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diy solar

Electric and propane water heater in series?

Scorch

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Need a quick response here as I'm having plumber hook up water heaters today.

My initial plan was to have an electric tank water heater to pre-heat my water using excess solar (I have a 16kw system). My plan was to then have the output of the electric WH to go to the inlet of a propane on demand heater which will then be used as a back up, or if the solar just cant heat the water enough.

When I mentioned this to my plumber, he insisted that this won't reduce my propane cost because the propane on demand will fire up the moment water goes through it regardless if it's pre-heated or not (yet it has a thermostat on the output which regulates the temperature, so where would all the extra heat go if my water was going in at 120F and coming out at 120F?)

Anyone have any experience with this?
 
Anyone have any experience with this?
Yes, I had a similar setup years ago and my particular demand water heater did not come on. There are also demand water heaters that do not function well if the inlet temperature is on the low side where lukewarm water would not get heated. I do not know specifically what your plumber was talking about. You are correct the heat has to go somewhere and it would be dangerous to send scalding hot water to a domestic hot water system. At the minimum a tempering valve should be installed to prevent that.
 
Yes, I had a similar setup years ago and my particular demand water heater did not come on. There are also demand water heaters that do not function well if the inlet temperature is on the low side where lukewarm water would not get heated. I do not know specifically what your plumber was talking about. You are correct the heat has to go somewhere and it would be dangerous to send scalding hot water to a domestic hot water system. At the minimum a tempering valve should be installed to prevent that.
thanks for the input..... on further thought, I can also just shut off the propane heater when not in use.... maybe even measure water temp in the tank and use my smart hub to turn on the tankless heater if water temp in the tank is less than 90-100F
 
Need a quick response here as I'm having plumber hook up water heaters today.

My initial plan was to have an electric tank water heater to pre-heat my water using excess solar (I have a 16kw system). My plan was to then have the output of the electric WH to go to the inlet of a propane on demand heater which will then be used as a back up, or if the solar just cant heat the water enough.

When I mentioned this to my plumber, he insisted that this won't reduce my propane cost because the propane on demand will fire up the moment water goes through it regardless if it's pre-heated or not (yet it has a thermostat on the output which regulates the temperature, so where would all the extra heat go if my water was going in at 120F and coming out at 120F?)

Anyone have any experience with this?

I use a Rinnai Infinity on demand propane HWS downstream of a heat pump electric HWS. If the inlet to the Rinnai is hotter than it’s setpoint it doesn’t fire up.
 
Need a quick response here as I'm having plumber hook up water heaters today.

My initial plan was to have an electric tank water heater to pre-heat my water using excess solar (I have a 16kw system). My plan was to then have the output of the electric WH to go to the inlet of a propane on demand heater which will then be used as a back up, or if the solar just cant heat the water enough.

When I mentioned this to my plumber, he insisted that this won't reduce my propane cost because the propane on demand will fire up the moment water goes through it regardless if it's pre-heated or not (yet it has a thermostat on the output which regulates the temperature, so where would all the extra heat go if my water was going in at 120F and coming out at 120F?)

Anyone have any experience with this?

Your plumber is an idiot.

The propane on-demand controls it's outlet temperature and will do nothing so long as the water leaving it is at the set temp.

Most on-demand water heaters are designed to run in series, they sometimes use a master/slave setup or you can just put them in in series without them talking to each other.
 
I can also just shut off the propane heater when not in use.... maybe even measure water temp in the tank and use my smart hub to turn on the tankless heater if water temp in the tank is less than 90-100F
If that will work with the smarthub great. If not a clamp on thermostat can be used with a relay to disable the propane and ignition if it has an electronic flow switch to trigger the gas; many have to be powered to have the gas valve open.

Having said that, I would think the unit itself would be able to be set for a temperature slightly less than the electric hot water heater output so in theory that shouldn’t come on.
 
...

