Converting from propylene glycol to ethylene glycol

I should add...domestic baseboard and in-floor radiant heating system. Copper piping with steel tube fin baseboard (1950's single zone construction with huge cast iron boiler, converted to multizone system you see pictured) along with two in-floor zones using PEX.

Ahh. Excellent catch on the backflow preventer. While I can leave the makeup valve closed most of the time, that Honeywell BP900 is a double check vacuum breaker and wouldn't meet code...not something I considered. Thank you.

I also plan to install a flow switch in the short boiler loop and tie that back into the gas valve circuit. The low water cutoff is important but if the main boiler circulating pump stops, there will be a lot of noise and bad things happening before I lose enough water/glycol for the low water cutoff to trip.
Scott

No, I have never had a problem with it short cycling - in fact the boiler rarely cycles separately from the zones except for the one in-floor zone. 160k max input BTU, about 2000 sq ft home (1955 brick home and not extremely well insulated either). I think the large 1-1/2" C loop provides a decent thermal mass and having individual zone pumps along with the close-coupled boiler loop keeps the flow rate through the boiler high enough to prevent the short cycling. In addition, the Laars Mini-Therm boiler puts out higher BTUs (close to max BTUs) when the boiler temp is significantly less than the cutoff temp but then regulates the burners back as the temperature comes closer to the cutoff temp (I think I have the cutoff set to 180 or 190 deg F at the boiler). I assumed it was designed this way to reduce short cycling. I think the boiler is also sized about right for the home - a larger BTU ouput would probably want to short-cycle more - I assume. I'll look into the Axiom product though. Thanks.
Scott

It get cold here in Michigan and like I said, older home with "50's" vintage insulation

Actually, one of the in-floor loops is an unheated garage that gets turned on occasionally in the winter when one of the cars needs some work so the added capacity is very welcome then as I have to heat a sub-zero slab of concrete. I was worried that the boiler would short cycle for normal house use but as mentioned earlier, this Mini-Therm regulates the burners down quite a bit as you start approaching the cutoff temperature and I really haven't experienced any short cycling.

Short Story:
You should have seen the original 1950's vintage American Standard cast iron oil-fired hot water boiler with a natural gas conversion burner...it looked liked the olympic torch inside that thing when it fired (and made a nice rumble when you laid on the family room floor above). I'm on our local volunteer fire department and I went on a run that was reported as a basement fire. I went in the basement only to discover the same type of set up I had at my home...an old cast iron oil-fired hot water bolier with a gas conversion burner that somehow failed and blew up the cast iron boiler (must have filled the bolier cavity with gas and ignited it). It blew out all the basement windows and took the basement dorr off the hinges and looked like it lifted the house off the foundation for an instant as there were signs of that. The piping, pump, etc. was a twisted manged mess. It was then that I decided to upgrade. I got my boiler for free so couldn't complain and decided to make it work. My old boiler had a 150K BTU input so I figured I could make the copper tube fin Mini-Therm work even though it was more efficient and might be a bit oversized. Sometimes you have to compromise a bit on a retrofit with some free equipment. It's worked out suprisingly well.