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: http://www.cemline.com/product-list/water-heaters/ffh