MIXING TANK on a Tankless water heater good or bad?

[DeewNikoms420]

Hello everyone!

Today Ive came here for some guidance with a buying decision Im about to make.

Im going to be swapping out my water tank heater for a tankless option but have some questions to get answered first.

I have a small 24’ class C. Come September my girlfriend, my pup and I will be setting off for full time. The goal is starting at a year on the road, but if all goes well it will probably be extended.

I am a big bath guy, I love to soak! Of course so does she, and with today’s portable bathtub options we are trying to see if its worth the trouble to figure out a bathing option while traveling.

I think if we both could have 1 bath a week, that would make this trip more like home and one less thing to have to learn to live without.

My thoughts so far are to buy one of those privacy tents, unscrew the head on my outside shower, adapt it to a hose, and run it to the collapsable bathtub we will put inside that tent. Im sure we can find a way to set all that up where its a nice treat once in a while.

With a tankless water heater I’m not worried about the amount of water to fill the tub, but I am worried about a couple other issues so I’m here looking to learn what I don’t understand.

The company I have chosen offers 3 main options for me. I need your help picking the most reasonable unit.

A 48,000 btu 2.5 gpm unit w/o mix tank
A 56,000 btu 2.9 gpm unit w or w/o mix tank
A 66,000 btu 3.6 gpm unit w mix tank

While I ASSUME the 48k unit would be all we need, even for filling a bath (albeit slowly), I wonder if without cost being considered, a higher flow rate would be a little more satisfying. However I’m definitely still considering this unit as I do believe it could work with a little adjustment of expectations.

If Im honest, I do find myself more attracted to the 66k unit. I know it’s overkill for 2 people in a 24 footer, but sometimes that means it won’t have to work as hard. I don’t know if that’s true in this scenario. If I’m completely honest, I’m just looking at the flow rate of +1gpm over the 44k unit.

I would probably just buy that unit without coming here for help, except it has a feature that I don’t quite understand the point of and thats where I need you guys to educate me.

The 66k unit is only available with a mixing tank.

I don’t understand why thats a good thing on a tankless heater, and seems to me if it was mixing with cold water, then it would have to be hotter than the desired set temperature and so more likely to use more gas than a unit with no mixing tank set to the same temperature output.

If I set the 66K unit to output 115°, and it has to go through a mixing tank, I assume the heated water would actually have to be much higher to temper down to 115°. Again, this is just my assumption, I can’t find a lot of information about it online.

However, if I set the 44K (or the 56k) non mixing tank units to output 115°, they only have to heat to 115° since no cold water is tempered in thus using less gas to achieve same result. Have I got it right so far?

So if all that is true, the 66K unit will require more gas for every day operation, and will inevitably cost more in gas than it’s worth for an occasional 3.6gpm bath.

I would much more prefer filling the occasional bath up at a slower rate if for the rest of the time it’s a much more efficient unit. Again 90% of the time we will only be using sink and shower.

Am I misunderstanding something? I definitely want to be set straight if I am.

What do you guys think?

What unit would you choose if you shared our hopes of an occasional hot soak?

Thanks to all

-Shane

[d23haynes57]

2.5 gpm will have trouble feeding a shower head.

[jacwjames]

Originally Posted by d23haynes57

2.5 gpm will have trouble feeding a shower head.

On a regular shower you'd use the cold to get to a usable temp, I think 2.5 would work but it also depends on the ambient temp of the water you are using. If you look at the displays in the big box stores the units are sized by region, further north you go the larger water heater you need.


SO not knowing where you will be traveling you may consider a larger one.




I'd also question the need for a portable bath, in the ~28 years we've been RVing I've only seen one set up for more of a permanent use. You not only have to worry about getting water into it but what do you do with the water when your done since it would have to be pumped to a sewer drain, doubt any campground would approve of it just being dumped to the ground.


I think for the trouble I'd just splurge on a hotel every once in awhile.

[DeewNikoms420]

I appreciate the feedback!

My concern is more about the mixing tank right now. Ill figure the rest out as I go but before I commit to a unit I need to understand the mixing tank part of it better

Thanks for participating!

[OldBaldFat]

My bigger question is the amount of water. Looking at these things on-line - they run from 60 to 130 gallons of water.

