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Plumbing a dump load heater.

42OhmsPA

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I'm probably over thinking this...
I currently have an oil fired water heater and excess solar.
If I were to add a small 120V heater before my current heater could I get the small heater to circulate and heat the oil fired tank?
It wouldn't work if I just plumbed the new hot out to the old cold in and reduced the temp, correct? I would need another loop to let it circulate?
Anyone have a visual?

I'm thinking something like this on a timer or a voltage based relay.

 
You'd burn off excess electricity, but I don't think it would be very efficient (or last very long) in the circulating configuration you're suggesting.

I'd go with a larger 40+ gallon tank and use only one of the elements if you want to stick to 120 volts and not worry about circulating. Just prewarming the water would cut back a LOT of oil usage.
 
Thanks for following up.

I'd prefer 240V to keep things balanced.

I figured a full size would make the most sense but I don't have space in the area for another full size heater and the other spot it would fit is already dedicated to the next battery bank. (I suppose I should have mentioned that in the original post).

I'll probably just replace the oil fired with a HPWH, but I'd have to frame out some sort of a filtered cabinet, the current heater is next to the stoker stove.
 
I think 20 gal tanks have 240v heating elements. You may need to dump a lot of power. How large is your panels?

If you get a hpwh, they have electric elements you can use. Maybe heat to 120 degrees with heat pump, and to 145 degrees with elements. Get a water mixer to keep 120 degrees.
 
If you just plumb a full electric tank water heater before the oil boiler then you won't have to circulate. The water will either be preheated by dump load or not when it flows into the oil boiler.

Possible legionella concerns though.

Edit: Oh I should read first, yes space concerns.
 
The problem with the circulation system would be how do you get the oil burner to wait to heat at all, and leave any heating work for the electric unit to do.
 
I think 20 gal tanks have 240v heating elements. You may need to dump a lot of power. How large is your panels?
I currently have ~11KW between ground and roof but I'll be adding another aray that's at least 3kW.
Good to know on the 20 gallon, I'll check that out, I could frame out a shelf to hold it above the pressure tank.

It's charts like these that make me realize I'm losing out on lots.
Screenshot_20240219-195541.jpg
If you get a hpwh, they have electric elements you can use. Maybe heat to 120 degrees with heat pump, and to 145 degrees with elements. Get a water mixer to keep 120 degrees.
That's a great idea.

I already see this turning into a much bigger project, replumbing the softener tanks to a new location to make room for a hpwh so I can easily box in the corner and have a filtered water closet...
 
The problem with the circulation system would be how do you get the oil burner to wait to heat at all, and leave any heating work for the electric unit to do.
Good point. The oil fired is around 20 years old anyway. I might as well do it right and swap to a new full size unit.
 
Looks like you need to dump 10kW worth of power. That would require turning on both heating elements of a 240v water heater. You will need custom controls to do that. Most only turn on one at a time.
 
When the small water heater is heating water, the pressure builds due to expansion of the water. It tends to push hot water into the larger tank. Put them in series, especially when adding a small tank dump load to a larger tank.
 
When the small water heater is heating water, the pressure builds due to expansion of the water. It tends to push hot water into the larger tank. Put them in series, especially when adding a small tank dump load to a larger tank.
Wish I had a better understanding of fluid / thermal dynamics.
Series would be hot to cold, correct?
Wouldn't you still need a return from the larger tank to allow the circulation?

I could relocate the cold air return / filter box I built for the stoker stove but I really don't want to deal with duct work and plumbing in the same project.
I'll have to take some measurements and stare at it again tomorrow.
 
Wish I had a better understanding of fluid / thermal dynamics.
Series would be hot to cold, correct?
Wouldn't you still need a return from the larger tank to allow the circulation?

Should not need it. With the dump load water heater in series ahead of the main water heater, the pressure will be higher inside the dump load water heater when it is powered on due to expansion. There is also the flow thru as hot water is used at the faucet.

Heat exchangers used on water heaters that use wood boiler heat rely on thermal siphon and convection to heat the water. No pump is used to pump hot water into the water heater, it's the pressure differential due to expansion and heat differential that will create circulation when water is not flowing out of a faucet.

I could relocate the cold air return / filter box I built for the stoker stove but I really don't want to deal with duct work and plumbing in the same project.
I'll have to take some measurements and stare at it again tomorrow.
 
Plenty of heating elements that could be used with existing boiler. You would probably be better off without an extra tank, extra heat loss and they don't last long. Heating elements can be directly put in just a pipe if you have a circulating pump.
 

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