Home>Discussions>INSULATION & HVAC>Bleeding three zone hydronic heating system
3 posts / 0 new
Last post
Bleeding three zone hydronic heating system

I had to drain my heating system yesterday, as I had to replace the expansion tank (diaphragm ruptured).
After replacing the tank,I was able to get heat circulating in the first two floors, but not the third.

I have one circulator, with three Taco zone valves (with manual valve opening levers).
I do NOT have shutoff valves , but have purge valves. No bleeder valves in any of the three circuits.

The layout of the three return lines is (top to bottom):

Purge valve, with zone valve below. All three zone valves connect to a manifold which connects to a 1 inch pipe. Water supply enters this pipe. The circulator is below this. Below the circulator is the boiler inlet.

Obviously I did something wrong with the third floor. I believe I have air trapped. Is there an order to fill the system? I've found procedures to fill systems when there are shutoff valves installed, but I don't have any.

The Outside temperature is 40F today. It was below freezing yesterday. Could I have a frozen pipe?


Re: Bleeding three zone hydronic heating system

When you say you have a purge valve is it just a boiler drain or a purge valve with a turn off? If it has a turn off it will be either a lever or screwdriver slot. To purge the zone that is not heating turn off the purge valve and connect a hose to the boiler drain and open it but keep a eye on the water pressure at the boiler, don't let it drop below 10 lbs. If the pressure drops the stop purging until the pressure climbs bask up. If the line are frozen you will get very little water out of the boiler drain.
When I say boiler drain I am referring to the hose fitting on the purge valve not the one to drain the boiler.


Re: Bleeding three zone hydronic heating system


Congrats on installing the expansion tank.

I would agree with john's purge procedure in general, but would use a slightly different method:

Not sure I follow your description of the return circuit components, but you should be able to turn off the boiler at the on/off switch, and bleed each of the 3 circuits individually using the zone valves as the open/close switch that you read about in purge procedures.

As you noted, manually lifting the lever on the 3rd zone (the one that need to be purged, will manually open that zone, while the other two will stay closed (Taco zone valves are normally closed); you could then attach a plastic hose to the purge valve, close off the lever or turn wheel to close off one end of the piping, & use the "quick fill" lever on the pressure reducing valve to force water thru the 3rd zone & out the hose into a bucket.

When air bubbles stop coming out of the hose end into the bucket, you know that you have most of the air purged out of the 3rd zone.

If there is no "quick fill" lever on the PRV, you can take a screwdriver and turn the long screw at the top of the PRV to increase flow thru the 3rd zone.

When done purging, you can turn the PRV screw back in until you get 12 psi on the boiler gauge.

Make sure to re-close the lever on the Taco zone valve & re-open the closed end of the purge valve to obtain normal service.

It's highly unlikely that your system would freeze at the temps you stated---check the "water altitude" of the system while you're at it to make sure that boiler water is getting up to the 3rd floor.

On 2nd & 3rd floor hot water heating systems, the boiler water height follows the formula of: water height = 2.31 X boiler gauge psi.

Thus, if the psi on the boiler gauge reads 12 psi X 2.31, that means water in your system will only go up to 28 ft. above the boiler; if your upper floor is 35' above the boiler, you won't get any water in the upper floor rads & will have to increase the psi pressure at the boiler (by adjusting the screw on the PRV) to 15 psi.

This sounds like a system that evolved from an older "open air" system with a steel tank in the floor joists, and an expansion tank was added---this is a good step toward making your system a "closed air" system by adding a few air vents especially at the top of the system on the 3rd floor (air in a HW system always gravitates to the highest point in the system; an "air scoop" and a vent is almost always installed at the boiler main supply pipe as it comes out of the boiler; a diagram of some of these component setups is forthcoming---this would eliminate the bothersome task of purging, which is not the best solution when you have a pressured expansion tank---introduced air during any purge contains entrained air in the water, which will quickly leach out of solution to cause air noise in the piping & lower heating efficiency.

For a pdf file of the diagram of eliminating air from a heating system, Google "air elimination from hydronic heating systems"---these components are low-cost & should be included with at least 2 convector vents on the top floor of the system.

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.