Water heater on inverter - great test of inverter efficiency

[richard5933]

Now that we've got our solar and inverter configured and running well, I thought it was time to push the limits and do some testing.

The system is comprised of: Four L-16 batteries providing 780 Ah of capacity, Samlex PST-2000-12 2000W inverter, Victron battery monitor, and 540 watts of solar charging at 30 amps. There is also a battery-to-battery charger which allows for pushing 70 amps @12v to the house batteries from the 24v chassis system while underway. We have a Seaward 120v-only 6-gal water heater. It has a 1500-watt element which takes about 30 minutes to bring the water from cold to shower temp. Our generator is a Kohler 12.5kw powered by a Perkins diesel, and our house battery charger is a Progressive Dynamics 9270 putting out a max of 70 amps @ 12v.

My goal was to be able to make hot water when we are dry camping without having to run the generator. I hate the noise of the generator, and the vibration of the Perkins puts my nerves on edge. We've done all we can to quite/calm it down, and I believe we've explored that route as far as possible. A newer/quieter generator is not in the budget at all.

What I did was use a small manual transfer switch to enable switching the power feed to the water heater between the main shore/gen system over to the inverter. For most of the time, it will be left powered by the shore/gen system. But, when we have solar deployed and running under the sun or while we are underway and charging from battery-to-battery charger I can now switch to power the water heater from the inverter.

The inverter seems able to provide power to the water heater well. Watching the monitor for the inverter and the battery monitor, I was able to see a great demonstration of the loss involved in the process. The inverter was outputting 12.4 amps @ 120v, and the battery monitor showed a draw on the batteries of 150 amps @ 12v. That's about an 85% efficiency in the process, which seems to be about normal. This somewhat hidden 15% loss might explain why some running residential refrigerators don't get the run time they expect based on wattage and amperage calculations.

On a fully charged battery bank, I should have no problem heating water for a shower. Would not want to make this my normal routine, but it's nice to have the option when I don't want to hear the noise of the generator.

To answer the obvious question - Yes, I have looked into installing an LP water heater. Problem is they all require venting out the side, and due to our pantograph doors on the bays it's not that easy to install anything requiring sideways venting. The only through-the-floor vented water heaters I've found are tankless, and since we dry camp a lot they are not an ideal setup - they don't do well with extremely low-flow showers or when taking Navy showers.

The whole project cost me nothing more than a few feet of wire to connect things. I already had the small transfer switch in my parts bin. I consider this a successful project, although possibly of somewhat limited value.

 

[jleamont]

Richard, you need this water heater, LPG and 12v for the electronics. It utilizes an exhaust pipe and can be in the basement of the coach, it vents through the floor. Its tankless so it would be endless hot water.

https://precisiontemp.com/products/rv-550-nsp/

 

[richard5933]

48 minutes ago, jleamont said:

Richard, you need this water heater, LPG and 12v for the electronics. It utilizes an exhaust pipe and can be in the basement of the coach, it vents through the floor. Its tankless so it would be endless hot water.

https://precisiontemp.com/products/rv-550-nsp/

 

I looked into that one, but it seems that our low-flow situation won't be enough to trigger the burner. There is also a problem on thankless heaters when you use the on/off feature on the shower head.

I'm surprised that no one makes a traditional water heater with a floor exhaust or possibility of remote mounted exhaust/intake.

 

[jleamont]

1 hour ago, richard5933 said:

I looked into that one, but it seems that our low-flow situation won't be enough to trigger the burner. There is also a problem on thankless heaters when you use the on/off feature on the shower head.

I'm surprised that no one makes a traditional water heater with a floor exhaust or possibility of remote mounted exhaust/intake.

 

Why low flow? We have this one in our coach, works fine with the shower head switch. This unit works flawlessly so far with every obstacle I have tossed at it.

[richard5933]

We do a lot of dry camping. When we do use water, we use it often on what others would call a trickle. We can make our 80 gallons last a week for two people.

[jleamont]

35 minutes ago, richard5933 said:

We do a lot of dry camping. When we do use water, we use it often on what others would call a trickle. We can make our 80 gallons last a week for two people.

Ahh, ok

[dickandlois]

Richard, Looks like you have beat up the water heater issue . The fact that you use so little water, is there a possibility that a solar unit would work doing the summer ?

      Regarding the Generator noise, Mechanical vibration coupling.

Take a look at this link. there are a number of suppliers.

           https://www.mecanocaucho.com/en-US/

Rich.

[richard5933]

I was posting more to report how things went than to ask questions about water heaters. Thought others might find it interesting, especially the part about the 15% loss.

[wildebill308]

5 hours ago, richard5933 said:

Yes, I have looked into installing an LP water heater. Problem is they all require venting out the side, and due to our pantograph doors on the bays it's not that easy to install anything requiring sideways venting.

I don't think it would be that hard to vent through the door.

Bill

[kaypsmith]

With a very low flow situation anyway in warmer weather, a black hosepipe can be laid out in the sunlight, attached to the hot water heater input, pump cold tank water to the hose and then the inverter will not use as much 12V for warming the water. I use a 100 foot hose for this purpose, just leave the hose coiled up, no need to uncoil.

[richard5933]

59 minutes ago, kaypsmith said:

With a very low flow situation anyway in warmer weather, a black hosepipe can be laid out in the sunlight, attached to the hot water heater input, pump cold tank water to the hose and then the inverter will not use as much 12V for warming the water. I use a 100 foot hose for this purpose, just leave the hose coiled up, no need to uncoil.