My initial plan was to have an electric tank water heater to pre-heat my water using excess solar (I have a 16kw system). ...
How will the electric tank know when you have excess solar? You do not want to design a tank that has no temperature control. That means it must cycle on and off to warm the water but stop before you lift the over pressure valve or have a steam explosion.

If your solar setup has enough regular power to use an electric water heater than why add a gas one to it? If it does not have enough power than anytime the water heater turns on it will overload you system.
 
If your solar setup has enough regular power to use an electric water heater than why add a gas one to it?

For the same reason many offgrid guys have backup generators.

Earth is tilted on it's axis which means insolation changes through out the year and might not be enough during certain weather conditions on times of year when insolation and length of days are shorter.
 
If that will work with the smarthub great. If not a clamp on thermostat can be used with a relay to disable the propane and ignition if it has an electronic flow switch to trigger the gas; many have to be powered to have the gas valve open.

Having said that, I would think the unit itself would be able to be set for a temperature slightly less than the electric hot water heater output so in theory that shouldn’t come on.

If placed close enough, it won't come on. If it does come on, it will be for a 1/2 second until the output temp is reached and it throttles back and shuts off.
 
I have my electric heater hooked up before my Navi Combo boiler. I put a 3 way valve that allows me to bypass the boiler. When my electric tank is above the minimum temperature the boiler doesn't have flow so it doesn't kick in. When the water is below temp but still preheated the boiler comes on and brings it up to temperature. The Navi is good for this, I don't think all models of instant heaters can handle preheated water

I also have a mixing valve that tempers hot water down to 40c (104f). With this I can heat my electric tank up to 80c (176f ) when I have excess solar. That means my electric tank has roughly double the capacity of hot water because to get 1 gallon of 40c it takes out 1/2 gallon of 80c and mixes in cold water to bring it down to 40.

My electric dump load is DC element with a solid state relay and a pulse width modulated signal. I control the PMW singal to keep the battery voltage just below the solar charge controlers absorb setpoint so my charge controler never switches out of MPPT mode. That way if there is only 100w of excess power it still captures it.
 
I have my electric heater hooked up before my Navi Combo boiler. I put a 3 way valve that allows me to bypass the boiler. When my electric tank is above the minimum temperature the boiler doesn't have flow so it doesn't kick in. When the water is below temp but still preheated the boiler comes on and brings it up to temperature. The Navi is good for this, I don't think all models of instant heaters can handle preheated water

I also have a mixing valve that tempers hot water down to 40c (104f). With this I can heat my electric tank up to 80c (176f ) when I have excess solar. That means my electric tank has roughly double the capacity of hot water because to get 1 gallon of 40c it takes out 1/2 gallon of 80c and mixes in cold water to bring it down to 40.

My electric dump load is DC element with a solid state relay and a pulse width modulated signal. I control the PMW singal to keep the battery voltage just below the solar charge controlers absorb setpoint so my charge controler never switches out of MPPT mode. That way if there is only 100w of excess power it still captures it.

Could you tell me what kind of 3 way valve is between your electric heater and the Navi? I'm familiar with manual three way valves but the way you described it sounds automatic. I checked out that Navi and it looks awesome.

Edit: Looks like a 3 way diverter valve. I snapped a shot from my text book, I'm guessing you're doing something like this?

 
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I ditto some of the post above. I have similar set up with 30gal tank WH with 2000w upper and 1500w lower heating elements. It feeds to Reem on demand water heater. I have a valved bypass so i can manually adjust the mixed temperature that goes to Reem.

XO1iQLA.jpg

(T in Celsius)
I can tell by the way my ODWH sounds how hard its working on heating the water. In the Reem manual it says that warmer the inlet water will result in more GPM of hot water on the outlet. My ground water on average 54F to kick to 116 if a lot of BTUs but when my tank is heate to 130F (that's what it set to) my resulting mix is about 80-87F. It's a welcome T delta of ~30F. So i say its worth it for a Solar dump.
 
