Question about DHW Plumbing w\Unpressurized Storage

I'm setting up a wood boiler\oil burner parallel system. I have an unpressurized storage tank with both space, and dhw heat exchangers. This post is about the dhw plumbing.

So, starting at my current mixing valve... Cold water comes in the top, directly from my well. At the bottom of the mixing valve is hot water from my oil boiler. Out the back of the mixing valve is a copper pipe that turns ninety degrees vertical, goes straight up about about 8 feet or so, and converts to red pex which goes out to all the areas of my house that require dhw.

My initial plan was to put a tee in the copper pipe that has the mixed dhw a foot or so before where it converts to pex. I would then connect that tee to the hot water output of my storage dhw exchanger.

Here's my question. Let's say it is summer and the oil burner is essentially basement temperature. The tank is heated. It seems to me, when there is a call for dhw, cold water from the well is going to be pushed into both the oil furnace, and the cold water input to my dhw heat exchanger in storage. Hot water (that has already come through an anti scald valve on the storage exchanger) is going to go into the tee I described in the previous paragraph, and mix with unheated water coming through the oil burner.

Am I over thinking this, or do I need to incorporate a zone valve below the tee that I am planning to add? The zone valve would only open when the oil furnace was running.

Thank you for your time.

So you have a low limit boiler with a tankless coil?

I believe the tempering valves will allow cold water through. Perhaps you could use a third mixing valve?

Have one for storage and one for the oil boiler, then have the output of each go into the two inputs of the third valve. Not sure that would work, just speculating.

Yes. I should have made that more clear. That is the current setup.

I am not sure from your description, but you should not have cold water passing through the oil boiler if it is shut off.
I checked into using a zone valve for DHW. My understanding is that this is Kosher, but maybe someone else can state otherwise.
I have not tried it.

I would use a three way zone valve which would give priority to the wood tank if it is hot enough. There are a lot of ways to do this.
The simplest thing to do is use a manual shutoff.

A 3 way zone valve selects either solar coil or boiler coil. A setpoint control measures the solar tank temperature, at say 120F it allows the flow thru the ZV to the 3 way thermostatic valve for blending. Pay attention to the valve you select the ports are sometimes different from brand to brand. Flow goes into A and B and comes out of AB.

If the solar drops below a use-able temperature the zone valve opens and allows flow thru the boiler coil. The end switch in the zone valve could call the boiler on if you don't want it hot all the time, but you have the start up time.

The Z One 3 way zone valve from Caleffi is an IAMPO approved no-lead valve if that is a concern. Several states require all no-lead for domestic water use, it will be a nation wide requirement in 2014.

A non electric method uses several thermostatic valves together. This Caleffi valve, built for the European market is basically two valves assembled together. since you already have one 3 way thermostatic, you could just add another.

This valve is also offered with 3 thermostatic valves, that limits the temperature going to the boiler if it is an instantanous and has a max. temperature inlet.

This valve is built for solar, but just replace the word "solar" with "wood fired"

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