Steam Heat and Indirect Hot Water

steve_173

What are some good general considerations when sizing an indirect versus a stand alone water heater with specifics in regard to steam? How does the indirect limit the boiler's available sq ft of steam? Does an indirect need to be the same size as ordinary tanks? Does anyone know of comparable efficiency data? Are the indirect tanks really much more reliable in terms of leaks/floods down the road?

Here's the link with some details for my situation.
http://forums.invision.net/Index.cfm?CFApp=2&Message_ID=415426#Message415368

TimS

The indirect does affect the amount of BTUs available for steam, but there are too many variables to get a system to make steam and feed the indirect at the same time. Make the indirect a priority circuit with an aquastat. Otherwise you'd need to balance the BTUs such that the water was making steam but not so much that it overcame the head needed to stop the circulator from cavitating. If the return temp from the indirect was constant, or you had a modulating flame with a feedback signal sourced from the return temp, you *might* be able to engineer this.

steve_173

Let me see if I understand this. You size the sq ft of steam. You size the indirect to the demand and boiler. Then you just prioritize the hot water? That would not work if you needed both at the same time.

In my situation I plan to use a 2/3 and 1/3 boiler. I’m still unclear as to how to size the indirect tank? If an indirect should be the same size as a standalone hot water heater than I would need to tie the indirect to the 2/3 boiler. This means that the 2/3 boiler will be very busy maintaining steam and hot water.

Are you actually gaining any efficiency, or are you just reducing the BTU input and gaining efficiency that way? In my case it is 301 MBH in verses 366 with a 65 MBH hot water burner. Of course, the boiler fires longer to make hot water so maybe you don’t gain anything net, although the longer cycles are more efficient. It might make the small boiler turn on more which might hurt efficiency.

TimS

You're installing two boilers?

The indirect is sized to your demand. Find the storage capacity and first hour ratings that satisfy your hot water needs.

You can't make steam and feed an indirect at the same time. Either the returning water from the indirect lowers the boiler temp below boiling, or you cavitate the circulator by making a huge fire, which leads to extreme short cycling when the indirect isn't calling.

jim

steve_173

Yes, per the board's recommendations I am using one boiler at 1/3 the sq ft of steam and one at 2/3. The idea is to run both and then shut the 1/3 after the piping has heated up, using the bigger one to maintain.

Is an indirect usually the same size as a ordinary hot water heater or is it usually smaller. We had a 50 gallon gas water heater and it was fine although some might put a 75 in the home if they use a lot of water. What size indirect does that translate to?