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Old 01-15-2019, 10:03 PM   #1
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Electric Heating Bias on Combo Water Heater

Hey Everyone,

I have an Atwood DSI water heater and I have a question. As far as I know, there is no way to have a separate turn on temperature for the electric and gas heating, however, would there be any technical way to have the water heater prefer the electric heating option and only start the gas after it gets down to a certain temp?

Iím fairly savvy and welcome some technical discussion on how to make this possible.

If it already is, someone please enlighten me! 🙂

Thanks,
Dave
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Old 01-15-2019, 10:49 PM   #2
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Since 2004 Atwood redesigned controls/functions for their combo water heaters.
12V DC is used to control electric & propane with a DC Relay that triggers the 120V AC to element


Both electric & propane use common DC source, the SAME circuit Board and the SAME set of t-stats for temp control
T-stat Opens at 140*F to stop heating cycle/Closes at 110*F fro reheat
ECO (Energy Cut Off---High temp t-stat) Opens at 180*F and stops ALL heating sources...triggers the Red Fault Light/locks out DC circuit.
ECO will Close when water temp drops below 150*F but you have to manually reset the 'lock out' by turning switch OFF---wait 30 seconds then turn back ON (switch for whichever heat source was being used)


Electric is slowest recovery time....6.2 gal/hr based on initial heat cycle with cold water
Propane ....10 gal/hr
BOTH can be on at same time for fastest heat recovery---16 gal/hr
Since both use same set of t-stats.....they will function at same time


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Old 01-16-2019, 09:20 AM   #3
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Hi Dave! Welcome to IRV2! We're sure glad you joined the gang!

Listen to Old Biscuit! Have fun and keep her between the ditches!

Good luck, happy trails, and God bless!
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Old 01-16-2019, 11:54 PM   #4
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Leave the propane switch off until you anticipate needing a quicker recovery / higher demand. (I know, not what you wanted to hear)
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Old 01-17-2019, 11:19 PM   #5
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Thanks for all the info, everyone!

I knew the answer was, ďitís not meant to work that way/leave the switch off until youíre ready to use it.Ē

Iím more curious as to how it could be done. Really seems like this should be an option, as the gas kicks on and off when the hot water isnít in use. Seems like the electric element should be able to keep up on standby on its own.

What if.... I wired in some sort of pressure switch on a hot water line that sensed when it when there was flow to the shower and activated a relay that closed the circuit for the DSI?

I think I can engineer most of it, but does anyone know what type of switch would sense water flow in a hose?
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Old 01-18-2019, 08:02 AM   #6
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An adjustable water pressure switch....used on well pumps etc


DC wiring would need to be on ORANGE wire (DC from propane on/off switch to circuit board)


Water pressure drops.......press switch closes----propane lights off
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Old 01-18-2019, 10:46 PM   #7
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Eureka! I think weíve done it!

Thanks for the help, Iíve worked on well pump system like that in the past. Totally forgot about that.
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Old 01-19-2019, 12:17 AM   #8
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Post update when you do the modification..
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Old 01-20-2019, 09:41 AM   #9
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I'm surprised there isn't a separate switch for propane firing. Have you tried disconnecting the orange wire to see how well the element keeps up by itself? If it does, then could a switch be installed in that part of the circuit to fire up propane when needed?

Asking because the Hott Rod I installed some 15 years ago does just fine with the three of us taking nightly showers. Granted, ours are usually shorter showers with water shut off between soaping and rinsing. I have turned on the propane a few times over the years to assist when incoming water was frigid or if I felt like taking a longer hot shower. It stays off otherwise,, except when we're boondocking.
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Old 01-20-2019, 10:44 AM   #10
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Quote:
Originally Posted by Mudfrog View Post
I'm surprised there isn't a separate switch for propane firing. Have you tried disconnecting the orange wire to see how well the element keeps up by itself? If it does, then could a switch be installed in that part of the circuit to fire up propane when needed?

Asking because the Hott Rod I installed some 15 years ago does just fine with the three of us taking nightly showers. Granted, ours are usually shorter showers with water shut off between soaping and rinsing. I have turned on the propane a few times over the years to assist when incoming water was frigid or if I felt like taking a longer hot shower. It stays off otherwise,, except when we're boondocking.

OP's water heater DOES have separate ON/OFF switches for electric & propane
Electric can be ON
Propane can be ON
BOTH can be ON at SAME time


OP wants to have BOTH ON but have propane ONLY fire up when water temp drops from using the hot water
Doesn't want it cycling to just reheat water due to natural cool down cycles
Electric & propane on his use same set of t-stats for temp control so when water cools down BOTH come on..