Could you tell me what kind of 3 way valve is between your electric heater and the Navi? I'm familiar with manual three way valves but the way you described it sounds automatic. I checked out that Navi and it looks awesome.

Edit: Looks like a 3 way diverter valve. I snapped a shot from my text book, I'm guessing you're doing something like this?

You could do it in a couple of ways. Either with thermostatically controlled valve or you could get a electronically controlled valve hooked to a temperature operated relay. Something like that:

12v Valve on amazon and this Temp controlled 12v relay.

Hope it helps.
 
Your plumber is an idiot.

The propane on-demand controls it's outlet temperature and will do nothing so long as the water leaving it is at the set temp.

Most on-demand water heaters are designed to run in series, they sometimes use a master/slave setup or you can just put them in in series without them talking to each other.
Not to mention recirc systems which are commonly used with on demand units.
 
Need a quick response here as I'm having plumber hook up water heaters today.

My initial plan was to have an electric tank water heater to pre-heat my water using excess solar (I have a 16kw system). My plan was to then have the output of the electric WH to go to the inlet of a propane on demand heater which will then be used as a back up, or if the solar just cant heat the water enough.

When I mentioned this to my plumber, he insisted that this won't reduce my propane cost because the propane on demand will fire up the moment water goes through it regardless if it's pre-heated or not (yet it has a thermostat on the output which regulates the temperature, so where would all the extra heat go if my water was going in at 120F and coming out at 120F?)

Anyone have any experience with this?
You got a lot of good advice from the previous posters.
FYI, I implemented that type of system with the following additions/changes:
I use a 40 gal indirect propane hot water tank and an 80 gal solar/electric tank.
I added a circulator pump and automatic valve and a differential controller such that when the solar/electric tank is 8 degrees hotter than the propane tank it turns on circulating water between the two tanks bringing the temperature differential down to less than 4 degrees.
The solar/electric tank is 80 gals and the propane tank is 40 gal. On very sunny days I can get a total of 120 gallons of water at 120 degrees or more all on solar. On less sunny days the solar tank just preheats water for the propane and I get 40 gals of hot water at a discount.
I never bothered to hook up the electric to the solar/electric tank.


The system works great and has been in service now for over 30 years.
I did it all myself and got it approved by the township inspector who also complemented me on the system.

Plumbers plumb, DIY's innovate and should definately do their own plumbing!

Good Luck.
 
Read post #4 in this thread - that is what i use.
I would be interested knowing what type of HWS heat pump you are using.

How much energy the heat pump requires and how much in-rush current it has.

What kind of DC/AC inverter do you use? Do you turn off the inverter when the heat pump is not running for better efficiency?

Thank you.
 
I would be interested knowing what type of HWS heat pump you are using.

How much energy the heat pump requires and how much in-rush current it has.

What kind of DC/AC inverter do you use? Do you turn off the inverter when the heat pump is not running for better efficiency?

Thank you.
I use a Rheem mpi 325.

Maximum power is just over 4kw, running power is 500 - 800w.

I use a Victron Phoenix 48/5000, DC sag is eliminated via a maxwell supercap.

The inverter is used constantly, for the HWS as well as many other loads depending on PV availability. It is never turned off.
 
I use a Rheem mpi 325.

Maximum power is just over 4kw, running power is 500 - 800w.

I use a Victron Phoenix 48/5000, DC sag is eliminated via a maxwell supercap.

The inverter is used constantly, for the HWS as well as many other loads depending on PV availability. It is never turned off.

Thank you for your reply.

Are you totally off grid?

How much battery Cells capacity do you have?

How big Is your Maxwell Supercap? Is it connected directly in parallel to your battery Cells?

Do you use the 4 kW backup heating element of the Heat Pump
or only use the heat pump econo mode (and the Rinnai Infinity on demand propane HWS when needed) ?
 
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