirect water heaters piped primary secondary for domestic hot water and radiant heating. The primary loop had a Spiro-vent installed. The water lines in the basement were 2" red brass ( that had copper on etched on the side of the piping) but it was threaded. The 2" brass fed 14 bathrooms all with old 6? gallon flushometers.Every time someone flushed the basement toilet the spiro-vent would let loose a spray of water.It drove me crazy, because the feed line of the boiler was protected by the check valve in the feeder, plus a Watts 9D. The expansion tank was oversized and I also installed a domestic hot water expansion tank. The distributor that supplied the boilers and water heater thought I was crazy. Then they told me how this was a water heater like no other because it was designed to flex to break off any coating from hard water on the inside of the tank. It then dawned on me that the flexing of the tank was causing a type of cavitation, creating a pressure fluctuation. When I explained to the distributor my theory they stopped taking my calls. I knew I had to be right there was no other explanation. I also had to spend several hours adjusting the water temp in the heaters by moving the temperature bulb, up and down the internal tube  (on the lowest temp setting I was getting 160 degree water out of the factory) I had to move the capillary tube up a few inches then run water and see when the boilers would cut out....very frustrating day!!

6 gpm flush valves

Did you ever eliminate the boiler pressure fluxuations?