Tankless DHW Pre Heated With Stoker

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tugcapt
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Post by tugcapt » Sat. Jan. 03, 2009 2:58 am

Hi Everyone
I installed a Rinnai Tankless Hot Water heater in my home last summer.I was thinking of placing a hot water coil in my stoker and preheating the water before it enters the tankless unit.I am wondering about a couple things.If the water is sitting in the coil inside the stove could it get to hot?Could it be so hot as to damage the tankless heater?
When I installed the hot water system that I used to have. I added a water mixing valve to the output water line from my oil furnace.It had a knob on it,all I had to do was set it and forget it.That will not work as well in this application.What I was thinking.Is there a water mixer that acts like a thermostat in your car.One that will open and close automaticly.When that first burst of hot water hits it it will open and when the water gets cooler from just passing through the stoker it closes.That will also prevent any one using the tap from getting burnt..

I called Rinnai and the guy (cluless Guy) said he never herd of any one trying that.So theres no help there.
Thanks
Keith

 
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Freddy
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Post by Freddy » Sat. Jan. 03, 2009 6:43 am

A coil in the stove needs to have the water moving all the time. You'd be asking for it to sit there until someone uses water. That small amount will simply quickly come to a boil and blow the pressure relief valve that you added right next to the stove... you did add a PRV in your plans I trust? As soon as it blows, new water will cool it & the cycle will repeat. It will hammer & slam...not good.

Your idea is sound, but I do believe you'd have to incorporate a tank into the design. The coil warms the water in the tank, the water goes from the tank to the tankless heater.A mixing valve could go after the tankless, but I think I'd put it between tank & tankless.

 
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Post by TimV » Sat. Jan. 03, 2009 6:54 am

Sometimes, but not always, when you feed water into a tank from a heated source it will be hot enough to "kick out"the that units "high limit"(in your case the "tankless") then when you rely on it for your hot water you will need to reset the "snap switch" if electric.
If it doesnt have electric I am "clueless"...like your installer .
Usually when you have a boiler equiped with DHW its best to use that standing alone.


 
CapeCoaler
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Post by CapeCoaler » Sat. Jan. 03, 2009 2:03 pm

Needs a tank and a heat dump. Cold water to tank as a 'preheater' and if the tank water is hot enough the tankless will not run but if the tank did not have the water hot enough the tankless would then kick in.
This would also work with solar.
No standby losses in the tankless unit.
Without the tank or a heat dump the water in the coils would just boil off if there was no DHW demand.
A boiler with a coil is a different story, the boiler water only hits the low 200* mark and DHW would not boil off in that enviroment in the coil.

 
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tugcapt
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Post by tugcapt » Sat. Jan. 03, 2009 10:56 pm

Thanks Fred I think you know exactly what I'm talking about.I was thinking the water would sit there like a paper cup full of water on a hot surface, it wont burn until the water has evaporated.I forget to add in the pressure that would build.It was just a idea I cam up with sitting around doing nothing.I'm always thinking about getting as much as possible out of a lb of coal as I can.But, even if I added a tank I think it would still build up to much pressure.there must be someway to get a limited amount of BTU's to warm the water even a little.The chart that came with the heater shows how much propane it takes to raise the water temp.The warmer the water entering the inlet,the less it has to heat it to DHW temps.Any suggestions?

 
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WNY
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Post by WNY » Sun. Jan. 04, 2009 8:45 am

Put in a hot water storage tank and circlulate the stove coil thru it, then run it to the On demand Hot water. SO it is then pre-heated to the on demand, and it may not even come on if hot enough.

You cannot leave the water sit in the hot water coil at all in the stove, you must circulate it since it will boil pretty fast and kick the PRV open.


 
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coalmeister
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Post by coalmeister » Sun. Jan. 04, 2009 12:10 pm

I have been thinking about doing the same thing but using my DHW tank hooked to my coal boiler as a preheater. How do you like your tankless?

 
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tugcapt
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Post by tugcapt » Sun. Jan. 04, 2009 1:15 pm

I'm very happy with the tank less.Before I got it in the warmer months we would turn off the oil furnace and only turn it on when we needed to take a shower ,wash laundry,etc. We have been doing that for years although the manufacturer said not to.It would take 5-10 Min's to warm up so it wasn't too much of a inconvenience.Last year I made 100 gals of oil last for 11 months.None the less I still got little remarks from the wife and kid so I bought the tank less.And still got remarks over the price.I just cant win.

As far as propane consumption it hasn't been working long enough to give any numbers yet.It does seem to be working as advertised.
WYN,any ball park figure on the size of the tank?

 
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Post by coalmeister » Sun. Jan. 04, 2009 1:35 pm

tugcapt wrote:I'm very happy with the tank less.Before I got it in the warmer months we would turn off the oil furnace and only turn it on when we needed to take a shower ,wash laundry,etc. We have been doing that for years although the manufacturer said not to.It would take 5-10 Min's to warm up so it wasn't too much of a inconvenience.Last year I made 100 gals of oil last for 11 months.None the less I still got little remarks from the wife and kid so I bought the tank less.And still got remarks over the price.I just cant win.

As far as propane consumption it hasn't been working long enough to give any numbers yet.It does seem to be working as advertised.
WYN,any ball park figure on the size of the tank?
I have a 50 gallon but with the tankless a 40 probably would be fine depending of course how many teenage girls are in the house :D

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