Adding in series coil or electric tankless to indirect oil

[Stuartp]

1. 
   Stuartp Looking to increase hot water for shower use as it doesn’t last long enough as it is
   have a burn ham oil boiler with 40 gal indirect hot water heater
   Boiler has inoperative coil in it
   Looking to add either new coil in series with indirect system or electric heater in series for shower point of use
   

 

[Dana]

An electric tankless is a lousy solution, even lousier performance than a tankless coil in a high mass boiler.

Embedded coils are kept at high temperature most of the time, which promotes lime depositions on the interior surfaces the coil, eventually hampering flow. It also requires a high standby temperature, for high standby losses on the boiler.

A regular indirect with the heat exchanger inside the tank rather than in the boiler would allow you to keepboth the boiler and the indirect at a lower temperature for slower lime deposition, and since the potable water is not internal to the heat exchanger it doesn't reduce flow even when substantially limed-up.

Most oil boilers can be safely idled at 135-140F without risking excessive condensation in the fuel or on the boiler plates. To get any sort of hot water performance out of a tankless requires a low-limit of 155-160F, which adds a LOT to the standby losses of the boiler.

A tankless water heater is a HUMONGOUS instantaneous load, which is hard on the distribution grid infrastructure (including the transformer feeding your house). It may require an electrical panel upgrade or even a heavier service drop to your house to support the instantaneous load. Eventually (as has already happened for solar ratepayers in Eversource's MA service area) demand charges will likely become part of all residential electrical rates. Demand charges are typically based on the heaviest 15 or 30 minutes of power use during the billing period, and a separate line item from the kilowatt hour energy charges. An "extra" 15,000 watts during a couple of back-to-back showers at some point in every month can potentially end up costing a lot of money over a year, even if the total kwh used during those showers isn't very much.

Even though it's more expensive up front, either an indirect, or separate heat pump water heater is really the better way to go.

 

[Sponsor]

 

[Stuartp]

An electric tankless is a lousy solution, even lousier performance than a tankless coil in a high mass boiler.

Embedded coils are kept at high temperature most of the time, which promotes lime depositions on the interior surfaces the coil, eventually hampering flow. It also requires a high standby temperature, for high standby losses on the boiler.

A regular indirect with the heat exchanger inside the tank rather than in the boiler would allow you to keepboth the boiler and the indirect at a lower temperature for slower lime deposition, and since the potable water is not internal to the heat exchanger it doesn't reduce flow even when substantially limed-up.

Most oil boilers can be safely idled at 135-140F without risking excessive condensation in the fuel or on the boiler plates. To get any sort of hot water performance out of a tankless requires a low-limit of 155-160F, which adds a LOT to the standby losses of the boiler.

A tankless water heater is a HUMONGOUS instantaneous load, which is hard on the distribution grid infrastructure (including the transformer feeding your house). It may require an electrical panel upgrade or even a heavier service drop to your house to support the instantaneous load. Eventually (as has already happened for solar ratepayers in Eversource's MA service area) demand charges will likely become part of all residential electrical rates. Demand charges are typically based on the heaviest 15 or 30 minutes of power use during the billing period, and a separate line item from the kilowatt hour energy charges. An "extra" 15,000 watts during a couple of back-to-back showers at some point in every month can potentially end up costing a lot of money over a year, even if the total kwh used during those showers isn't very much.

Even though it's more expensive up front, either an indirect, or separate heat pump water heater is really the better way to go.

 

[Stuartp]

Thanks Dana
The right thing to do I guess is to replace the 45 gal indirect tank that is already there with an 80 gal tank At $ 1200-2000 cost
Was thinking that an electric tankless installed to just feed the one shower with input from the operating indirect would be a cheaper alternative thinking that it wouldn’t have to work very hard as it would have the 45 gal already heated water as it’s source or the coil option in the same series setup
The 80 gal tank although not an endless source of hot water is the most logical and efficient (in the long run) solution
Do you agree?

 

[Dana]

Assuming a 2gpm low flow shower, at mid-winter incoming water temps of 40F and 105F out of the showerhead that's a 75F rise, and about 1000lbs of water per hours, which is (1000lbs x 75F=) 75,000 BTU/hr, which is ~22,000 watts.

Got an "extra" 100 amps of 240V of capacity in your panel?

If the tank is already hotter than 105F it can deliver the whole shower. If it drops to 70F after multiple back to back shower's the tankless has to deliver 12,000 watts, which is still a huge load. A standard electric tank only draws 4500 watts, a heat pump water heater even less.

Regarding long term efficiency...

If it's showering capacity you're after and you have more than 5' of vertical drain downstream of the shower, a 4" x 4' drainwater heat exchanger can return about half the heat from the water that's going down the drain, which would turn a 45 gallon indirect into an "endless shower" situation even with a pretty crummy boiler setup. That's also a grand or so, but unlike an 80 gallon indirect the extra "apparent capacity" in showering mode doesn't use any fuel.

It won't enhance tub-filling capacity but it'll more than double the apparent capacity in showering mode if the tepid water output is feeding both the cold side of the water heater and the cold potable feed to the water heater. I have a 4' x 48" in my house, but if you have the vertical space for more the additional return efficiency would deliver both a faster return on investment in reduced oil use, and higher apparent capacity for the water heater.

Renewability's PowerPipe series are listed as pre-approved potable fixtures in MA, and can be purchased direct from the manufacturer or through Home Depot. Sometime they are available on Amazon, sometimes not. Some of EcoInnovation's Thermodrain series are also approved in MA, but no longer available for direct purchase, and I'm not aware of any US distributors. Any others would require a variance to be code-approved in MA. Ecodrain's V1000 series have really great specs for both return efficiency and minimal pressure drop, but aren't currently listed for MA (or weren't the last time I looked.)

 

[Stuartp]

Hi Dana
I have only a coulple feet of vertical drain and a lot of horizontal and on the other side of the basement from the well
Not optimal
Thanks for the info

 

[Dana]

What is the highest operating temp on the existing indirect? Got a model number?

With a thermostatic mixing valve on the output most indirects can store water at temps up to 180F, which substantially increases the apparent capacity of the tank when it's delivering 115-120F water. That's a heluva lot cheaper than an 80 gallon indirect!

What boiler is this? (Make, model, burner size...)