Piping without an external central heating circulator (TRX150 / TRX199 and TRX199C)

What is going on

Experienced guys will tell you this layout works but has limits. The boiler's internal circulator feeds the heating system directly, so there's no external central heating pump. It suits zone valve or non-circulator systems with small flow needs, up to about 9 gpm at 9 ft of head. Go past that and you starve the heat exchanger, so confirm the numbers first.

Likely causes

• Zone valve system with low flow needs
• Job with no space or budget for an external circulator
• High-resistance or long piping that exceeds the internal pump's capability
• System needing primary/secondary piping instead

Check in this order

1. Confirm the system flow is within about 9 gpm at 9 ft of head before using this layout.
2. For direct piping (not primary/secondary), install the expansion vessel on the return side of the boiler.
3. Do not put check valves or other flow-restricting devices between the boiler return and the expansion vessel.
4. Keep minimum flow of 2 gpm through the heat exchanger.
5. Keep the 12-diameter straight-pipe run upstream of any circulator.
6. Check flow after startup by comparing supply and return temperatures.

More boiler guides

• Access the low voltage wiring connections
• Blocking errors versus lockout errors
• Blower not running
• Blower runs at high speed with burner off
• Boiler bangs or hisses; Lockout 81 delta T
• Burner not running on a call for heat