Sidearm poor performance
Ant, I too am the owner of a CB OWB. It is an E-Classic 2400. Performance of the stove is rated so far at a 10 based on about 2 months use since install. I have a 185k HE in a plenum and a sidearm on 40 gal HW heater that CB rates at 25K-30K BTU. My propane HW heater is 34K BTU when in operation. My household is a family of 4: my wife, 10 yr old son, 9 yr old daughter, and myself. My daughter loves to take a 20 to 25 min shower if you aren't monitoring her. My son therefore would always get stuck finishing with a cold shower. Talking to CB, they told me usage was considered "excessive".
The sidearm is meant to "build" up the temperature over time; actually it will take the sidearm longer to recover the temperature than it would if the tank was being heated with propane because the sidearm delivers fewer BTU’s than if the propane were firing. I found it that when my son finished with his cold shower, it would take nearly 2 hours before the water was warm enough to be comfortable for me to get in and shower, and you notice I said warm, not hot! The sidearm is a very passive means to heat water. My CB dealer could not believe I could run out of hot water. I was the first case he heard of in 14 years of selling CB units. He said he and his wife could both take showers and he has never run out of hot water. However, he is a 2 person household, both adults, using a 80 gal electric HW heater, taking about 10 minute showers each. No wonder! That is not comparing apples to apples!
Now for the truth: If you are a "normal" size household, performing "normal" daily activities such as laundry, showers, dishes, etc., and you don't have all day to do it, you may, out of sheer luck and by practicing some major conservation practices, get by with a sidearm only, but don't bank on it. Here is what you must do if you need to get 4 people to church on Sunday before the closing hymn at the end of the sermon. Get a plate exchanger big enough to supply you about 140 degree hot water at your estimated gallons per minute usage while performing several functions at the same time, for instance, doing a load of laundry while taking shower and the dishwasher is in operation. Run your cold supply line from your water softener, hopefully you have one if you water is bad enough to be a concern for a plate exchanger, into the plate exchanger. If your boiler operates in the range of 185 degrees the water should exit the plate exchanger ideally around 140-160 degrees and dump into you hot water heater. By doing this, you are not diluting your water heater that was already heated to 165-170 deg by the sidearm before you started using water for your duties, with water straight from the well which is 45-50 deg. Now your water will flow from the water heater through a mixing valve, don't leave this little contraption out, and to your faucets. Basically, the plate exchanger will work like a hot water on demand water heater and your 40 gal HW heater is preheated stored water just waiting for you to enjoy it. The water in your HW heater will never get colder than the water exiting the plate exchanger, and even if you used so much water that the water heater was full of 140 degree water, as soon as your water usage subsides, the sidearm will begin bringing up the water temp until it reaches say 165-170 degrees over the next couple hours waiting for you to tap into it and enjoying even more hot water. The biggest problem in most cases is that the dealers are always dealing with customers who want to get out as cheap as possible so I feel they are hesitant to suggest any additional equipment in fears of scaring you away from even the most basic of options.
Word of advice: Don’t buy a boiler that “might” heat the sq. footage that you want to heat, buy the next size up; install a larger heat exchanger than you think you will need to heat your house and you will never have a cold feet; install the proper equipment to meet your demand for your hot water heater, and you won’t be the unfortunate last person in the shower wishing you had been first!