Water make up

EBEBRATT-Ed

I have noticed recently that there are quite a few hot water systems that have posted pictures, and, in some cases, the Make-up water is piped to one side of the circulator, yet the expansion tank is piped to the opposite side of the circ.

I am surprised no one but me seems to question this. "no hot water to baseboards" is the most recent one.

Jamie Hall

Good point. The makeup, if it's automatic, should be close to the expansion tank.

hot_rod

If the fill is on the suction side and the pump pushing towards the expansion tank, with enough delta p it may over pressurize the system.
Unknown is if the fill valve is valved off, or not able to open due to old age?

Flushing second floor hydronics, post is an example.

I,d expect to see more relief valve tripping questions if they are allowing excessive fill?

EBEBRATT-Ed

With the MU water on the pump suction and the prv set to 15 psi and ex tank on discharge set to 15 psi pump starts decreases the suction pressure by the amount of head on the pump over pressurizes the ex tank (effectively making it smaller)

hot_rod

I think you would need to define the circuit to know how much, the inlet side pressure drops.. Or measure it
if the fill valve is valved off, it wouldn’t change pressure 

Teemok

Doesn't it change the system pressure once? On first pump start it drops suction side pressure below 12 and the fill valve recovers making the pump off static pressure higher. After that initial increase the pumps suction only reduces the pressure to the fill set point so no new water enters and the static cold pressure remains slightly elevated and stable while the acceptance of the expansion tank is reduced.

edit: I get what you were saying. fill valve off. Ignore the above.

hot_rod

In this example a low static fill was use to demonstrate how a negative pressure is established. A higher head circ could also pull the negative on a system with a 12 psi fill.
Where the tank attaches establishes the PONPC, so unless water leaves or enters the piping that pressure point will not change. Pumping Away taught all of us that concept.

If you knew all the piping, fittings, valves in the circuit you could put exact numbers to this example. Also the head of the circulator needs to be considered. The Circuit Simulator module in HDS software is useful for "what iff" examples.

A small low head circ in this hypothetical example doesn't establish much ∆P. Looks like an 007 by the color:) Although graphic this was generated from old B&G diagrams