Might help in warmer weather, but not while underway of course. Not sure I'd get buy-in from all involved on something that required changing the plumbing to make hot water. I wanted to see if something could be done at the flip of a switch, and apparently the answer is sort of.

[richard5933]

4 hours ago, WILDEBILL308 said:

I don't think it would be that hard to vent through the door.

Bill

Here's what the current water heater setup looks like. They mounted it on a riser since the waste tank discharge goes underneath the water heater. If you look at the photo of the door, you will see that the pantograph door setup results in a multiple layers of door on the upper half (inner door/outer door), with two rubber hinges going across the door. It is possible to vent through the lower section of the door, but it's too low for standard RV water heaters. Also requires quite a bit of fabrication since the aluminum is double layered on the outer door. Beyond my pay grade.

[wildebill308]

Building a vent through the door doesn't look that hard. Modifying the compartment to mount the water heater would be more of a problem. 

That is why I quit trying to make/do conversions. There is enough work just keeping something that was (semi) designed to be a motorhome working.

Bill

[richard5933]

43 minutes ago, WILDEBILL308 said:

Building a vent through the door doesn't look that hard. Modifying the compartment to mount the water heater would be more of a problem. 

That is why I quit trying to make/do conversions. There is enough work just keeping something that was (semi) designed to be a motorhome working.

Bill

Most RV water heaters normally use about a 12x12 opening for the access panel. Since I can open the door to gain access, I'd only have to make the opening large enough for the air intake and exhaust output. However, I would then have to fabricate some type of collar to go between the water heater and the door once it closed. Since the door has thickness due to the multiple layers, I'd also have to build a collar on the door itself to direct gases outwards. Of course, most of the units would end up with the venting positioned at the portion of the door which has the center hinge for the pantograph action, making things ever more difficult. I've played with the specs on the various RV water heaters for quite some time on this, and none would work without substantial modifications to either the location of the water heater, the plumbing, the door, or some combination of all three.

For all these reasons, I'm still on the lookout for a water heater which can be remotely exhausted through the floor. Seems like someone would have taken the high-efficiency style water heaters used on residential units and adapted it for use in a motor home by now, but I guess that most wanting to do that have just gone to tankless water heaters.

Another option I've toyed with is going with a Webasto type unit which could make both domestic hot water and provide space heating. Right now we've got a working traditional LP furnace, but if/when it gives out I have enough space to install a Webasto. It could send a loop through the motor-aid coil in the Seaward to make domestic hot water, and I could use a air-heating coil to provide space heating inside the coach on the existing ductwork. Of course, Webasto systems tend to be pricey, so I'm not in a rush to do that. Probably could just install a modern quiet/smooth generator for about the same cost as the Webasto.

Good thing I enjoy all this designing and experimentation. For me it's part of the enjoyment of owning a coach like ours.

[jleamont]

Got it, buy a few of these, place them on rocks around your site so they warm up. You can have the drip shower you desire and no serious mods needed to the coach other than a hook to hang it in the shower area.

https://www.amazon.com/Coleman-2000014865-5-Gallon-Solar-Shower/dp/B0009PUT20/ref=asc_df_B0009PUT20/?tag=hyprod-20&linkCode=df0&hvadid=167119746601&hvpos=1o1&hvnetw=g&hvrand=5753640239665129012&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007366&hvtargid=pla-310230575649&psc=1

 

[manholt]

Bill E.  Uses something similar for outdoor showers..

[richard5933]

8 hours ago, jleamont said:

Got it, buy a few of these, place them on rocks around your site so they warm up. You can have the drip shower you desire and no serious mods needed to the coach other than a hook to hang it in the shower area.

https://www.amazon.com/Coleman-2000014865-5-Gallon-Solar-Shower/dp/B0009PUT20/ref=asc_df_B0009PUT20/?tag=hyprod-20&linkCode=df0&hvadid=167119746601&hvpos=1o1&hvnetw=g&hvrand=5753640239665129012&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007366&hvtargid=pla-310230575649&psc=1

 

I actually have one of those that we carry just in case. I'd be in trouble if a shower was needed and I was totally unable to produce at least a little hot water. Wouldn't want to rely in it for more than backup use though.

Here's something I found today which might actually have a place in my system: https://www.amazon.com/Whale-Gallon-Hot-Water-Heater/dp/B00XCIJN78

It's only got a 300w element and is designed to run on 12v. This would pull only about 30 amps from the batteries, something that the solar could keep up with on its own. I'm thinking that it could be installed next to our existing 120v water heater. It could be installed parallel to the existing water heater and a single diverter valve would determine which water heater was being fed from the cold water supply. Not saying I'll do this tomorrow, just that it's an option for the future.

[manholt]

Boon dock by a moving stream, use a hose with a pump and replenish shower water as needed.  I got my rig, so hat I could bring most comforts of home...a shower or two per day, makes me and those around me, very happy!  Tent is cheaper! 

[richard5933]

I'm not really looking to camp in a tent by a stream. Not really sure where you're going with all this. Besides, if I boon dock near a moving stream I'm pretty sure that even the FMCA roadside assistance wouldn't cover me getting pulled out of the mud.

I started this thread because I thought others might find it interesting. I wasn't asking for a solution, I wasn't complaining about anything, and I wasn't even asking for suggestions on what else I could be doing other than what I did. I simply thought that someone might find interest in the process I went through and the results I got.

We each have our own way of doing things, a budget that limits spending, and our own priorities in how we set up our rigs. In this case, I wanted a quiet and simple way to make hot water without running the generator. The parts were already on hand, so I gave it a try. Then I posted the results.