Converting to gas, considering DHW options

ForTheGlory said:

Thank you for the info! We have 2.5 bathrooms for 2 adults and 2 children, no high flow fixtures. Definitely would like to be able to have one person in the shower and one person in the kitchen or something at the same time, probably will need to be able to have two showers going at the same time in the future. I have never had an issue with the current setup providing enough hot water though my wife has mentioned waiting a little bit to get in the shower after I get out. I do notice the water temperature is a bit uneven at times. The current tank is 50 gallons.

Based on this information I wouldn't recommend the #3 (or #2) option, getting a heating only boiler and using the existing electric tank heater stand alone. It's not going to handle two concurrent showers and being electric the recovery time is slower than gas. If you had an 80 gallon I'd be more optimistic and say you could go that route for now and replace it with an indirect in the future when necessary due to failure. But you don't. From personal experience I can tell you 4 people won't be happy with a stand alone 50 gallon electric unless you're all into 5 min showers. ;-) At this point with the information provided, I'd recommend you go with option #1, get a combi boiler and continue to feed the electric tank until it fails. At that point you can replace it with another electric, an indirect or remove the tank entirely. To remove the tank from the design you just need to ensure the combi you install can provide the GPM you desire. DHW GPM on combi boilers is determined by the high end BTU output. Honestly I don't have a preference for any of the three options, it would come down to a financial analysis. Combi with lower GPM cost + electric tank every 5-7 years amortized. Combi with lower GPM + indirect tank amortized. Combi with higher GPM (3-4) amortized. We can't discuss pricing here on The Wall, but I just did some quick calculations using some current online pricing and the option with the lowest cost amortized over the same time period is going with the higher GPM combi, not replacing the electric tank with another buffer when it fails. Something like this combi. I'm not recommending any specific brand, just using it as an example of one that provides 4 GPM hot water which is in the range I think you should be targeting. You can use the following formula to determine GPM:

P=(Tm-Tc)/(Th-Tc)
P= hot water ratio
Th= Supply hot water temp
Tc= Cold water inlet temp
Tm= Desired mix temp at fixture

Example: Scenario #1 dead of winter Portland OR

P=(110-40)/(140-40)
P=70/100
P=.7

.7 x 2.5 gpm = 1.75 gpm of 140 degree water

In the scenario above in order to take (2)concurrent showers where you need 100 degree rise you need 3.5 gpm. If you know your cold water inlet temp then you can figure the degree rise you need and plug into the formula above. If you don't know you can always measure temp of the cold water at a tap or look it up based on geographic location & if private well or municipal supply. The varying inlet temps and configurable SWT will change the P value, or the hot water necessary at any given fixture based on total flow. As well as flow rate of the fixture itself. Lower flow shower heads 2 GPM and lower will also reduce it. In the example above, if you have 2 GPM shower heads instead of 2.5, hot water needed per minute per shower becomes 1.4 GPM. Or 2.8 GPM for two to shower at the same time. I would apply this formula to every fixture in my home then decide how many fixtures I desired to use at the same time and add up GPM to figure out the combi output I needed.

ForTheGlory said:

The current setup was in place when we bought the house, so I do not know why it was chosen. I hope that info helps.

Likely because the existing tankless coil in the boiler can't deliver enough GPM to meet demand. The electric tank is just buffer storage, a hack to allow the tankless coil to produce the hot water since it's faster than the electric, with enough in the tank to allow the demand to be met. With enough buffer any draw exceeding the replenishment is transparent to the end user.