You certainly don't have the ability to carry that much water in most RVs, certainly not one that small. The average I can find on 24' sprinter chassis RVs is about 30-35 gallons of freshwater.

And the same sized grey water tanks.

Certainly, your fellow RVers are not going to be happy about that much water being dumped on the ground at your campsite.

Maybe I'm totally wrong, but I'm seeing that you need the largest water heater possible, and a supplemental freshwater hose to fill the tub.

And a way to dump the water into a sewer connection.

Your 'mixing tank' will be the tub itself. Those on RV's seem to be trying to find a way to stop the person in the shower from being scalded from the hot water coming on.

Back in 1982 when the US Navy transferred me and my family to Japan, we had a tankless propane water heater. When it kicked on, the water was scalding hot. We needed the cold water to save us from burns.

We also had an approximately 1m x 1.5m Ofuro (designed to hold four people). The landlord showed us to fill the tub approx 1/3 with cold water, then add hot water until it mixed with the cold to near a room temp, and then add a little hot water to reach a temp we could stand.

Good luck. I still miss that tub even today. Never had anything like that since.

[DeewNikoms420]

Thank you for such great information shipmate!

Apparently theres a ton of confusion with what Im asking so let me try and clear it up.

I probably wouldn't ever try to fill the tub from my freshwater tank, the bath nights would definitely be for when we are connected to a water source, and i would never dump it on the ground afterwards in a site near others, the tubs drain has a water hose connection and would be drained into the sewer system of the site. if the site didn't have both a water source and a septic drain then it simply wouldn't be a bath night.

Also i definitely understand the point of a mixing tank, just not when the temperature is adjustable. If the propane heater just heats a tank to 140° then its a very good feature to have, but these heaters are thermostatically controlled. If I set it to 115° then they output 115°… as a matter of fact what you said is what I'm worried about. If it uses a mixing tank then that tells me the water is heated to a much higher temperature than the set point and has to be tempered back down (requiring more gas then a unit that just heat it to the set point).

In my mind it would take less gas to use a tankless heater that heated to 115° without a mixing tank vs one that heats to 140° but then gets tempered back down to the same 115° with a mixing tank

I guess thats what I’m asking here.. i just don't understand if the unit with the water tank would use much more gas as one without.

So #1) Am I correct that a tankless heater that uses a mixing tank would have to heat water well above the set point to mix back down?

#2) If thats is the case then one that did not use a mixing tank and would only heat to the set point would use significantly less propane?

I tried to get the manufacturer to explain to me what exactly the benefit was of that mixing tank models (they sell for a premium) when you can set it to a comfortable 115° on the cheaper non mixing tank units, but of course they’re Chinese and seem to be just as confused as I am.

It just doesn't make sense to me why you would need a mixing tank if you could control your water temperature output to a comfortable temperature.

My best guess is that the 66k unit cant stay in a safe temperature zone and HAS to be tempered down. If thats the case it may not be what I need. I admit I like the flow rate, but maybe the btus are just too high for what I need.

I still don’t have what I consider to be a good understanding so I hope I get more insight from you guys soon!

Thanks for participating!

[Persistent]

I am guessing you miss-understand the specifications you posted. On the other hand you may be correct. Ask the company that designed the heaters. I certainly am not an expert on "On Demand Water Heaters".

I bought a new Air Stream in January. It has an on demand water heater. It has a 0.5 gallon tank as a part of a recirculating system.

The system has a small recirculating pump the keeps the water in the pipes hot. The small tank is part of that system. It mixes the cold water resting in the hot water pipes with newly heated water from the burner chamber until the return water is hot enough.

It does not mix water from the cold side with water from the hot side.

It is true that some tank type water heaters have a "mixing valve" that mixes cold side water with the hot water that is too hot for safe use. This effectively increases the amount of water available from the tank. This of course is not what you are asking about.

So, what does the "mixing tank" do. Ask the company that designed it.

On Demand Water Heaters only light up and heat when water is flowing. Water in the pipes and water in the heater get cold while sitting idle. When the burner cycles "OFF" and back "ON" due to stopped or low flow, there is a brief period when the system passes cold water through.

