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[Corngrinder]

Anyone using their boiler in conjunction with an on-demand water heater?  I have my plumbed in and ready, I just haven't opened the bypass yet - I'm not sure how it will affect the tankless heater. 

Reading on this site has led me to believe that I'll get about a 20 degree temp swing in the water to water plate HX.  So, by that theory, my tankless heater will be getting feed water around 160-170 degrees (using CB5036 with set point at 185-190 degrees).  Since my on-demand system is only set to heat up to 115 degrees, I theoretically shouldn't be using any electricity to heat my water... right?  It should just flow through my tankless system and not require a second heating?  This also means that my DHW will be coming out around ~160 degrees at the shower, sink, etc. 

I understand I could put a mixing valve in to keep it closer to 145 degrees, but I think I would rather mitigate the hotter water by mixing cold in at the shower or using scald preventers.  I think I like the idea of 160 degree water getting to the sink and dishwasher though.

So... to the point.  Is my theorizing correct?  Will I indeed be getting 160ish degree water after the HX?  And, if so, will feeding 160 degree water into my tankless have any adverse effects on the tankless system's integrity?

[slimjim]

You would be better off to consult wit the manufacturer, I would bypass the heater and use an Aqua stat to kill it's ability to operate when your wood boiler is up to temp.

[RSI]

You will get up to 190 degree water into the water heater if that is what the owb is running at. The 20 degrees is what you will see on continuous flow through it. What sits in the plate will eventually get to the same temperature.

You have to use a mixing valve with the setup you described. Most anti-scald fixtures just balance pressure. They don't compensate for actual temperature. They just prevent a pressure drop on the cold or hot line making a sudden temperature change.

If you want to not use a mixing valve you will need to add a buffer tank.

[Corngrinder]

It will always be a continuous flow.  Aside from the cup-full that sits idle in the exchanger prior to use, my system will be on-demand.  That's why I expect to see an average continuous flow temperature around 160 degrees.  I believe my water to water HX is rated for up to ~7ish GPM, which is more than enough to accommodate my DHW needs. 

Using a mixing valve seems counterproductive to me, unless I'm confused...  With a mixing valve, I would heat my DHW with the HX, then cool it with the mixing valve, then heat it again with the on-demand system?  Or, heat it with the HX, then "heat" it again with the on-demand system, then cool it with the mixing valve?

Using an aquastat seems like the best idea... perhaps in conjunction with a 3-way zone valve?  Only problem is that I'm not sure how to wire that.  The set-up would go something like this: flow to the on-demand system when feed water temp is <X and flow bypasses the on-demand system when >X, but again, no idea how to wire that in.

[RSI]

Put the mixing valve after the on demand water heater.

[slimjim]

If you wish to use an Aqua-stat, get a break on temp rise, strap on Aqua stat, Honeywell's  6006C 1018 is the one I use, the break on rise terminals are R+B, use those terminals to interrupt the power wire to the wall hung.

[LittleJohn]

You could have an untempered (potentially up to 180f+) line running to appliances (Dish Washer & Clothes Washer), but I would not run that hot of water to faucets, showers or sinks. 

I recently installed side arm and thermostatic mix valve (adjustable, but set to 145F); trust you me, it is not safe to have DHW that hot, took about an hour of futzing to figure out a temperature that was accetpable to me and the misses.  If you can not stick you hand under a running faucet it could WILL be a problem; just remember you might know the HOT water is that temperature, but what about guests and small children.

[Corngrinder]

That's a good point, LJ.  I'm leaning towards using a mixing valve now, on account of I could get one installed in less than 5 minutes.  I like the idea of an a-stat, but that's an undertaking for another day.

Anyone know if there's a mixing valve with a manual bypass available?  In other words, one that I could "skip" if I wanted un-tempered water without actually installing a bypass line?  Running separate lines to appliances isn't an option I want to use; I have one main hot supply that branches out to the rest of the house (sinks, showers, dishwasher, you-name-it).