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[LittleJohn]

Are thermostatic mixing valves a must?

Not REQUIRED, but HIGHLY recommended, because like CountryBoy eluded to -> depending on setup, the temperature of you water heater will approach the temperature of you BOILER.  I don't know the temp of your OWB, but mine is about 180f, that might make for a very brutal shower  I think if your run a secondary heat source in excess of 140f to your water heater it is highly recommended, if no CODE to mix down to 120f.

In the state of Minnesota, as a landlord I am pretty sure I am required to have the wate heater set between 110-130 on a rental property!   ...don't ask me how I know 

[ST98]

Is there any way to just heat your water through an exchanger without having a hot water tank, and be hot enough?  Like an on demand system.

[CountryBoyJohn]

Yes. Just plumb a plate exchanger after your water heater. It will heat it as you use it.

[ST98]

How many plates would you recommend?  I have a 50 in the house and a 70 in my garage.

[CountryBoyJohn]

Woah!  20 plate is all you'll need for domestic hot water.  If you go with the "instant" setup I could see a 30 plate giving you more readily available hot water, but I would be going with a mixing valve in that case for sure! 

[ST98]

Pardon my ignorance but I have another question(s)?  Would my well water then run into my heat exchanger then into my hot water line?  What is the normal temp of hot water in a house?  My electric hot water heater is only a couple years old and I don't want to replace it. It's got some funky connections that don't look like other people's who can bolt on a side arm.

Thanks

[CountryBoyJohn]

Hey, everyone is on here for more information!  The plate exchangers have 4 ports.  The left two are for your domestic water and the right two are for your stove water (when holding the exchanger vertically.)  Your well water will enter the exchanger at the top left and exit the bottom left.  The stove water will enter the exchanger at the top right and exit the bottom right.  (Some people have the flows go opposite, I fail to see how it makes much of a difference.)  After the well water exits the exchanger, it will enter your water heater.  The only modification you will have to do is cut your supply line, attach it to your plate exchanger, and attach the exit to your water heater where the supply used to enter.  Very simple, very minimal changes to your water heater.  You will not have to replace it. 

Normal high temps for a water heater is 120 degrees.  The setup I described above can get your water up to approximately 140 degrees, depending on your stove temp.  Most people do install a mixing valve to prevent scalding. 

[mlappin]

If you have a large enough exchanger cross flow won't matter. It does make a difference though according to most manufacturers.

[ST98]

Sounds easy enough. Thanks for your assistance!

[slimjim]

I always plumb them the way the manufacturer suggests for the most efficient exchange and that is with countercurrent flow.