Hydronic System Pressure

28W

Just curious . . .

I've noticed various posts where it is recommended that slightly increasing hydronic system pressure helps reduce/eliminate air bubbles.  If the system pressure is increased, does the air pressure in the expansion tank need to be increased also, to match?  For example, if a system starts out at 12#, and you raise it to, say, 14# does the tank pressure need to go to 14# also?

My system is running fine, so I'm not going to touch it.  This question is just for my education.

Mark Eatherton

Only IF....

If the expansion tank is marginal or EXACTLY sized, then yes, diaphragm pressure MUST be adjusted to the system fill pressure. With that said, there is no penalty (other than monetary loss) for over sizing an expansion tank, and in many cases, the tank is oversized, so adjusting the fill pressure without adjusting the diaphragm pressure has no real consequences.



Not adjusting the diaphragm pressure, and turning up the fill pressure affects the ability of the tank to accept expanding fluids. Its called the acceptance factor.



Another factor to your benefit is the maximum operating temperature, which is one of the factors used in sizing the tank and its acceptance factor. It is virtually impossible for ALL fluid to be at the same operating temperature, hence there is some "error" factor already built in to the calculations.



There are other "tricks" that can be employed prior to raising the fill pressures to eliminate gasses in suspension. For example, injecting Dawn Dish detergent will do wonders for getting rid of nuisance air pockets.



If it ain't broke, don't fix it ;-)



ME

jumper

high pressure trouble

Yes higher pressure dissolves more gas. But that means that the water also has a greater affinity for air. Generally the pressure should be the height of hydronic system unless that is too little for your pump's suction requirement.

Jamie Hall

Which it won't be

if you are pumping away from the tank...

jumper

internet is dangerous

How can one know whom to believe? If there is insufficient pressure in a closed loop a centrifugal pump will suffer no matter where an expansion tank is teed off. On the other hand if there's sufficient height of water above the circulator it won't. Location of tank is immaterial. This is why a good pressure gage with the correct height of water marked is a good idea.



Another incorrect idea is that low pressure will ingest more air through pump seal than high pressure. So I repeat that generally the pressure should be the height of hydronic system unless that is too little for your pump's suction requirement.

hot_rod

I'd suggest

the height of the system plus 5 psi, if you have any float style air vents up top on a radiant manifold or air handler for example.



Seems most fill valves and expansion tanks are pre-set at 12 psi, may as well start there.