Difference between boiler and tankless

[hvacbs1]

What is the difference between a tankless hot water heater (such as the Noritz 0841) and a typical small boiler? Both are mod-con.

[Corpsie]

The basic answer is that boilers are designed for space heating and tankless water heaters are made to heat potable water for cleaning and food use. The long answer is all about the advance controls built in to the boiler.

And the answer reading between the lines of your question: No, you will not save money getting a Noritz 0841 installed vs a small boiler such as a Munchkin. More likely you'll pay more in the long run in the fuel usage since it probably doesn't have all the advanced controls as a boiler (reading their literature, it doesn't).

[skippro]

The difference is that they're designed to do two entirely different functions. A boiler is allowed the luxury of heating water slowly, heating many gallons of water from start, which could room temperature or higher, to high limit. A mod/con boiler, depending on the distribution medium, can operate between off (room temp) and maximum high limit as set for the system. That could be 200-degrees or higher.

A tankless water heater is designed to heat relatively smaller volumes of water instantly! That is, water enters the heater well below room temperature, normally 50-55*F and must be heated to a maximum of 130*F before it leaves the unit. That could take a huge flame if 2-showers are running simultaneously. So the gas pipe must be sized for a Btu input that greatly exceeds the Btu input for the average boiler in an average home. The tankless also has a minimum flow rate to fire the burner. It does not operater just on water temperature.

Two different units, two different missions, two different installations, two different costs.

[propmanage]

HVAC bs1
When you say typical small boiler are you talking about an independent domestic hot water heater with a storage tank or a boiler with domestic water heating ability or a boiler with a separate storage tank. It may help if you tell us why you ask?

[hvacbs1]

Corpsie,

Thank you for your reply. “Reading between the lines,” is it a no-brainer to use a mod-con boiler to provide domestic hot water or is it ill-equipped to handle this job efficiently (i.e. high-delta high-temperature rise for domestic hot water versus low-delta temperature rise for home heating)? I am thinking about replacing the boiler and hot water system in a 5-plex. Would I be better off separating the two functions or integrating the two functions? They are separated today...

[johnsp]

Probably save you energy over the year as one heat source is producing both heat and hot water. But you will need a goog deal of storage of domestic hot water for the load requirements of 5 apartments. There will be periods of heavy demand (morning showers) where a tankless WH or combo could not keep up. You need to store HW, and have enough buffering, for the boiler to
re-charge the DHW tank. You're probably looking at using at least a 160-180
gal indirect storage. You're probaly going to have to do an 80 gal primary tank with a secondary 80gal storeage tank hanging off of that.

[hvacbs1]

Propmanage/johnsp,

Thank you for your replies. Can a boiler and a tankless hot water heater work together in unison. I have noticed that Munchkin advertises the ability to enslave another boiler. Could the slave be a tankless? For example, could you size the boiler for typical need and have it control the tankless for extra horsepower on extra cold days? In conjunction, could you configure a tankless to be the primary source for hotwater, but call the boiler for “extra kick” when the demand outstripped the tankless capacity? Or, for example, if one failed, could the other backup…albeit less efficiently. Or, would the best scenario be to use to smaller boilers?

[hotwaterworld]

you will not need 2 tanks to do the amount of hot water you need for a 5 unit building. Depending on your local mean low outside temps I would think you are a max of 175,000 btu input required for that size of building with a mod/con boiler. Then you probably will want a single 80 gallon max indirect water heater as long as you have somewhat conservation minded shower heads. I do a lot of apt building jobs, I have one 28 unit 1 bedroom units with 2 60 gallon indirects w/ single 399,000 btu modcon boiler. No problems. Another 12 unit fairly large 2br units and very leaky building w/ same set up for heat and hot water. Main criteria is the boiler needs sizing for heat load 1st then you look at recovery and storage capacity for domestic hot water with a priority setup on boiler for potable water supply. Easy calcs to do if someone pays attention. If you have large units and in very cold climate you may have to go to a 250,000 btu input boiler but highly doubt it. This would be the most reliable and economical system to put in with lower operating costs. Pick a good mech. contractor and look at Triangle tube condensing boiler and Heat transfer superstore indirect water heaters. That is my preference for this setup. Good luck