When water flow reaches the preset flow rate, the burner goes through a safety cycle and then lights. If water does not reach preset flow rate, the burner does not light. The flow rates you cite may be the minimum flow to light the burner, not the maximum flow rate.

There is usually a delay time before burner lights. Next is the time to reach operating temperature. And finally the time to purge cold water from the pipes.

Filling a bathtub would probably not be a problem. But, washing hands or dishes at a kitchen sink can be a problem. "Navy" showers require water on for short times. On demand heaters may not work well. Water is always cold. To get hot water, you always waste lots of water and waste a lot of time waiting.

Some of the people who get on demand units complain a lot. Some don't mind the cold water.

Recirculating systems often solve the on demand problem. A small recirculating pump can be activated by a switch, or by temperature. The water in the pipe is always hot. The small tank supplies hot water until the burner lights.

My system has a dedicated return water line built in. Some systems use the cold water pipe to return the water for reuse.

Ask to see typical plumbing schematics. That may explain the issue.
https://library.suburbanrv.com/wp-co...03-01-2023.pdf

[DeewNikoms420]

Originally Posted by Persistent

I am guessing you miss-understand the specifications you posted.

No Sir, apparently it's you that has completely misunderstood this whole thread, I don't know if it's from lack of reading, or just comprehension, but I have stated it multiple times over and over, I don't know how I can be any clearer.. It's even in the title!

I bought a new Air Stream in January. It has ...

Correct, because YOU are talking about a recirc pump, NOT A MIXING TANK as I am!

It is true that some tank type water heaters have a "mixing valve" that mixes cold side water with the hot water that is too hot for safe use. This effectively increases the amount of water available from the tank. This of course is not what you are asking about.

Wrong again, this is in fact exactly what I am asking, but instead of a mixing VALVE, it has a mixing TANK

So, what does the "mixing tank" do. Ask the company that designed it.

Another thing you might already know if you would've READ my post was that I HAVE already consulted the manufacturer, or at least the agents they hire to answer the emails, and they are Chinese and I can tell they have even less understanding about the product than I do. They are useless for an answer, thats why I came here.

On Demand Water Heaters only light up and heat when water is flowing. Water in the pipes and water in the heater get cold while sitting idle. When the burner cycles "OFF" and back "ON" due to stopped or low flow, there is a brief period when the system passes cold water through.

When water flow reaches the preset flow rate, the burner goes through a safety cycle and then lights. If water does not reach preset flow rate, the burner does not light. The flow rates you cite may be the minimum flow to light the burner, not the maximum flow rate.

The numbers I gave are the output flowrates, not the minimums.

There is usually a delay time before burner lights. Next is the time to reach operating temperature. And finally the time to purge cold water from the pipes.

Filling a bathtub would probably not be a problem. But, washing hands or dishes at a kitchen sink can be a problem. "Navy" showers require water on for short times. On demand heaters may not work well. Water is always cold. To get hot water, you always waste lots of water and waste a lot of time waiting.

Some of the people who get on demand units complain a lot. Some don't mind the cold water.

Recirculating systems often solve the on demand problem. A small recirculating pump can be activated by a switch, or by temperature. The water in the pipe is always hot. The small tank supplies hot water until the burner lights.

My system has a dedicated return water line built in. Some systems use the cold water pipe to return the water for reuse.

Ask to see typical plumbing schematics. That may explain the issue.
https://library.suburbanrv.com/wp-co...03-01-2023.pdf

Again, I just don't think you understand what I'm talking about. I do appreciate you took the time to reply, I just wish you took the time to read what you were replying to first!

Regardless, thank you for your participation.

Maybe some visual aids would help...

[DeewNikoms420]

here in this photo you can see the 10 oz mixing tank that has a cold water supply for it to temper down the water in the mixing tank.

[DeewNikoms420]

So if my understandings are correct, mixing tanks are only used to bring scalding temperatures down to a comfortable level.

if "on demand" heaters are thematically controlled they shouldn't need a mixing tank.

So why does this unit have one? It's not available without a mixing tank on this flagship model. Seems counterintuitive to me, so I came here for someone to explain to me why.

Not only that, but the fact that it has a mixing tank tells me that the water is heated well above set point, and tempered back down. that tells me it would use a lot more propane than a model without the mixing tank that just heats up to the set point.

maybe time for another visual aid..

