[tbonehedge]

I'm no plumber so take my words with a grain of salt, but I just went thru this a couple months ago after I purchased a house. The pic you attached is a tempering valve (looks like a Watts, very popular brand), it takes the hot water from the boiler (190°) and mixes it with the cold city (or well water for you country folks) to make whatever temperature is set on your little black dial that you removed. My tempering valve went bad so I replaced the guts in it, the sweated valve body attached to the pipe was still OK and stayed in place.

So, if your looking to lower your water temp coming out of the faucets, replace the guts in the valve (VERY easy to do, adjustable wrench was all I needed). You can also turn your boiler down from 190, you'll only need it that hot to heat your house in the winter.

I eventually installed an electric hot water heater and now turn my boiler off all summer. Costs me almost $300 for the heater and about $30 more a month in electric, but with the cost of fuel oil now I'm glad I did it. Good luck!