Will this work?

JimmyJam

I have a customer that would like to remodel his bathroom and replace CI rad with a Quik Trak system. The catch is that the existing system is gravity. Can I close gravity loop and install a secondary with a 3-way tempering valve and a circulator or am I looking for trouble? Heat loss is 2,049 BTUH, requires 114 degree water with a flow rate of .47 GPM. Could I use a strap on aquastat on gravity main to cycle Quik Trak or am I just dreaming about the whole set up?

Mike T., Swampeast MO

As long as you keep flow really low there shouldn't be a problem adding in a [very small] circulator in the way you describe.

Here's an idea for an elegant approach. Instead of a 3-way tempering valve, use a two-way thermostatic valve like available from Danfoss, Oventrop and possibly others. It will vary the flow to keep the loop temp within reason--the hotter the supply, the lower the flow.

To control your circulator use a simple mechanical adjustable setpoint controller with its sensing bulb outdoors. Have the circulator kick on whenever it's below 60° or so outside. Bring the piping for the new bath as close to the boiler as possible (as shown in your drawing) and insulate it nicely. Install a VERY small fixed bypass between supply/return to the bath right after the circulator. A good manual-adjusting flow-control valve in the bypass would be a nice touch. Since gravity boilers have a high water content, you'll get to "live" off the heat in the boiler when it's not firing and you'll have a nice proportionally controlled, constant circulation system with indoor reset for the bath with a really happy and comfortable customer.

Just keep your circulator SMALL! As you say, you only need to move ½ gpm at the head loss of the bath circuit. Use too much circulator and you'll get too much bypass and it might interfere with the gravity circuulation.