clunk at the end of heating cycle resulting in release thru pressure relief valve

Effer

Specifics: Navien NHB-80 LP boiler in closed hydronic system, ~20psi system pressure, and 30 psi pressure relief valve.
Doesn't happen all the time but occasionally at the end of the heating cycle when the boiler shuts down there's a loud clunk that results in the pressure relief valve purging slightly. The system pressure has maintained around 20 psi since the initial start up and doesn't appear to decrease when the relief valve spits. I have 2 automatic air vents, one on the boiler and one on an air scoop. Neither appears to release any trapped air when the caps are loosened. System pressure increases approx. 1-2 lbs when the circ. pump is running but I haven't witnessed if there's a spike in system pressure when the clunk occurs.
Any ideas what's causing the clunk?
Thanks for your input.

Jamie Hall

Somewhere there is a valve which is closing very fast. The clunk is a water hammer.

HomerJSmith

If the clunk is a fast closing valve, perhaps a Honeywell zone valve, and the circulator has not turned off, the added pressure from the pump may be pushing the pressure relief valve open. Sometimes a circulator was incorrectly installed that has too high a head pressure for the application.

20 psi sys pressure is too high especially when the sys is cold. Drop your pressure to 12-15psi and adjust the expansion tank air pressure to that pressure. A 1-2 psi rise when the boiler is hot is to be expected and is normal.

Effer

There aren't any zone valves in the system, only a check valve on the circ. pump. Would that do the same thing?

Effer

This is as far away as I could get.

bburd

How many stories is the house? Is it all radiant, or is there something like a hydro air coil mounted in the attic?

Effer

It's only 1 level (all radiant).

HomerJSmith

I don't want to be picky, but I'm not too fond of your setup. I like a primary-secondary setup for the boiler. I like pumping into the highest pressure loss not away from it, that would be into the manifolds, I would think. I assume you have a single pump. You should have two flow paths, one thru the boiler and one thru the heat emitters(manifolds). That doesn't appear to be the case. Having one pump is right for one and wrong for the other.

The flow thru the pex can be a lot higher than the max of 2 gal/min. The flow in one part of the circuit is equivalent to the flow in any other part of the circuit.

Perhaps the clunking is the pump shutting off too fast and there is flashing in the boiler HX?

I think I would tie the pump into a delay on break relay or some kind of post operation of the pump for so many seconds after the fire box in boiler shuts down, cooling the HX in the boiler. Just my thoughts.

EdTheHeaterMan

Agree with Homer. The installation manual calls for the boiler to be on a secondary loop of its own. The circulator for the secondary (boiler) does not need to overcome the system pressure drop... only the boiler pressure drop. That Circ is too powerful for the boiler. You can live with it that way but the lifespan of the boiler will be reduced for several reasons. One being, as those little spurts of water leave... over time they will be replaced by fresh water from the auto-feed. This will reduce the boiler life span. Also, the water hammer will cause mechanical parts (like the relief valve, Extrol tank bladder, and some air vents) to stress and fail prematurely. Just to name a few.

If you read the installation instructions, starting on page 18, every illustration has some form of this diagram.



The bottom of the boiler is where the primary/secondary piping is illustrated.
I'm not sure the relief valve is properly located either.

Respectfully submitted,
Mr.Ed

PS, if you want to leave the piping as is... try removing the check valve from inside the pump. The water won't stop so fast without a check valve.

bburd

Effer said:

It's only 1 level (all radiant).

Then 12 psi should be an adequate cold fill pressure. The pressure should not rise more than a few more psi when the system is hot.

Effer

So after digesting the above suggestions and asking multiple other heating professionals (of which had has many different opinions) it made sense to me that circ pump was shutting off too soon (as Homer suggested) causing the water in the heat exchanger to hammer. The Taco controller was shutting off as soon as the thermostat hit the set point not letting the water cool in the heat exchanger. For $hits and giggles i connected the circ pump directly to the boiler control to see if that possibly would delay the pump shutting off and Bingo, it worked. The pump now stays on after the boiler completely shuts down eliminating the clunk/hammer.
I also dropped the system pressure to ~17 lbs. I left it a little high in case it drops with the purging of any more air. Time will tell if those steps will stop the relief valve from spitting but I suspect it will.
Thnx for the help.

