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boiler system with pipes

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Original Post: Weird hydronic issue with 2 zone boiler system

kirmool - Member


I have a Weil-McLain Gold CGT boiler. I have a weird issue that I can produce every single time. The system has two zones (upstairs and downstairs) and plumbed with a single Taco 007 circulator pump.

When both zones are calling for heat at the same time downstairs zone will never heat up. The minute I prevent upstairs from calling for heat downstairs starts to warm up. I have replaced the zone head and zone body but the problem still prevails. My circulator pump was replaced about a year ago.

My house is on a concrete slab thus some of my piping for zone #1 goes under the slab. I'm starting to think if the boiler was plumbed correctly when it was installed. I have no idea when it was installed as I bought the house with it about 10 years ago. I have never really noticed it until a couple winters ago. But now it's driving me mad.

I have attached a few pictures of the system for your review.


boiler system in a basement

boiler system in a basement

boiler element

PJmax - Group Moderator

Welcome to the forums.

Thread moved to the boiler forum.

I reoriented/resized your pictures.

The one thing I see that stands out is the furnace flue into the wall?

Is that a chimney in there?

It shouldn't be open.

What is the system pressure?

kirmool - Thread Starter

The boiler flue goes directly outdoors. There's no chimney back there as it's an exterior wall.

The system pressure is 15 PSI.

lawrosa - Super Moderator

That boiler has a mixer I believe that often caused issues. Give me a moment.

lawrosa - Super Moderator

That boiler heats your hot water too. I know that system is problematic. I'm trying to find documentation. Here's the manual.

lawrosa - Super Moderator

Testing the 3 way valve.

kirmool - Thread Starter

I have ordered a replacement cartridge for the 3-way valve and I should receive it within a couple days. I do have an extra actuator head for this 3-way valve as it has failed me twice where-in I had no domestic hot water. Thus, I had one on standby. I swapped it out with the old one and monitor the problem for a couple hours but no dice. I'll wait until the 3-way valve body cartridge comes in and change that out and see how it shakes out.

I'll report back in once I change the cartridge out.

Additionally, the 3-way valve lever position is always down even when the tankless t-stat is calling for heat.

kirmool - Thread Starter

I received the part in today and made the repair. She's now humming along nicely! Both zone are now providing adequate heat even when they are running simultaneously.

Getting the existing cartridge out was a major pain but In the end a little elbow grease and hacksaw blade saved the day.

Many thanks to you Lawrosa and PJmax.

old boiler parts

replacement boiler part

lawrosa - Super Moderator

Hey man glad you got it fixed... Yeah they are a pain. I don't see too many of those boilers anymore. I worked on my fair share

PJmax - Group Moderator

Good job. Thanks for letting us know how you made out.