There is a faucet-like valve on the bottom that takes a standard garden hose. Attach hose, open valve, let several gallons of gunky water out. Also look into "sacrificial anode" for it.

Here is what I would do if I was you. You don't need that many btu's to heat up them two rooms. Plus you want to get hot water also. haved a certified takagi tankless water rep. come to your home. ( this will be free ) tell him you want a tk-3 pro. ( better heat exchange in the unit itself) it can fire up to 199,000 btu's and down to 11,000 btus. then you want to use a bronze bump with a heat exchanger on one side, plus boiler trim. (1/6 horse power. with a mixing valve for your domestic water. you want to then put two zones on other sided of a heat exchanger with a zone board for 2 zones with 2 pumps one for the baseboard. Ask for high trim baseboard so you can set the water temp down and get max btu's per lininer foot. set the temp for 150 degress durring the winter cold month. Now for the radiant floor get a viega dealer in ur neighborhood. get climate panels that you can put cement nails in and the put a flooring on top. If you wanted to know more about this rout just leave a post. you can have all 3 comming off one unit. The best thing about this type of set up is that you dont need 180 degree water like a boiler. and in summer time you can set the water temp down to 120 or 115. and when your away on vacation you use no gas. The down side is hat the climate panels can be expensive, the tankless water heater has to have a nice size gas line. the equipment to do this job can get pricy. I mean really pricey. with nice equipment comes a good price. Takagi & viega are my brand of choice. but you could use navian and wirlsboro if you wish. all 4 manufactors will be happy to come to your home and sit dowwn and talk to you. ( I sell all four)

bump...down you go, lad.

I replaced a 50 gallon tank with the Takagi T-K2 about two years ago. Best money I ever spent. The Takagi supplies more than enough hot water to 3 baths, laundry, and the kitchen. We no longer have concerns about doing laundry and running the shower at the same time.