#### wwhitney

##### In the Trades

I have no experience but recently installed my first hydronic system (air source heat pump) and have wondered about the pressure vs temperature behavior of a closed hydronic system. So here's a simplistic analysis from first principles, corrections welcome:If I fill the system to 19psi when cold it does reach 30 psi by the time the water reaches about 160-170 degrees. But if I drop the pressure much below that, I find the top floor radiators will take in air and wind up cold. I feel like we shouldn't have a pressure change that large.

For the pressures involved, I believe treating water as incompressible is accurate enough. If you raise the water from 20C (68F) to 75C (167F), then a random website tells me the volume increases 2.4%.

If the closed system is made of all steel components, then steel has a linear coefficient of expansion of around 1*10^(-5) / degree C. The volumetric coefficient of expansion is 3 times that, and so for a 55C change in temperature, the components should expand in volume by about 0.16%. So let me ignore that volume change. [If much of the volume of the system consisted of plastic pipes, the effect would be an order of magnitude larger.]

There's air in the expansion tank, which is rising from 19 psig (34 psia) to 30 psig (45 psia). That means the air volume has changed by a factor of 34/45 = 75%. I.e. from the 20C start, the air volume has gone down by 25%, while the water volume has increased by 2.4%.

I conclude that at 20C, the volume of water in your system is about 10 times the working volume of air in your expansion tank (25%/2.4%). Is that at all close to being true? If so, seems like the pressure increase you are seeing is within expectations.

Cheers, Wayne