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[rhugg]

Installing an OWB with hot water coil.  I now use a Rinnai On Demand propane water heater.  I'd like to run the hot water from the boiler coil into the inlet on the tankless heater so that I get hot water quicker (Rinnai heats the cold water in the line to the OWB) and it is ready to go without adjusting valves to redirect water when the OWB is 'down'.

Any experience with a setup like this?

[RSI]

That is the way it is usually done on the tank type water heaters so should work fine on the tankless. Is the wood boiler coil out in the boiler with a separate set of lines going to it? If so, I would add a pump and keep it continually circulating so you don't have to wait for the water to get there from the boiler. If you do this you would want to add some type of a temp switch so the pump shuts off when the boiler is not hot. You could either use an aquastat or just a cheap snap switch would work.

[MTJAG]

My Rinnii is fronted by a 50 plate HX.  The only problem I am having is that at times, when there is heat demand, the Rinnii will run for up to 20-25 seconds before it realizes the hot water is there, then it will shutdown.  I think part of the problem has to do with my circulating pump from my OWB.  I have it set to run at 2GPM. 

[rhugg]

My setup is that the HW Coil is in the boiler, The boiler is probably 60' of 3/4" PEX away from the Rinnai.  So when the Rinnai senses flow it comes on which reduced the hot water lead time to the normal time.  The hot water coming from the OWB goes thru a mixing valve that is set higher than the Rinnai so as soon as the Boiler water makes it to the Rinnai it effectively shuts off.  As long as you keep the mixing valve temperature high enough it doesn't fool the Rinnai into running.  The benefit is that I have seamless hot water and a reasonable delay.  I am happy with it so far, I think the mixing valve also works as a check valve so I can't send hot water back the cold line or back the boiler line.  The Rinnai fan does run after it senses hot water but that is to cool the exhaust.

[mgw44]

I am in the early stages of planning an install of a gasification OWB.  How does this work with the Rinnai?  I read the previous comments but I don't get it.  I see how the OWB's work with the tank water heaters but i can't seem to figure out how they work with the tankless.

Is there a heat exchanger made to go over the water line that will heat the water to temperature as the house water passes through the exchanger?  Would this be placed in-line before the Rinnai?

[RSI]

I am in the early stages of planning an install of a gasification OWB.  How does this work with the Rinnai?  I read the previous comments but I don't get it.  I see how the OWB's work with the tank water heaters but i can't seem to figure out how they work with the tankless.

Is there a heat exchanger made to go over the water line that will heat the water to temperature as the house water passes through the exchanger?  Would this be placed in-line before the Rinnai?

If I was doing it I would use a plate heat exchanger (probably after the water heater) and a mixing valve to control water temperature. Then put a snap switch or aquastat on the boiler pipe that disables the on demand water heater when there is hot boiler water. (snap switch is much cheaper)
If you use a 20 plate heat exchanger it will be able to supply more hot water than the on demand heater can.

[rhugg]

I have a domestic hotwater coil in my OWB.  Cold water goes to the boiler and hot water returns from the bolier.  The hot water then goes tru a mixing vave which I have set to be a little hotter than the Rinnai setting.  Rinnai claims that the unit can see 5 degree accuracy or something like that.

Typically what happens when I ask for water is that there is 60' of cold water in the line between the boiler and the Rinnai so the Rinnai comes on and heats the water for maybe the first minute therby getting me hot water sooner.  Once the Rinnai sees the hot water it shuts off (quits firing but exhaust fan runs to cool it).  If the boiler is cold the Rinnai continues to run.  I also have it setup to bypass the Rinnai if I like.  I do not know if a plate exchanger will heat the water fast enough, I have no experience, but I assume it will so why not go from the plate exchanger to a mixing valve to the Rinnai?

Mine is seamless and I'm happy so far.  Today the water is going to a cold OWB (80F) and then to the Rinnai as the boiler is off and I have not bypassed the Rinnai.  The annoying thing about an on demand HW heater like the Rinnai is that every time someone turns on the hot water to washe their hands for 5 seconds it runs but the hot water is never used!

[mgw44]

Thanks for the information.  I like our Rinnai except it takes 53 seconds to get hot water to the kitchen sink.  Other than that I have learned to brush my teeth first in the morning to get the hot water upstairs before I shave and shower.  I also learned to just wash my hands with cold water.  If I knew I was going to consider a boiler I would have replaced our electric tank water heater with another one.  However, Your solution sounds good.  I will consider it if I do an install.

Thanks,

mgw44

[RSI]

What would happen if you put a 1 gallon electric water heater after the on demand heater? I would think it would operate very similar to a tank setup that way. All it would have to do is keep the gallon of water warm. The on demand would be putting hot water into it once it kicked in so it wouldn't need to do much.