Water Heater on Inverter

[richard5933]

Still trying to find a way to make hot water without running generator. We have Seaward electric 6-gal water heater currently which heats in about 20 minutes.

I'd install an LP heater, but they all require side venting, except for tank less and we don't want that.

So, I was thinking that I could possibly use what I already have on hand. We have a Samlex 2000-watt (continuous) inverter and four Trojan L-16 making 790 Ah of capacity.

We also have 540 watts of solar that can easily push 30+ amps back to the batteries.

I've got an extra manual transfer switch I can use to switch the power input to the water heater between pole and inverter.

Running the water heater on the inverter seems like it would be possible without pushing anything near its limits. Has anyone done this? Or, do you see problems with the plan I've missed?

[windtrader]

What's the rationale against tankless?

[TomC]

I have a similar setup. I have 2-10gal electric water heaters (straight from HD). The first one is wired straight to the breaker box and the second one is wired through the inverter so we can have hot water going down the road, and have hot water in the morning when boon docking. Works well. Good Luck, TomC

[buswarrior]

How much power does the hot water tank use to get the temperature change you want?

Is that 20 minutes from cold, or warming yesterday's still warm water? Best case / worst case needs to be considered.

Is there enough power available in the time that the hot water tank is drawing it?

Simple calculation, 30 amps of solar at 12 volts is only 3 amps at 120 V.

A significant hole will be dug in the battery bank, is there enough left over for other needs / is there enough sunlight left to re-fill?

Heating water is an energy devil.

It's all in the math.

Happy coaching!
Buswarrior

[richard5933]

What's the rationale against tankless?

From what I have read, they don't typically do well in very low-flow situations or when taking Navy showers. We often dry camp so water conservation is important. With very low flow burner may not kick on. I know some have put recirculation systems in, but there would still be other issues.

We can both wash up in the morning on the 6-gal tank. We'd only use it if we were expecting a sunny day or a travel day (we could easily recharge batts underway).

Twenty minutes is from cold water to scalding.

Thanks for the info guys.

[chessie4905]

Try it and report back.

[bobofthenorth]

How much power does the hot water tank use to get the temperature change you want?

Is that 20 minutes from cold, or warming yesterday's still warm water? Best case / worst case needs to be considered.

Is there enough power available in the time that the hot water tank is drawing it?

****** Simple calculation, 30 amps of solar at 12 volts is only 3 amps at 120 V. *******

A significant hole will be dug in the battery bank, is there enough left over for other needs / is there enough sunlight left to re-fill?

What ^^^^ he ^^^^ said.

There's never enough sunshine to make solar useful.  We had a 3 way switch so that we could switch the water heater to run off the inverter but we rarely used it.  Even running down the road with the alternator pumping into the house bank the water heater would still eat into our SOC.  The smart way to heat water on the road - IMNTBHO - is with engine coolant.  You could buy a marine water heater with built in heat exchanger loop or rig up your own using a flat plate heat exchanger.  Either way there's heat being shed off the engine that is free for the taking while you're moving.

[Oonrahnjay]

What's the rationale against tankless? 

      At my home, I have a moderate sized tankless (propane) water heater.  Since I live in a hurricane area, i did a survey on what I needed to have a connection to run on a generator and I found that the electronic controls on the tankless heater use less than .8 of an amp at 120V.  It seems to sip propane (but I'm the only person living there full time, so several people in a bus conversion would see higher/more expensive use but i've seen very low propane use). 
      I had already installed an Atwood 120V/propane/engine coolant loop 10 gallon heater in my bus before I had the experience with the home tankless but -- based on my results from the heater at home -- I would have no reluctance to install one in my bus if I needed to re-do my install.
      I should add that I don't have the papers with me now, but my memory is that the tankless heater was about three times the price (installed) of the Atwood heater.   Also, single person at home meant I never was washing dishes while I was taking a shower.  I have never had a single problem with low water flow, etc. 

(Another discussion we've had before ... and before.)

[Jim Blackwood]

Having owned a thankless water heater my advice is: Just Don't. It's a great concept that just doesn't work well in the real world. Maybe some day it will but the problem is the enormous quantity of heat that has to be compressed into a very small space. (That was an electric unit however, propane may be significantly better.)

There are however a lot of good options all depending on your lifestyle. Once you know exactly what your needs are it  gets a little easier to configure. But this is one place where it really is a good idea to stick with old reliable tried and true methods. Doesn't mean you can't combine them in new and different ways though.

For my money, a completely conventional dual heat RV water heater is hard to beat. You have your electric for shore power and your propane otherwise, plus you can run both for rapid heat-up. A manual switch and indicator light for the electric side is a great convenience. A second switch to go from pilot to burner on the gas would be that much better. Left on pilot they keep the water pleasantly warm. But that leaves the venting issue. You're not going to be able to mount it inside the bay very easily, certainly you aren't going to butcher your bay door for it. That does leave the option of mounting it in the cabinet under the sink however and cutting through the signboard to mount the cover. Might be a little tricky to get that to match but it can be done. Considering the small appetite of the pilot and the short delay to go to full hot as well as the extended shower time by kicking on both burners a few minutes before stepping into the shower, I don't see the extra expense of a propane tankless as being justified.