Hence the idea to control the DC to propane via pressure switch so propane ONLY fires upon drop in water system pressure from usage NOT from cool down
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Old 01-21-2019, 12:10 AM   #11
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Quote:
Originally Posted by Old-Biscuit View Post
OP's water heater DOES have separate ON/OFF switches for electric & propane
Electric can be ON
Propane can be ON
BOTH can be ON at SAME time


OP wants to have BOTH ON but have propane ONLY fire up when water temp drops from using the hot water
Doesn't want it cycling to just reheat water due to natural cool down cycles
Electric & propane on his use same set of t-stats for temp control so when water cools down BOTH come on..


Hence the idea to control the DC to propane via pressure switch so propane ONLY fires upon drop in water system pressure from usage NOT from cool down
I understand the OP doesn't want to waste propane keeping the water warm during non use. I also understand that when a faucet is turned on, that he wants a faster recovery. My question, has the OP tried using the water heater with only the electric element turned on? On ours, the element keeps up so I wouldn't want a flow or pressure switch firing the propane when it wasn't needed.

I wonder if a 2nd thermostat set a bit lower than the primary element's thermostat might be an idea? Maybe having it close at 105į or thereabouts? Would probably be fairly simple to mount on the tank and splice into that red wire..
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Old 01-21-2019, 08:19 AM   #12
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You guys are both on the right track. I would like to install something minimal enough as to not void my warranty. The electric does keep up when outside temps are normal, however, when I want a longer shower, or someone is doing dishes while I’m showering it would be useful for the propane to kick in as it is basically on demand hot water at that point. My system will literally produce hot water indefinitely with the propane.

Another time it comes up is when washing the dogs outside. I use the external shower for them.

It’s a bit of a lazy mans upgrade. But, it seems like a very useful mod for many people. Might even market it if I can make a easy to install system.
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Old 01-21-2019, 11:49 AM   #13
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Quote:
Originally Posted by Mudfrog View Post
I understand the OP doesn't want to waste propane keeping the water warm during non use. I also understand that when a faucet is turned on, that he wants a faster recovery. My question, has the OP tried using the water heater with only the electric element turned on? On ours, the element keeps up so I wouldn't want a flow or pressure switch firing the propane when it wasn't needed.

I wonder if a 2nd thermostat set a bit lower than the primary element's thermostat might be an idea? Maybe having it close at 105į or thereabouts? Would probably be fairly simple to mount on the tank and splice into that red wire..

OPs Atwood model uses ONE set of t-stat/ECO for both electric & propane

Design change by Atwood in 2004 so all functions where controlled by circuit bd vs separate control systems.
2nd set of t-stat & ECO would need a complete design change due to the DC circuit and circuit bd controls


Quote:
Originally Posted by Davejowen65 View Post
You guys are both on the right track. I would like to install something minimal enough as to not void my warranty. The electric does keep up when outside temps are normal, however, when I want a longer shower, or someone is doing dishes while I’m showering it would be useful for the propane to kick in as it is basically on demand hot water at that point. My system will literally produce hot water indefinitely with the propane.

Another time it comes up is when washing the dogs outside. I use the external shower for them.

It’s a bit of a lazy mans upgrade. But, it seems like a very useful mod for many people. Might even market it if I can make a easy to install system.

ANY change to wiring/controls etc will VOID the 2 yr warranty. (Per Atwood)
Especially a 'control scheme' change.


The water pressure switch wired into Orange Wire circuit would allow propane to fire off WHEN water system pressure dropped below the pressure switch set point.....would also shut down propane when water pressure recovered vs t-stat control.
So could get into an ON/OFF propane cycling situation which is the 'downside' of idea and on second thought a real issue.
Propane fires ..then water pressure recovers ...propane shuts down.......repeat. On/Off...on/off cause



Think about pump cycling due to low water demand

Then you would have to have a water system pressure control like adding an accumulator but then that might not resolve the propane on/off scenario so that means a bypass system to control pressure cycling issues
This is turning into a Rube Goldberg Project


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Old 01-22-2019, 05:45 AM   #14
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So these parts may or may not exist but ... Why not a temperature sensor on the hot side water lines that would toggle the propane switch when the line heats up. IE Hot water on, line heats up, propane comes on, water off, line cools down, propane goes off. Since it doesn't involve the wiring on the water heater itself no warranty issue.
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