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[dieselpolaris]

Gathering all my stuff up to start piping in my basement in preparation for an owb I'm building for next season. First decided that i will be using either 1 inch al pex or 1.25 inch 5 wrap pex for my supply and return for my boiler.Leaning more towards the al pex as the compression fittings seem easy to use and its barrier pipe and seems alittle cheaper than the 1.25. Either will acheive 100,000 btu at 10 gpm with a taco 0011 pump at a 100 feet distance from boiler to pump.  I want to run a primary secondary loop system with my furnace plenum having its own seperate pump, and also a pump for some in floor in my upstairs. My question is has anyone ran there water heater on its own aquastat that turns on a seperate pump to heat the water in tank when its needed. I figure this way it will function the same as it would normally. This way its only taking heat off my boiler when hot water is needed and i wouldnt need to run a mixing valve to keep the tank at a reasonable temp. Still debating on sidearm vs plate exchanger. I can build the sidearm for basically free but not sure it will work as good as the plate. Thanks for any suggestions.

[RSI]

Either size will work fine.
If you heat your DHW like that a sidearm will not work very well unless you pump the domestic water too.

[dieselpolaris]

Will a 30 plate work in my scenario for keeping the water heater to temp?

[juddspaintballs]

Short answer, yes.

Long answer is that no matter what plate number you're after, you want to get the wider and deeper one.  Some aren't as thick as others and therefore have less surface area per plate to exchange heat through.  5x12" by however many plates you want is the normal size you want to look for, I think.  A quality normal sized one would probably suffice with as few as 20 plates. 

Then there's the issue of water temps, the boiler water and domestic water.  The flow rate of both waters, the overall delta T of them after they pass through the exchanger, etc. are all factors that affect the effectiveness of the plate exchanger.  You could use Newton's Law of Cooling and figure it all out if you wanted, but to go back to the short answer, a 30 plate exchanger running with normal boiler temps and a decent flow rate will heat your domestic water just fine on demand.  They don't thermosiphon as well as a sidearm, so eventually the water in your water heater will cool off a bit unless you leave it on to maintain the temperature.  Once again, the speed at which that will happen is related to so many factors, but it will take a long while to cool.

[agriffinjd]

Partially off topic, but since you mentioned you're still unsure between al pex and 1.25 inch, I can tell you my al pex was nice to unroll because it doesn't coil back up.  One little benefit that nobody told me about that I discovered as I went.

I got 5-wrap 1" pex al pex from z-supply, have a 200 foot run to the house buried 3 feet deep and I lose at most 1 degree, if that.  It's been working great.  The dealer I used was in Wisconsin, and I highly recommend him.  http://www.cozyheat.net/Contact-Us_ep_5.html  It was half the price of logstor and I'm very happy with its performance.