So I am in the process of building a glycol chiller. 10k BTU window A/C with a 100Qt cooler. I have been running tests with just water and seems to have no problem keeping the water at 40 no problem. Do I need to add glycol to the mix?
I've been planning to do the same thing. though I've not built anything yet, my understanding is that it depends on what you intend to do with your chiller and how cold you need to get in order to do it. If you're planning on controlling ales vs lagering vs crash-cooling and how often and how long your chiller runs to keep the reservoir cold.
if you need to get your beer anywhere near freezing or want to change temperatures quickly (as in cold-crashing prior to to racking) you need your chiller to be at or colder than freezing to effectively make that happen..
The colder you keep your reservoir or the longer your chiller has to run, the more likely you are to freeze your solution around the condenser coil. The coil is quite a lot colder than the solution you are using it to chill, so freezing will definitely happen.
as soon as you approach or start freezing, you really need glycol or you'll end up with an ice block around your condenser coil -- which will prevent your chiller from working efficiently. (which will result in the chiller running constantly)
it is not uncommon, in the case of controlling ferm temps, to keep the chilled reservoir very cold (say 15f or 25f) so that you can limit the circulation time needed to keep your fermentation at temp. you don't want your circulation pumps running all the time, right?
For the time and $$ you're investing in building a chiller, I think it makes sense to add *Something* to help prevent icing and issues.
