Hydronic Heating Diagnostic and Repair in South San Francisco

South San Francisco — Service Call
Hydronic Heat issue diagnostic and repair

We were recently called out to a 12-unit apartment building in South San Francisco. The centralised heating system stopped working and tenants were left without heat.

✅ Initial Diagnosis
The boiler — Teledyne Laars Mighty Therm — would not start.
The boiler itself was in good condition, but there was no circulation.
Because of that, the flow sensor did not detect water movement and blocked ignition.

The cause: a failed Bell & Gossett circulator pump

✅ Pump details:
• 1.6 HP
• 1725 RPM
• 115V
• Single-phase
• Flanged connection

✅ Pump Replacement
We sourced the exact same model and returned the next day to replace it.

Important feature:
This type of Bell & Gossett pump requires oil in three lubrication ports, and all ports must be oriented strictly upward, or the pump may seize.

✅ Next Challenge — Air in the System
After replacement we restored circulation, but the boiler still would not fire.
The issue turned out to be air binding.

Here’s what we found:
• A lower air vent exists in the basement, but it isn’t working
• No bleed valves on the radiators in the apartments
• No access to the attic, where the upper automatic vent is likely located

✅ Creative Problem Solving
With no access to the top of the system, we had to improvise.

We temporarily jumped the flow switch, ran the boiler and increased temperature to push circulation hard enough for air to escape through the presumed attic vent.

After several attempts and around 30 minutes of runtime, the system finally purged, the boiler fired normally, and heat was restored to all 12 units.

✅ Next Step
We walked the owner through the findings and recommended upgrading to a modern system — a high-efficiency condensing boiler cascade with updated controls.

The client is interested and waiting for our quote.

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