Piping a DHW Coil Into a Electric Hot Water Heater

coldinmaine · Post by **coldinmaine** » Mon. May. 18, 2015 4:52 pm

I've searched a bit, I was wondering if there have been any good schematics how to pipe a DHW coil into a electric water heater so the cold water flows through the DHW coils, preheating the water, and a small pump is used to recirculate water to maintain temp. I have a rough one sketched out...but to be honest when ever I try to design something, I find one of you have already completed a similar project better than I would have!

tsb · Post by **tsb** » Mon. May. 18, 2015 5:25 pm

Pump the water from the bottom drain of the DHW heater.
Go through the boiler coil into the top of the DHW heater.
Use a thermostat to turn the pump on and off.
Simple.

Post by **Lightning** » Mon. May. 18, 2015 5:57 pm

Do you plan to keep the electric tank powered? Is it possible to just have a valve on a thermostat and just let it thermo syphon instead of using a pump?

lzaharis · Post by **lzaharis** » Mon. May. 18, 2015 9:02 pm

My domestic hot water coil was simply plumbed with the cold water in and then feeding the heated water to the in side of the hot water heater. my replacement Propane hot water heater is still the same way.

One thing you should remember is that most hot water heaters now have a check valve on the
cold side to prevent the hot water from exiting the hot water heater through the cold side so
keep that in mind B4 you do anything.

lsayre · Post by **lsayre** » Mon. May. 18, 2015 9:15 pm

I posted this to a different thread a few months back. See attached file.

blrman07 · Post by **blrman07** » Tue. May. 19, 2015 5:35 am

Shouldn't the mixing valve be located after the Hot water heater? That is the final device before it goes out to the system and touched by the people? Aren't you heating the water in the boiler and then tempering it back down before sending it to the electric hot water heater?

lsayre · Post by **lsayre** » Tue. May. 19, 2015 5:54 am

I would rather have 140 degree water going into my HWT (which is maintained at 140 degrees) than 215 degree water (which is how hot my boiler can get before it activates the dump zone).

blrman07 · Post by **blrman07** » Tue. May. 19, 2015 6:42 am

lsayre wrote:I would rather have 140 degree water going into my HWT (which is maintained at 140 degrees) than 215 degree water (which is how hot my boiler can get before it activates the dump zone).

Ok just wondering why you chose to place the mixing valve there.

McGiever · Post by **McGiever** » Tue. May. 19, 2015 7:01 am

For anyone who is following along,
Just to be clear...the DHW through a coil is not a boiler zone. This DHW water is moved only by house water pressure upon manually opening a faucet or starting a hot water using appliance.
An automatic dump zone generally needs to activate a boiler zone.

coldinmaine · Post by **coldinmaine** » Tue. May. 19, 2015 11:35 am

Great advice...thanks! I was also wondering about where to place the mixing valve...larry's reasoning makes sense. I may alos put a couple of ball valves in so I can isolate the coil from the electric heater and vice versa in case one of the two fails...but it looks like I have a plan. I'll post the boiler at some point .....man it throws some heat!

Post by **Rob R.** » Tue. May. 19, 2015 12:40 pm

coldinmaine wrote:I may alos put a couple of ball valves in so I can isolate the coil from the electric heater and vice versa in case one of the two fails...but it looks like I have a plan.

Make sure you can bypass the coil...you do not want to be running cold water through it when the boiler is not in use.

Post by **Rob R.** » Tue. May. 19, 2015 12:53 pm

lsayre wrote:I would rather have 140 degree water going into my HWT (which is maintained at 140 degrees) than 215 degree water (which is how hot my boiler can get before it activates the dump zone).

Fair enough...but the small amount of 215 degree water going into the tank would be quickly diluted/tempered by the 140 degree water. My preference is to have the mixing valve last in line so I can temper the water going to the fixtures.

Post by **Lightning** » Tue. May. 19, 2015 2:12 pm

What about an arrangement like this? No extra pumps needed, electric tank powered off while the boiler is running. Natural convection heats the tank. Temperature recovery of the tank with the boiler would be pretty fast I'm thinking.

Have a valve between the drain on the tank and the input of the DHW coil to stop flow during the off season (like Rob suggested) and power on the electric tank. This valve would be needed so that the electric tank didn't try to heat the boiler during the off season.

Flyer5 · Post by **Flyer5** » Tue. May. 19, 2015 2:30 pm

Rob R. wrote:
coldinmaine wrote:I may alos put a couple of ball valves in so I can isolate the coil from the electric heater and vice versa in case one of the two fails...but it looks like I have a plan.
Make sure you can bypass the coil...you do not want to be running cold water through it when the boiler is not in use.

Just curious. Why? ( Scratching my head)

Post by **Lightning** » Tue. May. 19, 2015 2:57 pm

Flyer5 wrote:Just curious. Why? ( Scratching my head)

Just a guess.. It would make the firebox cold and more prone to condensation?

Piping a DHW Coil Into a Electric Hot Water Heater

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