Sizing water heater to boiler

[CLEAVER97]

Looking to replace water heater with a new indirect model, either superstor or triangle tube and trying to make the best decision on sizing the unit for my home. Currently have a oil fired peerless boiler with output of 119,000 btu to feed the unit and have a home of 5 people that need to shower, wash dishes and perform laundry during the day. When performing the calcs I cannot seem to determine which would be more efficient, a smaller unit such as a 30 gallon which falls within the btu input requirements for the boiler but is on the verge of the first hour rating (which I estimate requiring 180 to 200 gallons) or increasing to a 45 - 50 gallon unit which exceeds the first hour rating requirement but is oversized for the boiler. Would the purchase of a larger unit be a waste of money or would it be better to have a larger unit for the water storage with the understanding that the boiler would need to run more to keep up with the unit.

Any input would be great appreciated

[jadnashua]

Think of it this way...a standalone WH often has 30-50K BTU inputs, you have 119K. You can easily heat any of the size tanks you are interested in...I'd get one that can meet your first hour needs. Now, if the tank spec sheet specifies a larger boiler for the first hour rating, you'd have to discount that value since you won't be able to heat the incoming cold water as fast, but it still will - thus, you'd have to start with a larger tank. Often, the spec sheet shows the ratings for various heat inputs so you don't have to interpolate.

If your system is set up with the indirect as a priority zone, while it is calling for heat, the rest of the house won't be being heated, but unless you have a lot of infiltration, that normally isn't a problem. The other option is to treat it as a 'standard' zone, but you'd lose even more first hour ratings when the other zones were also calling for heat. This may not be a big issue, since, unless you have a really big house with lousy windows and no insulation, you probably don't need all that 119K in the first place, so treating it as a standard zone probably won't affect operations much. Still, the norm is to treat it as a priority zone, and it will recover the fastest that way, and then be able to resume heating the rest of the house.

[Gary in NJ]

I have a W-M Gold 40 that has a 36 gallon DHW capacity. With five people in the house (3 full baths) we've never run out of hot water. In fact, this summer I shut down my boiler and re-piped it to add a zone and to fix some odds-n-ends. The Gold 40 was emptied in this process. After the boiler was restarted I had hot water after just 15 or so minutes.

Make sure you pipe it with 1" copper to the boiler.

[CLEAVER97]

Thanks for the input.

As usual I am over thinking this since I have a 1,200 sq. ft. home with cast iron radiators that over heat the house within 30 minutes of the boiler kicking on and the probability of all five of us showering within the first hour is a rarity, typically it is maybe 2 people showering first thing in the a.m. with maybe the dishwasher running so my first hour load is minimal, just want to make sure I have enough hot water on that rare occasion that everyone in the family has to run through a shower quickly so if the water heater is set up as a primary zone the heat shutting down for a short while would not be an issue but I guess it come down to the efficiency, would a smaller unit such as a 30 gallon super stor (or approved equal) be more efficient unit since it heats the water quicker due to the size or is it better to upgrade to a larger unit that would store more "hot" water. A larger unit would be the easy response but the price difference between a 30 gallon and a 50 gallon (average) is about $400 dollars which is a huge cost difference and I would love to try to save as much fuel oil as possible during the winter months on long island.

Once again that you for the time spent responding and providing your thoughts.

[jadnashua]

If you make it a priority zone, all of the heat goes into keeping the water in the tank hot, you can go with one of the smaller tanks. Go by the first hour rating.