Can anyone recommend a thermostatic mixing valve?

I've attached a rough diagram of my domestic hot water system. One thing I'm lacking is a mixing valve (anti-scald) on the hot water line between my Superstor tank and the Takagi heater. I think I'd like to keep the water in the superstor at about 170 degrees when the boiler is running. The Takagi is working great at not coming on when the water passing through it is already up to temperature. When the boiler hasn't been running, the Takagi kicks on and we don't even know the difference (which is great). But now that we're going into our first boiler season with this setup, I know I'll need a mixing valve to avoid the scalding risk for my kids... and unsuspecting visitors.

Questions:
1. can anyone recommend the best mixing valve? I have 3/4 copper lines and never more than 10 or 12 gpm.

2. do I need a check valve? I saw a retail outlet that recommends a check valve on the cold water inlet to the mixing valve. To prevent hot water going into the cold water side I suppose? Is that a real or imagined problem?

3. Also, I still don't need an expansion tank, do I? I don't have an expansion tank dedicated to the hot water tank presently, since I'm on a well and any expansion in the hot water tank can currently be taken back up in well pump's very large expansion tank. I think I still won't need an expansion tank, so long as the hot water tank pressure can "see" its way back to the well pump's expansion tank. ( there are no check valves in the way.) I do have a pop-off valve on the Takagi. Perhaps I should add a pop off valve to the superstor, since in theory someone could close the supply shut off valve between the well tank and the hot water tank while the boiler is dumping heat into the hot water tank.

4. Has anyone done anything like this and placed the mixing valve downstream of an on-demand hot water heater like the Takagi? One reason I might want to do this is to be able to plumb super-hot water to my dishwasher and clothes washer. I'm off the grid and I really hate to exercise the heating elements inside these appliances. Also, the hotter the water, the shorter those appliances can run.

Thanks for any advice or thoughts or even just questions to make me think twice!

ps. I noticed my diagram is not 100% accurate - in reality the DHW has priority over the radiant floors on the primary loop.

In spite of my past poor experiences with Pexsupply.com I recently purchased a Taco mixing valve from them. It works great and their price was the best around.

I did not install a check valve. I'm not sure how you could have reverse flow unless you loose house pressure.

Looks like you are "pre-heating" in the tank before the on-demand unit? Do you not have any kind of expansion tank on the wood burning side of your diagram? I assume you do. I suspect you shouldn't really need a domestic expansion tank for the reason you suggest - you pretty much already have one. But others may disagree and I have seen setups that have them on the DHW loop....

A couple thoughts. First check with SuperStor about that operating temperature I've noticed some tank manufacturers are noteing max operating temperature.

Temper before the feed to the Takagi, I'm not sure they want to see 170F inlet?

Look at the ASSE listing on various mix valves as there are different listings for different functions, temperature only, temperature and pressure, etc.

Sounds like an ASSE 1070 is what you want on the tank. This valve includes checks as part of the listing and can never exceed 120F outlet during the testing and listing, regardless of inlet temperature, usually defined as 1080F for the test.

The Cv is the flow rate a valve will allow with a 1 psi pressure drop. Typical residential valves have a 3.5 Cv high flow valves 5, some 7.5Cv.

When you start playing with high temperature DHW, be sure to chose the correct valve! I also advise shower valves that have a "scald guard" features. The best ones watch temperature and pressure fluctions. This could be used even with the mixer back at the source as extra protection.

You need to do this properly so guest, or future owners are protected also.

This link explains a bit more on how the valves work and size. google around for other info.

www.caleffi.us/en_US/Technical_brochures/01092/01092.pdf

You may need a thermal expansion on the DHW if you check all sides of the valve.

hr

Iam using honey wells sure any would work as long as it has the range

I am using a Taco valve seems to work pretty good with my standard gas hot water heater that gets up around 170 or so. I do not have a check valve or exp tank on the DHW side.

If your DHW tank ever gets close to 118, then you'll want the Honeywell AM101. It can regulate its output to within a couple of degrees of the hot inlet - much closer than the Taco mixing valve.

Thanks guys. Based on your feedback, I'm probably going to buy the Honeywell(sparco) AM101, unless there is a huge price difference between it and the Taco unit. I'm also going to try it w/o a check valve going to the cold water side and see how that goes. While I'm installing this, I'm thinking I'll add a pressure relief valve on the Superstor, but not an expansion tank. (the relief valve is just in case someone, like me, does something bonehead, like shut off the flow from the well-pump expansion tank, while the boiler is heating the Superstor.) (Smee, I do have an expansion tank on the boiler set up).

I have noticed one little weird thing about my Takagi on-demand hot water heater, that might require check valves to fix. If someone is running only COLD water the Takagi will sometimes cycle on and off briefly. I haven't noted if it happens when they turn the cold water on, or when they turn it off - but it only happens during a transition. (I'm in the basement when it happens) Best I can figure is that a pressure wave travels from the cold supply through the hot water side, and triggers the flow meter in the Takagi.