Hybrid steam and FHW system rumbling

NickP178

Hi All,
Have a situation that has some smart heating guys at a bit of a loss, would love some input; I am the homeowner, but I'm trying to help my older, my less internet savvy heating guy with some research. Sorry if my terminology and descriptions are not perfect...

Background: Single family, previously one story brick bungalow (900 sq. ft.) that had a small second floor (600 sq. ft.) added recently. First floor runs single zone steam off an 8 year old Peerless EC boiler and original 100 year old piping and radiators with no problem. New second floor uses an Everhot exchanger with one zone of FHW baseboard.
When the second floor calls for heat, there is a significant rumbling (sounds like boiling) from inside the exchanger for about 30-45 seconds. Both the heat exchanger and FHW loops have NO AIR in them as they have been bled multiple times. The exchanger is plumbed in well below the top of the water in the boiler and the hot water loop has a properly operating air scoop.

Some observations:
1. The noise only occurs when the second floor circulator calls for heat, and the boiler is currently or recently running (HOT).
2. The return piping from the second floor is quite cold when this happens (New second floor is well insulated and only calls for heat about 1/3 of the times that the first floor calls)
3. I'm not sure if the circulator for the heat exchanger runs off the aquastat or only when the circulator for the basesboard calls, I will look into this. (Both steam and hot water zones run off Taco zone valves)

Can cold, air free water from the second floor baseboard zone entering the heat exchanger be making the noise just due to the temperature differential?

If the circulator for the heat exchanger only runs when the baseboard calls, could this be wired in to run any time the aquastat fires the burner to keep both the boiler and exchanger hot?

Any help or input would be appreciated.
Thanks,
Nick

Danny Scully

Hmm…Both steam and hot water zones run off Taco zone valves you say? 🧐 I’d like to see a picture of what you’re talking about there. In general, let us see what’s going on. 

NickP178

Jamie Hall

How far below the boiler water line is the intake for the heat exchanger circulating pump?

NickP178

Sorry, not zone valves. Switching relays.
I'm a former Manufacturing Engineer (that worked as an apprentice for my plumber/HVAC guy in college) who went back to his roots and bought a car repair shop; and knows just enough to hopefully ask the right questions.

NickP178

Circulating pump is at the bottom of the tank on the "outlet" on the back, The "inlet" is 3 to 4 inches below the top of the water in the front.

Jamie Hall

The reason I ask is that unless you have three or four feet of head at the suction inlet of that pump, you may be getting flashing and rumbling (really cavitation) at the pump inlet.

Danny Scully

One thing I’m immediately noticing is that The circulators are mounted incorrectly @NickP178. They can be just about any orientation except the way they are.

NickP178

OK guys, just caught it rumbling and ran downstairs.... Definitely coming from the heat exchanger, cracked open the coin vent on the exchanger and bled out a bunch of steam. Tried this before when it wasn't circulating and only got water. Where is the steam going when it isn't hot?
Exchanger is well below the level in the boiler (by 3-4") so how is the steam is bubbling in from the boiler?
Would a regular old Taco Hy-Vent be a proper replacement for the coin vent?

NickP178

Jamie Hall said:

How far below the boiler water line is the intake for the heat exchanger circulating pump?

See below

TonKa

I don't see a bypass. Is the circulator for the heat exchanger loop on the supply or the return? I can't tell from the pictures, but I'm guessing the return. It matches your symptoms.

212 degree water will boil at the slightly lower than atmospheric pressure within the heat exchanger created by the circulator if the circulator is on the return and there is no bypass. Even if the aquastat on your heat exchanger loop is set to 180, the water will be at 212 if you are making (or have recently called for) steam when the loop starts. The solution is to move the circulator to the supply side of the heat exchanger loop with nothing in between it (check valve, dirt separator or the like) and the boiler.

NickP178

TonKa, I wish there was a "Buy this man a beer" button on this forum! The circulator is indeed on the return side of the exchanger.
So what you are effectively saying is that we need to "push" the water through the exchanger as opposed to the current setup "pulling" it? How does this solve the rumbling? Not questioning your advice, just curious of the science. Weird stuff.
Thank you.

TonKa

Yes, "push" the water through the heat exchanger. "Pull" it directly from the boiler with no restrictions.

Water at atomospheric pressure boils at roughly 212 degrees. The head loss through the heat exchanger can create enough of a pressure differential drop to cause the water to boil if the loop is not pressurized. Putting the circulator first stops this from happening because the circulator will create positive pressure within the heat exchanger. The head pressure of the water in the boiler will prevent this from happening at the intake of the circulator if the circulator is placed first in the loop below the waterline without anything which causes significant head loss before the circulator, including too many pipe fittings and pipe lengths.

https://en.m.wikipedia.org/wiki/Cavitation