Zman

It sounds like you have found a "workaround" kind of solution.
The I&O instructions for that boiler require primary secondary piping to assure minimum flow through the boiler is achieved.
It is always a good idea to require in the signed contract that the contractor will install the boiler per manufactures instructions. In your case it would be a just a matter of pulling out the manual when he hands you the final invoice. You can then require a do-over. As installed, your warrantee is likely void.

STEVEusaPA

On a side note, I don't think the insulation is allowed to be exposed like that. Might need to cover with drywall and at least 1 coat all seams and screw heads.

EdTheHeaterMan

@Effer, Did the installing contractor do the steps that resolved the noisy spurts? Have you consulted the installing contractor regarding the issue? The craftsmanship is decent, I would be happy to call that my work (except the incorrect piping). Neat and orderly, plump and level and clean piping joints. That said, this installer should also be informed of the problem so he can learn from this problem. He does nice work, so it would be nice if it was also problem-free work. If I made the piping mistake you have pictured, I would want to know about it. I would want to get to the bottom of it and I would want to know so I don't make that mistake again. I would also make the repipe at no charge to you. But that's me. I also know of installers that would not make the repairs and just tell you that's the way it is, live with it, then walk away.

I wonder which type of installing contractor you have?

Mr.Ed

Effer

I have the DIY type contractor and at the moment am satisfied with his work, even with the exposed insulation. If I run into further problems I'll be sure to call him back, I know where he lives.

HomerJSmith

I can't see the type of pump that you have. Whether there is a check valve in the volute.

I would say off hand that when the sys shuts off the tridicator gauge pressure goes above 30 psi. What does your two tridicator gauges say about pressure when it goes klunk and spits boiler water out of the PRV? I don't much care for where the PRV is mounted it is too far away from the boiler.

Re-pipe as Ed's diagram shows, a primary and secondary sys, which you don't have now. Put in the missing pipe.

EdTheHeaterMan

@HomerJSmith
That would mean adding a circulator. There is not enough room for a secondary (Boiler) circulator. Since it won't fit, then it must not be needed.

After the boiler has a premature failure then he can get a new one under warranty. Does the Warranty cover the labor to replace the boiler? Oh well, we will cross that bridge when we get to it. The noise is gone for now, so everything must be fine.

This has been a message from the NSS.

Like we need your support!

Effer

The pump is a Bell & Gossett NRF-25 with check valve installed. With the pump now hooked to the boiler control the gauges don't move at all when the system shuts off. They used to bounce when the pump shutoff prematurely (when going thru the Taco controller) but not anymore.

So in simple terms why is the primary/secondary loop needed when I only have the one heat zone?

HomerJSmith

I would say the boiler flow needs may be different than the heating loops needs.

Maybe the pump is putting too much velocity into the sys and when the pump turns off and the check valve closes a slug of water forces the PRV to spurt water. What speed is the pump operating with?

So what is your delta T as shown on your tridicators when the boiler is in full operation?

I know what sarcasm is and I don't engage in sarcasm. The warranty will be denied because the boiler wasn't set up as a primary-secondary sys, I think. There's always room for another pump.

I don't know the pressure loss thru the HX, but the Expansion tank, I think, should be connected to the inlet of the pump. The pump appears to be pumping into the E-tank.

motoguy128

Why are you using a zone panel off that boiler. just wire the pump direct and call direct to the boiler and then it will do an off delay on the pump in the settings.  I believe whenever possible to boilers control the primary pump

Pressure only needs to be set to 10-12psi with a single floor radiant set up like that.  

motoguy128

Probably been mentioned but the air scoop won’t work very well the way it’s pipe that should’ve been mounted down below near the manifold so there’s 10 pipe diameter length,  leading into it.  Expansion tank could’ve been mounted remotely upside down. 

Plus the air scoop and expansion tank should be at the pump suction at the point of lowest pressure not after the heat exchanger before the manifold.  It’s not the worst spot but it’s not optimal