Air in Hydronic heating system

RobertK

This is interesting... My friend had her boiler replaced about 3 years ago. Shortly after, I added glycol and successfully purged out all the air. It has been running "silently" since then - until last week. A 3 story, 3 Zone house and now all 3 zones are loaded with air and gurgle continuously. The 3 Hi-Vents in the Mechanical room were not replaced during the boiler change-out and they are all pretty corroded, but not wet. Is it possible they are sucking in air? The pressure at the boiler has been at 20 psi since day one and still is. I'm an HVAC/R contractor and familiar with Hydronics, but at a loss on this one.

hot_rod

It does take time to get air out of glycol solutions. Ramping the temperature to 180F for some hours can speed up the air removal. Is the boiler on outdoor reset control that it is just starting to run hotter?

You really need a good microbubble air purger on glycol systems. Hi vents or ramp purgers just don't grab the microbubbles which over time form into the larger air bubbles that you are hearing.

I know it is a pain to re-purge a glycol system, but if the air purger cannot eliminate it, you need another power purge.

What size purge pump do you have? It takes 1/2 hp or so to get up around 10- 12 gpm with a glycol solution, for an adequate purge.

Got a pic of the near boiler piping? If you are pumping at the expansion tank, air removal gets much tougher.

EdTheHeaterMan

I remember taking a one day seminar offered by @DanHolohan back before the turn of the century. (Boy does that sound old). There was this old man from the 17th century, he is so old that he is dead now, that wrote a paper about an experiment he did that proved that the amount of gas (AIR) that can be held in suspension in a liquid is proportional to the temperature that water is subjected to. Robert Boyle or Boyle’s law states: higher temperature less gas in suspension, Lower temperature more gas in suspension.

YEA… There is AIR suspended in the water. But in the 19th century this other old man proved another fact that Boyle’s experiment discovered, specific to oxygen dissolves in water, Henry’s law High pressure more air in suspension Lower pressure, less air in suspension.

So if you want to get all the air in a liquid to be released to the gaseous state (no longer dissolved in the liquid) all you need to do is lower the pressure and raise the temperature. And guess what happens in a heating system. The temperature gets heated up, and the pumps cause lower and higher pressure zones.

SO if you can put BOTH the lowest pressure and the highest temperature in the same place at the same time, then put a microbubble air separator right there, VIOLA, all the air will exit. Now you can put that air in a old style compression tank (expansion tank) or if you have one of them new fangled tanks with the air pressure already in it with a bladder or diaphragm that separates the water from the air (Extrol is an example) then you can just let the air leave thru the vent.

This is all explained in a book that Dan wrote called Pumping Away and other really cool piping systems… available in the store on this website or on Amazon.com

So your system may have air from the original fill with glycol that is just now getting around to becoming released from being suspended in the liquid.

hot_rod

Does the system really need glycol? It brings along a lot of baggage.

RobertK

Thanks Gentlemen. I guess I will have to change out the Hy-vents and install something more robust. And yes the system should have freeze protection. The owner is away often in the winter and there is potential for freeze-ups. Appreciate your replies.

hot_rod

RobertK said:

Thanks Gentlemen. I guess I will have to change out the Hy-vents and install something more robust. And yes the system should have freeze protection. The owner is away often in the winter and there is potential for freeze-ups. Appreciate your replies.

So how are the plumbing lines in the house protect if the heat fails?

RobertK

Good point Bob. She leaves the well pump breaker OFF. I know that wouldn't prevent plumbing from potentially bursting, but may prevent a flood - from both potable and heating water. There's not a lot of iso valves, so it looks like I'll have to drain the system to replace the air eliminators. Are you suggesting refilling the system with straight water?

hot_rod

A few things about glycol. Beside the price and occasional maintenance, checking ph and adding inhibitors. It requires more pumping power, doesn’t transfer heat as well, and tends to leak around some threaded or crimped connections. It is also harder to get dissolved or entrained air out. You need a fill tank to maintain pressure, not a fill valve that could dilute the mix.

Anytime air purges out, you need to add some fluid to maintain the pressure. Without a tank of glycol to maintain pressure,you could get air problems. Axiom makes small 6 gallon glycol fill tanks. If you stay with glycol, add a fill system axiom fill tank


Maybe consider freeze alarms, or a thermostat with freeze alert. My Ecobee thermostat sends a txt to my phone when temperatures drop inside.

A pic or drawing of the boiler piping would help us see if the hy vents are helping or hurting, and where to best install an air separator instead of float type vents

psb75

You don't need to DRAIN the system to change out the Hy-vents. You just need to lower the system pressure to "zero" psi, change the vents and then turn the system fill valve back on and put a little bit of new water in. Glycol is a real 'bear' to work with. I tell folks that it is a "necessary evil." Avoid if at all possible.
Glycol is messy, expensive, lowers system efficiency, degrades components, must be monitored occasionally (yearly) etc. A micro-bubble resorber-style of air eliminator is essential. Put it in the proper location in a "pumping-away" system. Old fashioned air-scoops are not sufficient.