Old 1955 boiler 3 zone with 3 pumps 1/2 copper in basement floor radiant and the same in attic

Pressure depends on the height from from the boiler to the highest point in the system.

Measure that height and divide it by 2.31. Take that answer and add 4 psi to it.

Most systems can run at 15psi for a two story house. You have to adjust the expansion tank pressure to match this.

Example:

25' from boiler to highest pipe.

25/2.31=10.8 psi

add 4psi= 10.8 +4=14.8 psi 15psi would be fine

If it was 35' high

35/2.31=15.15 +4=19.15 so 20 psi would be fine

3 zones but how many loops? Is each zone only one 1/2" loop? Are there manifolds with multiple loops? Is there a mixing valve or two anywhere?

While pump purging, are you getting anything out of the return, or the pressure is rising too fast?

I've added antifreeze to systems with radiant, but in houses where there was also 5 fan coils and an indirect or 2, so I would isolate the radiant and fill the rest of the system with antifreeze. Then run all zones to check the level. In your situation, got a hand pump? There's snow melt glycol systems. Somebody here must know a trick to pump purge small radiant loops.

I was born in 1955, so I know all about those kind of systems.

You most likely have air in the loops that are not heating.

Let me see if I understand you correctly

What is working

• Zone 1 – Basement: Working properly.
• Zone 2 – Front of house: Appears to be working properly. The supply pipe from the boiler is very hot and the return is also hot.
• Zone 3 – Rear bedrooms: Problem area.

What is NOT working properly

Zone 3 has a very hot supply pipe coming from the boiler, but the return pipe is only slightly warm.

That suggests hot water is not circulating properly through Zone 3.

What you've already done

• Drained the system.
• Added antifreeze using a pump.
• Pumped antifreeze into the boiler drain.
• Opened the bleeders for each zone to remove air.
• Zone 3 may still have trapped air, especially because of the piping arrangement in the attic.
• You bought a smaller 1/12 HP pump to try purging the system again.

Your proposed plan

You are thinking of:

1. Put antifreeze in a 5-gallon bucket.
2. Pump it into the boiler drain.
3. Open the Zone 3 return bleeder and run the discharge hose back into the bucket.
4. Keep the hose below the antifreeze level.
5. Let the pump circulate/purge Zone 3 for a while to see if more air comes out.
6. If that works, possibly repeat the process on the other zones.

If I got that correct then there is one line in your statement that indicates that you have an air problem.

" When checking temps on 1-1/4 steel out and in pipes the pipe from boiler is very hot but the return is not very warm "

The 1-1/4" vertical pipe will not lend itself well to purging. You need to vent that zone at the high point.

The pump you use to force the antifreeze mixture into the 1-1/4" pipe is most likely connected to the system at the boiler drain with a 1/2" or 5/8" garden hose. If you somehow shut off the return pipe so that all of the boiler water and the water from your 5-gallon pail must travel through the 1-1/4" supply pipe and up to the top of the system, that part should work fine. There is a considerable amount of water in those pipes, but the flow should be adequate to push the water up to the system.

The problem occurs in the 1-1/4" return pipe.

The water flowing through that pipe will not be moving fast enough to entrain the air and carry it out through the purge station, which I assume is connected to the 5/8" or 1/2" hose returning to your 5-gallon pail.

If you are feeding the system through a garden hose at 2 feet per second, the velocity through the 1-1/4" pipe would be only about 0.32 feet per second, or roughly 4 inches per second.

At only 4 inches per second, I would expect a significant amount of air to remain trapped in the attic piping—particularly in those copper pipes buried beneath the vermiculite.