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Steam Fired Instantaneous Feed Forward Water Heaters

Hello all,

I was recently asked to help replace a 3-Way valve for DHW on a commercial feed forward steam fired water heater serving roughly 225 apartments. For these units, the steam is fed continuously at a constant pressure to the steam side and the mixing valve modulates according to demand. Pressure in the mixed supply drops due to usage and new hot and cold water are blended to maintain set point. This is what I found through research, so if someone has a better working knowledge, please chime in.

From what I can tell, there is no temperature sensor connected to the three way valve and I do not believe there is a self contained thermostatic element, it seems to be all done on pressure drop.

My question is, has anyone on here tried to replace the three way "blending" valve with something that is thermostatically controlled, either electronically or with a self contained element? The way I see it, the valve would still modulate open on demand (sensing mixed temperature) causing the same pressure drop through the valve. In some ways it is no different than a typical system (tankless, indirect, etc) but there is some mystery to me about the constant steam pressure. Is there a pressure or thermostatic element closing off the steam supply in on the steam side as well?

I hope my question is clear.

For reference here is a similar model to the one I was looking at:


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,708
    I am assuming the three way mixing valve is on the water side of things. I doubt it is only pressure balancing. It must have a thermostatic element inside or remote sensor or sensing bulb
  • JohnNYJohnNY Member Posts: 2,424
    What he ^^ said. There has to be something in place to sense temperature on the mix and have the valve actively respond. 225 units is going to be an engineered system with many Codes with which to comply.
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is the Boilers and Hydronic Heating Systems Course Instructor at NYC's Mechanics Institute, a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
    John also oversees mechanical installations and maintenance for metro-area clients with his family's company, Gateway Plumbing and Heating along with his brother/business partner.
  • ratioratio Member Posts: 2,279
    According to the linked model, there's a thermostatic mixing valve that is fed with hot water from the steam HX via a pressure-operated valve. It seems that the steam side is uncontrolled, and the mixing valve gets whatever temp of hot water that comes out of it.
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