Tankless combi application with future indirect if needed

[Mmoy]

I am upgrading my old boiler and gas water heater because of a remodel and relocating the boiler location. My first thought was to go with a tankless combi boiler however with six people (2 adults, 4 kids under 8) in our house the domestic hot water flow rate of 3.6 seems like it may be a challenge. Also the combi will be located in the basement with 2.5 baths (one bathroom on the second floor, one in the basement, and the half bath on first floor.)

I am thinking about going with the combi boiler and seeing how it works in our house. If in the future there is too much waiting for hot water on the second floor or hot water flow rate issues would it be possible to add an indirect in the basement for the half bath and the bath on the second floor off of the heat side of the combi and having the bathroom in the basement supplied by the on-demand part of the combi?

I also looked at a condensing boiler with an indirect setup but I was told there isn't an advantage to getting the condensing boiler over the combi boiler. I am not sure if that is correct. Are there advantages of a condensing boiler over the combi?

 

[Bannerman]

I was told there isn't an advantage to getting the condensing boiler over the combi boiler.

Many combi units are condensing. Combi means combined Domestic Hot Water and heating boiler. If the unit's heating efficiency is 90%+, expect it will be condensing.

What brands & models are you considering?

Tankless DHW units must heat the incoming water almost immediately from the arrival temperature up to the desired temperature chosen. The flow rate supported will be directly conditional on temperature rise. For example, a Navien NFC-200 or NFC-175 (both combi) with 199K BTU DHW capacity, can raise the water temperature 100 degrees F when the flow rate is 3.6 GPM or less. If the water temperature rise is less than 100 degrees, then the supported flow rate will be higher such as 5.1 GPM @ 70F rise, 6.0 GPM @ 60F rise and 9.0 GPM @ 40F rise.

If those flow rates are exceeded, then the water temperature arriving at the faucet may not be the expected temperature but somewhat cooler depending on how much the flow rate is exceeded.

Because the water is heated directly as it is consumed, hot water will continue non-stop for as long as needed.

Depending on the hot water temperature setting, most people while showering will mix some portion of cold water with the hot to suit their temperature preference. A typical single showerhead will flow 2.5 GPM max, but if only 60% of that flow is hot-water, then the WH will be supplying only 1.5 GPM or 1.875 GPM @ 75%.

If the tankless water heater or combi cannot provide the DHW flow rate required for the temperature rise chosen, then a 2nd tankless WH can often be added on so both will be cascaded to increase the total supported flow rate.

The time for hot water to arrive at the 2nd floor will mostly be a result of the piping distance. To reduce the time for hot water to arrive regardless of a tankless of tank-type WH, a recirculation pump will often be utilized. If your home is not equipped with a return line back to the water heater to return the flow during recirculation, often the home's cold water plumbing can be utilized for return flow.

 

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[Mmoy]

I'm considering the Lochinvar NKC155 combi boiler. My heat requirements are in line with this boiler and it provides 3.6gpm. I'm located in Pennsylvania so the incoming water temp can get brisk.

I think I am wondering if the combi doesn't keep up instead of adding another boiler can an indirect be added such as pictured below. So some of the DHW will come from the indirect tank and some will be on demand. See my attached pic to see if it helps explain what I'm thinking.

 

[Bannerman]

My heat requirements are in line with this boiler and it provides 3.6gpm.

The Lochinvar literature only shows 3 combi models with the NKC150N closest to the model you specified. The 3.6 GPM DHW flow rate specified is @ 77℉ temperature rise or 3.9 GPM @ 70℉ rise.

Lochinvar specifications are not as detailed as some other brands but the literature does state their NKC199N will support 4.8 GPM @ 77℉ rise or 5.2 GPM @ 70℉ rise which appears comparable to the Navien models mentioned above.

Unlike your existing oil burning boiler that will fire at one constant flame setting, these units utilize a modulating burner with a 10:1 turn down ratio. The flame setting (& gas consumed) is constantly variable in direct relation to the amount of heat needed at the time, to as little as 10% of the maximum capacity. The 199,000 BTU model will provide your family with higher capacity when needed for DHW and/or space heating, but will reduce the gas input down to as low as 19,900 BTUs when minimal heat is required for either usage.