Tankless to supplement tank for single bathtub

[Luke Maddux]

I'm in the process of repiping with a Manabloc home run system.
My wife and I live alone and have a 50gal water heater. We're interested in installing a large, clawfoot tub outside our house. It would also be a shower but we really want to be able to soak in it, and we want it to be large - enough for me, 6ft 195, to be comfortably submerged in it. Based on what I understand, this means we need a tub that is right at the capacity of our water heater or possibly even a bit larger.
So my question is this... If I install a small, tankless unit, something capable of 3-5gal/min, that ONLY feeds this tub as a supplementary unit for our main tank, and then run them both at the same time to fill the tub, is this a viable way to increase the output to the tub such that I can fill it and not completely drain our water heater? I've included a diagram that shows what I'm proposing. My proposal would be that the tankless-heated line and the tank-heated line would flow through a single valve so that both would be wide open when the tub was being filled, and then a separate valve/spigot would have the cold. I realize my diagram makes it look like there would be a mixing valve. I guess I haven't decided which route to take and am interested in feedback.
Open to any and all thoughts.
Thanks a lot,
Luke

 

[Twowaxhack]

Why not just install a tankless that will serve the entire household need and get rid of the tank ?

 

[Luke Maddux]

Why not just install a tankless that will serve the entire household need and get rid of the tank ?

You mean the whole need like my entire house or just the bathtub? Whole house is not a viable option. What sized tankless would you recommend for filling a large tub in a reasonable amount of time? 5GPM?

 

[pseudonym]

Is there a reason you want to draw from the current tank WH in this scenario? I have a supplemental tankless for an outdoor application as well. Just the cold water line supplies the tankless like any other installation, and it takes on the full job. Easiest way to go IMO. As far as what size tankless, it depends first on the gpm of your supply, then take a gpm below that number and calculate how long it will need to fill the tub. The higher gpm the higher the price, so you decide the sweet spot between waiting for hot water and your wallet.

 

[Twowaxhack]

You mean the whole need like my entire house or just the bathtub? Whole house is not a viable option. What sized tankless would you recommend for filling a large tub in a reasonable amount of time? 5GPM?

Where do you live ? What’s good for Miami isn’t good for Maine. It’s a relevant question.

 

[Luke Maddux]

Is there a reason you want to draw from the current tank WH in this scenario? I have a supplemental tankless for an outdoor application as well. Just the cold water line supplies the tankless like any other installation, and it takes on the full job. Easiest way to go IMO. As far as what size tankless, it depends first on the gpm of your supply, then take a gpm below that number and calculate how long it will need to fill the tub. The higher gpm the higher the price, so you decide the sweet spot between waiting for hot water and your wallet.

Basically I'm trying to optimize that sweet spot you're referring to. I want to fill the tub fast enough so that the water doesn't get ice cold, while also not completely draining our tank.
What sized unit do you have on your outdoor bath?

 

[Luke Maddux]

Where do you live ? What’s good for Miami isn’t good for Maine. It’s a relevant question.

Tennessee. We still sort of get four seasons here. It can get really cold (around 10F) once every couple of years, and it regularly gets below 30F every winter. Then in the summers it's super hot and we wouldn't be taking hot baths outside.
That said, the prospect of a hot bath when it's in the 30s/40s outside is appealing so we'd like to be able to push that boundary occasionally. This is a big part of why being able to fill the tub faster is appealing.

 

[Twowaxhack]

What kind of gas will you be using ? Propane or natural gas ? I don’t recommend electric tankless is most cases.

 

[Twowaxhack]

A tankless big enough to fill the tub fast is borderline big enough to serve a couple showers. You can’t pipe the heaters as you’ve illustrated. That’s why I would just get one tankless big enough to do the entire house.

It wouldn’t be that much more money and you wouldn’t have two water heaters to maintain.

But do as you wish. When you tell me what gas you’re going to use I’ll make a recommendation.

 

[Luke Maddux]

A tankless big enough to fill the tub fast is borderline big enough to serve a couple showers. You can’t pipe the heaters as you’ve illustrated. That’s why I would just get one tankless big enough to do the entire house.

It wouldn’t be that much more money and you wouldn’t have two water heaters to maintain.