I've not been impressed with the coolant loop incorporated into those RV water heaters however. That strikes me as very much a stop-gap bailer twine and bubble gum type of solution. It's light years from a proper heat exchanger. Until they start installing it as an internal loop or coil it's not really anything but a nod in that general direction. An extermal heat exchanger though, that's a decent idea but it requires the water to be circulated through the water heater tank to really be effective, adding in more complexity. Maybe it could be done with a thermosiphon loop but those are generally dependent on having plenty of time to do their thing, so not much better than the existing external coolant loop. However if hot water with engine on is a frequent need, maybe a copper line wrapped around the exhaust pipe would serve.

But as for solar heating of water, the most effective is also the most direct. Run the water through the solar collector box. Doesn't have to be a large line, a 3/8" diameter that has a long run could collect enough heat on a sunny day to give you a comfortable shower. But saving that in a battery bank and converting it to warm water when you want it is going to take a lot of battery storage. You can calculate how much, based on the volume of water needed, the degrees of temperature rise desired, and applicable loss factors. Water happens to be a particularly good heat sink though so it'll be more than you want to see.

Jim

[chessie4905]

Why not just add another small electric water heater. With a larger heating element, it'll heat water quickly. You have a generator, use it.

[oltrunt]

Lots of ways to skin a cat.  I'm happy with my tankless propane water heater now that I have a recirculating system and a thermostatically controlled on/off switch.  The heater takes 2 degree C water to 42 degree C (107 F) in about a minute and keeps it there until all the water is gone.  Jack

[richard5933]

The smart way to heat water on the road - IMNTBHO - is with engine coolant.  You could buy a marine water heater with built in heat exchanger loop or rig up your own using a flat plate heat exchanger.  Either way there's heat being shed off the engine that is free for the taking while you're moving.

Our water heater is a marine model and includes the coolant loop. I'm hesitant to cut into the bus cooling system, only to have hot water for the day we arrive in camp. Day two, three, and beyond the loop won't help.

Why not just add another small electric water heater. With a larger heating element, it'll heat water quickly. You have a generator, use it.

Can't stand the noise. Vibration of the generator puts my nerves on edge. We go camping for the peace and quiet. No, we can't afford a better/quieter/smoother generator.

Problem isn't how fast our water heater makes the water hot, it's the source of energy for it.

For my money, a completely conventional dual heat RV water heater is hard to beat. You have your electric for shore power and your propane otherwise, plus you can run both for rapid heat-up. A manual switch and indicator light for the electric side is a great convenience. A second switch to go from pilot to burner on the gas would be that much better. Left on pilot they keep the water pleasantly warm. But that leaves the venting issue. You're not going to be able to mount it inside the bay very easily, certainly you aren't going to butcher your bay door for it. That does leave the option of mounting it in the cabinet under the sink however and cutting through the signboard to mount the cover.

But as for solar heating of water, the most effective is also the most direct. Run the water through the solar collector box. Doesn't have to be a large line, a 3/8" diameter that has a long run could collect enough heat on a sunny day to give you a comfortable shower. But saving that in a battery bank and converting it to warm water when you want it is going to take a lot of battery storage. You can calculate how much, based on the volume of water needed, the degrees of temperature rise desired, and applicable loss factors. Water happens to be a particularly good heat sink though so it'll be more than you want to see.

Jim

I agree that a traditional RV electric/LP water heater is the best option. But, as you mentioned cutting up my doors is not a good option, especially with the pantograph doors. Higher up (outside the cabinets) is not a sign board on the older GM buses - it's either aluminum or steel with rivets about every two inches. Not keen on cutting into that either.

Running the 1500-watt water heater will consume about 135-150 amps/hour, which is less than the max discharge rate for our Trojan L-16 batteries. Running the water heater for 20-30 minutes would deplete the battery about 70 amps. We camped this weekend in northern Wisconsin with overnight temps cold enough to still require the furnace at night. Running heat, the compressor fridge, lighting, a little TV watching, water pump, etc. still totaled less than 100 Ah consumption overnight. Even if we added 70-100 more for the water heater, the solar panels could still have easily gotten us to full recharge by mid afternoon.

On days when solar charging doesn't look promising, we would either shower cold or skip showers till we had a larger reason for running the generator. Or, if we're traveling, we can charge house batteries at 70 amps while underway.

I've got all the parts on hand to give it a test. First step will be to temporarily connect a pigtail to the water heater so I can just plug it into the inverter and test it. Then I'll be able to record real-world data.

[luvrbus]

I thought you had a Webasto Richard ?

[richard5933]

I thought you had a Webasto Richard ?

That was on the 4106 - the bus that got smooshed.

LP furnace and electric hot water heater on this one.

[chessie4905]

Maybe something here may be an option.

https://www.amazon.com/Water-Heaters-Gas-Powered-Parts/s?rh=n%3A3754791%2Cp_n_power_source_browse-bin%3A542716011