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Converting to gas, considering DHW options

ForTheGlory
ForTheGlory Member Posts: 22
The gas company has run a line to my house and at the end of the heating season I am planning to replace my oil boiler with a condensing gas boiler. I've been considering my options with regards to domestic hot water and was hoping I may be able to get some info here. Here is the situation.

The way the house is currently set up, the oil boiler has a domestic coil which sends preheated water to an electric water heater. The electric water heater is only about a year old, so I probably am not going to replace it right now (though am willing to hear out ideas if someone thinks I should), so the way I see it I have three options.

1- Get a tankless coil in the boiler and use it to preheat the existing electric water heater.
2- Don't get a tankless coil and plan to replace the electric water heater when it reaches end-of-life with a standalone gas water heater (so would need to make sure there was a extra port for gas for the water heater for the future).
3- Don't get a tankless coil and plan to replace the electric water heater when it reaches end-of-life with an indirect storage tank (so would need to make sure the new system is designed to allow an additional zone in the future).

Does anybody have an opinion about what the best option would be?

Comments

  • Eastman
    Eastman Member Posts: 927
    Is there more information you could provide?
    Like: How much heat is required? The type of emitters? Is there zoning involved?
  • ForTheGlory
    ForTheGlory Member Posts: 22
    Thanks for posting. There are 4 zones (upstairs, 2 on main floor, basement), it’s baseboard hot water. The house is about 2300sf above grade.
  • gschallert
    gschallert Member Posts: 170
    What are your DHW needs? How many bathrooms, how many people in the home and what type of fixtures do you have? Any high volume & flow fixtures like whirlpool or walk-in tub, rainfall shower heads? How many fixtures do you want to use concurrently? The answers to these questions will determine which solution is best for you. There is no one size fits all option and the "best" option for you requires more information.

    Off the top of my head I would recommend ruling out #2 from the start since I just replaced a stand alone gas unit myself back in 2016 with an indirect. That being said, what you currently have is basically a tankless on demand feeding an electric buffer tank which mitigates GPM flow of the tankless coil that might not be able to keep up on its own. Does your existing setup provide you with enough hot water? What size is the electric tank? Option #1 would mean getting a combi boiler (DHW & heating in one) and continuing to feed the electric tank. That's perfectly reasonable and eliminates the biggest problem with tankless heaters that aren't sized correctly.

    Option #3 would mean you would have to use the electric tank heater stand alone until it was replaced with an indirect in the future. You could also go with option #1 now and have a combi feed the electric then when the electric tank goes replace it with an indirect. If the electric tank is new and it's big enough to buffer your GPM needs, personally I wouldn't replace until it fails.

    A 4th option would be to get a combi (DHW & heating) sized correctly to meet your DHW needs, go with option #1 and when the electric tank fails remove it from design since the combi wouldn't need the buffer tank that your current boiler & coil apparently do. You have lots of options but measuring your hot water needs in GPM/hr is pretty much the first step. :)
  • ForTheGlory
    ForTheGlory Member Posts: 22
    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. 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.
  • Eastman
    Eastman Member Posts: 927
    I'm biased against combi boilers. These systems are sized for DHW needs, and usually that requires way more power than what is required for the heating system.
  • gschallert
    gschallert Member Posts: 170

    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.

    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.
  • bob eck
    bob eck Member Posts: 929
    Get an heat loss for the house. See how many BTU needed to heat the house.
    First option
    Condensing combi boiler like the Lochinvar Noble boiler.
    Does heating and domestic hot water.
    Three sizes
    110,000 BTU input delivers 2.6 GPM domestic hot water
    150,000 BTU input delivers 3.6 GPM domestic hot water
    199,000 BTU input delivers 4.8 GPM domestic hot water
    With the Noble boiler you can turn the BTU input down on the heating side of the combi boiler and the boiler will still go to high fire for the domestic hot water.
    You can run the domestic hot water from the combi boiler into the electric water heater. The elements will keep the water hot all the time.
    When the water heater fails you can get another water heater or just use the combi boiler by its self.
    Second option
    Condensing boiler heat only with an indirect water heater.
    Match the condensing boiler to the heat load of the house.
    Put in an large enough indirect water heater to cover your domestic hot water needs.
    Third option
    Product like the HTP Versa-Hydro unit
    This unit combines space heating, domestic hot water, and total system control into one compact, high efficiency mod-con unit.
    No primary secondary loop needed. No near boiler piping. No indirect water heater needed. No isolation valves.

    I would look at these three options.