Appropriate PSI for glycol closed loop?

[sroof]

What's the appropriate pressure for a glycol closed-loop? I've been running at 30 PSI for years, but it's now working fine at 16 - 20 PSI. Are there advantages to running at 30 versus 20 PSI? Perhaps higher boiling temperature?

My system is two Stiebel Eltron SOL-25 collectors, a Stiebl-Eltron SBB300 tank, and new Grundfos 1/25 HP 3-speed circulator (plus expansion tank, check valves, etc). They are tied together with a fairly long run of 3/4 copper pipes from collectors on the roof to tank in the basement. The change in PSI occurred after I replaced a failed circulator. I re-charged the system to 30 PSI but it stabilized at 18 PSI after the air burped out. PSI has been stable for days, so doesn't seem like I have any leaks. The system is perfectly capable of running at 30 PSI and I can easily bring the pressure back up to 30 PSI if there's a good reason to do so.

[Mike90250]

no reason i know of to add PSI. I generally thought the only pressure was the height of the water column from the gauge to the top pf the roof. Without a pressure tank (bladder and Schrader Valve) you can't get higher pressure, without stressing the system. The glycol raises the boiling point, but higher temps ruin the glycol quickly.

https://www.engineeringtoolbox.com/propylene-glycol-d_363.html

[J.P.M.]

One question : What's the % glycol in what I'm assuming is a water/glycol mixture ?

As long as the maximum actual system pressure (probably at the pump outlet for a pump located at or near the system low point, is < the system design pressure for the highest expected (or design) temp., there is some advantage to operating at a pressure > than that needed to avoid boiling due to the vapor pressure of the mixture at some temp.

One reason for ensuring adequate system pressure is to avoid pump cavitation. That's not a worry for most solar thermal systems - mostly for atmospheric systems under low elevation heads, but needs checking at design stage.

Besides pump cavitation, the other common considerations are to have the system pressure high enough to avoid system boilover if the pump dies and the system stagnates on a hot summer day under bright sun, high ambient temps. and no wind. Or, for systems with large changes in elevation, having a system pressure high enough to avoid local operating pressures at the system high point that are below the vapor pressure of the glycol (mixture ?), probably at some failed pump stagnation temp.

In a correctly designed system, as long as the operating pressure is < system design pressure at max. design temp., high(er) operating pressures are not a problem and are usually the common way to proceed.

Just monitor the Ph of the glycol, especially after a heat induced pump failure or other high temp. event. As Mike writes, heat (and I'd add, time) screws up the Ph.

Also, take extra care to avoid leaks in glycol systems. Glycol can be fatal to felines and they seem to have a particular liking to the stuff.

[sroof]

Yes it's about a 50-50 mix of water + Hecules Cryo-Tek 100. I used the Hercules test strips and they show the mix is OK for pH and freeze protection to about -10 deg F. I checked the pH because I did have a stagnation event the other day when my local power went out for a few hours during bright sunshine. There was steam coming out the Watts Hy-Vent located at the top of the panels. It seems some of the plastic parts inside the Hy-Vent melted!

Thanks for the info - I'll take the pressure back up to 30 PSI.

Oh - is the Hercules Cryo-Tek really toxic to cats?

[J.P.M.]

Yes it's about a 50-50 mix of water + Hecules Cryo-Tek 100. I used the Hercules test strips and they show the mix is OK for pH and freeze protection to about -10 deg F. I checked the pH because I did have a stagnation event the other day when my local power went out for a few hours during bright sunshine. There was steam coming out the Watts Hy-Vent located at the top of the panels. It seems some of the plastic parts inside the Hy-Vent melted!

Thanks for the info - I'll take the pressure back up to 30 PSI.

Oh - is the Hercules Cryo-Tek really toxic to cats?

Understood. I'd get a vapor pressure curve for 50/50 glycol/H2O and use it to identify a pressure that's higher than the vapor pressure of the mixture at the highest likely system stagnation temp. and pressure at the highest point in the system.