Multiple tankless heaters in serial


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Old 12-09-15, 05:42 PM
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Multiple tankless heaters in serial

New house build. I am responsible for my plumbing. I would like to make use of the tankless heaters. All the charts are showing that my area will produce ground water at a low of 47 degrees. I will be using electric, not gas/propane. I have a simple house with a 3 and 2 piece bathroom right over each other and a kitchen sink 24 feet away. The washer is right beside the downstairs 2 piece. I am currently looking at installing my heater in the pantry, which is midway.

I would like to install a whole house tankless heater and 2 smaller area tankless heaters in series with the larger unit. This will have a higher installation and equipment cost but should work better in the long run.

I can not seem to find any good calculations for stacking these to determine size of primary and secondary.

Any body know how to select the right size? I would think having both areas would need the same high end temp as a design setpoint.

For whole house I have found a 27KW is recommended.

thanks!
 

Last edited by walkeasy; 12-09-15 at 06:15 PM.
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Old 12-09-15, 05:54 PM
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Welcome to the forums.


I would like to install a whole house heater and 2 smaller area heaters in series with the larger unit.
All tankless ?

I'm not a big fan of electric tankless heaters and I'm not the plumbing pro here but you are looking at a tremendous electrical load to supply three tankless units.

Just re-reading your post..... you'd need a heat rise of at least 80 to reach usable temperature.
 
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Old 12-09-15, 06:14 PM
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Yes, sorry. All tankless. It looks like I was wrong. Low temp would be 47. I will see if I can edit that.

Also I see 27KW water heater would do it for a single whole house water heater and that would be about 3 GPM on the coldest water days.

27KW heater is a 3 double 40 amp breakers. This is a new build and really doesn't add to the cost much as I am installing the plumbing and electrical. As hoped though I would rather reduce the main and use the multiple tanks in serial.
 
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Old 12-09-15, 06:24 PM
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Depending on what else is electric.... like the heating and cooking..... you'll need a minimum of a 200A service.... and maybe more.

Be sure to do demand calculations so as to have enough power.
 
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Old 12-11-15, 11:08 AM
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Are you positive you want to go three (????) tankless. I really don't see any benefit. Personally I would install a looped supply system, with an inline pump to a regular 40-50 gal Water heater tank. Cost of install of additional supply pipe and pump, far outweighs the benefit of numerous tankless units, IMO.

Yes you would heat all the water in one large tank but with the inline pump on a timer, the water is pumped hot as needed to any given faucet, within seconds, even faster than a closely placed tankless. Electricity cost for the pump would also be significantly less than the demand placed on the panel for tankless unit fire-up. With a looped service, pumps are placed close to the water heater drain valve, a virtually silent and typically use a dedicated 20 amp circuit.

Unless, of course, you have an underlying reason for going tankless, x3, which isn't clear.
 
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Old 12-11-15, 03:21 PM
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Be sure you tell the power company your plans to install an electric tankless unit because the transformer they supply may not big enough. I have seen them install a 15KVA transformer for a 200 amp service which would not be able to carry a 27KW load.

Depending your other electric loads (Dryer, Oven, A/C, etc) you might be looking at a 320 amp service to your house.
 
 

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