[DeewNikoms420]

These 2 options are the exact same heating unit. The only mechanical difference is the fact that the one on the right has a built in mixing tank while the one on the left does not! (hence the NO and YES on line 3)

[DeewNikoms420]

In the scenario above would you think the two heaters use the same amount of propane if the one on the left is electronically controlled to shut off at the desired set point, and the one on the right has a mixing tank which by design is used to temper excessively hot water down to the desired set point.

My thought process is if the one on the left is heated only to 115, and the other heats first to 140 and is then tempered down with cold water to 115, then the later is less efficient on propane usage. Thats my best guess.

Thats why it was one of my questions. If the mixing tank models are less efficient to get the same results then they aren't what I'm looking for despite the flow rate I was admiring.

So anyways I went ahead and took the plunge and purchased the flagship unit with mixing tank. I hope I just didn't shoot myself in the foot on propane usage, but there just isn't enough information out there on the subject of a tankless water heater with the feature of a mixing tank.

I guess I'm going to find out on my own.

-Cheers

[MikenDebbie]

I don’t have any info that will necessarily help with your questions on the on demand water heater…but for future reference, and possibly your next RV, I wanted to make you aware of possibly the ONLY RV a full bathtub and an on-demand water heater. I own one because DW saw it at a RV show in late 2017 and said “That is what I want! The bathtub”. Montana only made them for 2 years - 2018 and 2019…the 3920FB and the 3921FB (front bath)…see floor plan below. The 3920 will have a Norcold 2118 gas/electric fridge…the 3921 will have a Samsung RF18 residential fridge. Both units will have a Suburban IW60 on demand water heater. Montana makes a 3911 model and a 3931 model with a full closet across the nose - not the full tub. We do not dry camp…never! So water conservation is not an issue. On demand water heaters do not get much love on these forums, mainly because they are not dry-camp friendly. Lots of folks complain about the water “wasted” when you turn on the water and have to wait 15-20 seconds for hot water to come out. My shower at the S&B is 75’ from the water heater. I have to wait on hot water to get there. No big deal to me.

Suburban made a IW60RL a few years ago (recirculating on demand water heater that came on some Keystone Alpine units. I believe those were trouble. Not sure if they make those any longer.

Our IW60 water heater has performed well…we love it. I set the water temp to the max - 129* and temper the water at the outlet. I do not understand the need for a mixing tank on the unit you show above. We have learned to manage water flow for hot water. I have a variable water pressure regulator set for 55 to 60 psi. It works best to use hot water at ONE outlet only…be that the tub, the shower, or one of the sinks. More than one outlet calling for hot water degrades flow and temperature of the water…but hey, we are in a fifth wheel - not a house…no big deal for us. Also - the water temp you get at the outlet is related to the temp of the water coming out of the ground. We get REALLY hot water in the summer when the city water supply is warmer. In the winter - the hot water is at a lower temp. The IW60 is really good, but it has limitations. Know that going in and set your expectations accordingly. Diesel motorhomes with an Aquahot may not have these limitations.

I am not familiar with the water heaters you are looking at, but I would suggest you get the highest btu unit with the highest water flow, and skip the mixing tank.

Enjoy your Class C motorhome and have a great time traveling. Perhaps in the future, when you are ready for a unit with a full tub, half bath and tons of room - look for a used 2018 or 2019 Montana 3921FB…and if you consider one of these - look for a dually 1 ton truck to tow it. You will not be disappointed.

Edit: the 3rd image is a screen shot from one of my posts to the Montana Owners forum where I tested the temps at each outlet.

[MikenDebbie]

…one more thing for your consideration and just to know they are out there…
We have had 4 of these inflatable hot tubs over the past 13 years. DW used it primarily for water aerobics exercise. She has had a health issue that prevents her from using it for the past 18 months or so, and it has been stored. Technically, I suppose you could say these things are portable…but it would be more work than I would want to put in for setup and tear-down…plus it would take up a lot of storage area in an RV. Still, the thing works great, runs on 120volt power and it is cheaper than a small “permanent” hot tub typically $4k -$6k. DW is recovering so hopefully I can put ours up for her to enjoy later this spring.