Indirect or tankless for off-grid DHW?

[SteveJ]

I am modifying my off-grid setup described in this thread https://www.hearth.com/talk/threads/12118/

Adding 600 gallons of unpressurized storage and several flat plate collectors (3-4x10, 3-4x8, 3-3x8).

Configuration will be very similar to Nofossil - seems to have least number of components and smallest electricity draw - Thanks Nofossil

Question - referring to NoFossil's diagram http://www.nofossil.org/heating.html,

Would an indirect water heater (e.g. SuperStor) or a tankless with a pre-heat option (e.g. Bosch 1600PS) be more efficient?

I guess the main question is how to provide DHW at 120F and radiant down to 90F using the least amount electricity with a Seton W-130 and several flat plate collectors and as little propane as possible?

Thanks,
Steve

 

[Nofossil]

That's a complicated enough goal.

Problem with tankless heaters is that they don't modulate well enough. They're designed to heat pretty cold water, so if they're heating warm water you can get water that's too hot at the output. Add a mixing valve to prevent scalding, and the flow rate can drop below the trigger point and the heater turns off.

I'm planning on using a tankless heater as my backup for both DHW and space heating. It will charge my indirect tank as needed rather than providing DHW directly. To get faster response, I'm using a flat plate HX with a potable circ to charge the top of the DHW tank directly.

 

[SteveJ]

Nofossil,

Very cool - so the oil boiler is gone...

Tankless - I was worried about the internal HX corroding and about the lack of selection of those available that accept preheated water.
I like how you have the Tankless in the closed loop to address all the issues.

So, the tankless will keep the house warm when you are gone...

What is Z5 valve for in your diagram - isn't either the tankless or the wood boiler (or the storage tank) as the heat source? Is Z5 to allow for the tankless to mainly just heat the DHW - short path to prioritize?

Which Tankless model are you looking considering?

Thanks,
Steve

 

[SteveJ]

Nofossil,

Another question about tankless:

They seem to be able to only do a 40F differential, so will that be enough for backup?

Steve

 

[Nofossil]

Nofossil,

Very cool - so the oil boiler is gone...

Tankless - I was worried about the internal HX corroding and about the lack of selection of those available that accept preheated water.
I like how you have the Tankless in the closed loop to address all the issues.

So, the tankless will keep the house warm when you are gone...

What is Z5 valve for in your diagram - isn't either the tankless or the wood boiler (or the storage tank) as the heat source? Is Z5 to allow for the tankless to mainly just heat the DHW - short path to prioritize?

That was the original idea. I've since scrapped it, and I'm just using the zone valve for the DHW tank itself to provide a return path.

Which Tankless model are you looking considering?

Another question about tankless:

They seem to be able to only do a 40F differential, so will that be enough for backup?


Thanks,
Steve

I got the Takagi T-Kjr. It's rated at a maximum of 180,000 BTU/hr, and can be set for output temps up to 180 degrees. I expect to use it at 180 in the winter as backup heat, and 140 in the summer as backup DHW.


They are capable of a much greater temperature differential than 40 degrees, but there are limitations. The differential depends on incoming temperature, setpoint, and flow rate. They will modulate to attempt to attain the setpoint temperature. They can only attain their rated BTU output with fairly cold incoming water. In my case, I expect that I'll only be able to get about 40,000 BTU/hr when using it as backup heat for my living space.

 

[BrownianHeatingTech]

Would an indirect water heater (e.g. SuperStor) or a tankless with a pre-heat option (e.g. Bosch 1600PS) be more efficient?

I guess the main question is how to provide DHW at 120F and radiant down to 90F using the least amount electricity with a Seton W-130 and several flat plate collectors and as little propane as possible?

The Bosch has a standing pilot, which will suck down the propane. Use a Noritz N-0751M-DV or their new condensing tankless (N-0841MC). They're not cheap, but they're worth the price, in efficiency and quality. After having used their products, there's not another I would consider installing.

Set the tankless up using primary/secondary piping, with a flat plate connected for DHW.

Joe

 

[mpilihp]

So does the Takagi T-Kjr have a standing pilot or instant on pilot so not to use propane when not firing? I really like the idea of the HX for DHW with the tankless heater as a backup. Thanks for the ideas!

 

[Nofossil]

So does the Takagi T-Kjr have a standing pilot or instant on pilot so not to use propane when not firing? I really like the idea of the HX for DHW with the tankless heater as a backup. Thanks for the ideas!

The Takagi is instant-on - no standby propane usage at all.

 

[mpilihp]

Cool thanks for the info, Im diving in this weekend and plumbing the wood boiler into my system. Going simple to start and it will just be running to keep the oil boiler belly hot and stop the oil burner from running. Then next step is to get the DHW out of the oil boiler and then add storage.

~ Phil

 

[Nofossil]

Cool thanks for the info, Im diving in this weekend and plumbing the wood boiler into my system. Going simple to start and it will just be running to keep the oil boiler belly hot and stop the oil burner from running. Then next step is to get the DHW out of the oil boiler and then add storage.

~ Phil

I plumbed my wood boiler in parallel so that the oil boiler is cold when wood is operating. I essentially use an aquastat on the wood boiler to disable the demand signal that would normally go to the oil boiler.

My 'simplest storage' sticky above describes this approach, except that in that example the aquastat is on the storage tank.

 

[mpilihp]

IM well familiar with your sticky on simplest storage setup, have printed it off and refer to it so to make sure im plumbing to allow running in parallel eventually and adding storage. Unfortunately our DHW is in the oil boiler and I cant afford to make that switch this fall. So im essentually using the oil boiler as a cheap, very in-efficent indirect DHW heater. Its not a cold start unit has to stay warm and my goal this winter is to burn as close to zero oil.

Thanks ~ Phil

 

[Nofossil]

IM well familiar with your sticky on simplest storage setup, have printed it off and refer to it so to make sure im plumbing to allow running in parallel eventually and adding storage. Unfortunately our DHW is in the oil boiler and I cant afford to make that switch this fall. So im essentually using the oil boiler as a cheap, very in-efficent indirect DHW heater. Its not a cold start unit has to stay warm and my goal this winter is to burn as close to zero oil.

Thanks ~ Phil

Makes perfect sense. Take some of the savings and put them towards a future indirect or an electric DHW tank with a sidearm.