But do as you wish. When you tell me what gas you’re going to use I’ll make a recommendation.

This is another variable in the equation. We do have LP supply for our house but I don't think our tank (120gal) is large enough to supply our current usage plus this water heater. I think it needs to be electric, and getting up into the 4-pole, 60+ amp territory on the panel isn't something I really want to do.

What is the problem with piping the heaters as I've shown below? I'm not trying to be contrary I just want to educate myself and understand better why you can't have the tank heater and the tankless feed one line, because, if you could do this to obtain better flow, then my problem could probably be solved with a smaller tankless that draws a more reasonable amount of power. If I turned a 3GPM tankless line up to max and then used a valve to supply another 3GPM from the tank, then I'd have a combined flow of 6GPM, three from the tank and three from the tankless, that would fill the tub at a very reasonable rate.

 

[Twowaxhack]

Balancing the flow between the two heaters would require balancing valves. Otherwise you couldn’t regulate how much flow was coming from each of the two heaters.

You want to spend a few hundred on balancing valves and maintaining them ?

Ok, enough with that.

So gas supply is limited.

Get a 40 or 50 gallon electric and dedicate it to the tub. That’s your best option IMO.

Good luck with your project.

 

[Luke Maddux]

Balancing the flow between the two heaters would require balancing valves. Otherwise you couldn’t regulate how much flow was coming from each of the two heaters.

You want to spend a few hundred on balancing valves and maintaining them ?

Ok, enough with that.

So gas supply is limited.

Get a 40 or 50 gallon electric and dedicate it to the tub. That’s your best option IMO.

Good luck with your project.

I don't have the space for another large water heater, unfortunately.

I get the feeling your question about spending money on balancing valves was meant to be rhetorical/sarcastic but if that is something that makes this viable then I'd be interested to hear about it.

What about just having three spigots feed the tub? One from the tankless, one from the tank, and one cold?

 

[Luke Maddux]

Or how about this?
The tank and the cold feed a standard mixing valve that can be used for both the tub and also the shower head, but then there's a second, completely independent spigot that comes from the tankless and only feeds the bathtub? So for showering I just use the tank via the mixing valve, and for filling the bathtub I use the mixing valve from the tank but also turn on the tankless line so that it fills faster and hotter.

 

[Twowaxhack]

You could do all kinds of silly piping and things that would work.

We could talk about it for hours. But I don’t have time for that.

Get a bigger propane tank and install a tankless or build a shed to put an electric tank heater in.

That’s my answer

Good luck

 

[Luke Maddux]

You could do all kinds of silly piping and things that would work.

We could talk about it for hours. But I don’t have time for that.

Get a bigger propane tank and install a tankless or build a shed to put an electric tank heater in.

That’s my answer

Good luck

OK dude if you're just gonna be a curmudgeon and call my ideas silly then just stop replying. Just move on. I have a situation that I'm trying to find an elegant solution for and if you're not here to engage with people who are less experienced than you who are thinking outside the box then just dont. The end.

If anyone else would like to politely comment on the below suggestion then I'd love to hear your thoughts:

The tank and the cold feed a standard mixing valve that can be used for both the tub and also the shower head, but then there's a second, completely independent spigot that comes from the tankless and only feeds the bathtub. So for showering I just use the tank via the mixing valve, and for filling the bathtub I use the mixing valve from the tank but also turn on the tankless line so that it fills faster and hotter.

 

[Twowaxhack]

There’s a fine line between thinking outside the box and building a Rube Goldberg machine like you’re wanting to build.

I gave you several legit options.

Sure, you can install as many tankless water heaters, faucets and pipes to the tub as you want.

 

[Luke Maddux]

There’s a fine line between thinking outside the box and building a Rube Goldberg machine like you’re wanting to build.

I gave you several legit options.

Sure, you can install as many tankless water heaters, faucets and pipes to the tub as you want.

Ok so you're still replying which I can only assume means you want to help me.

If that is the case, help me understand why this is a Rube Goldberg machine? I am legitimately interested in hearing why. I've looked at the options for my situation and this is what I came up with. Whole home tankless isn't an option because I don't have the infrastructure to heat it with either gas or electric. Bigger tank and second tank isn't an option due to space. So I settled on this.

All I'm talking about doing is filling a tub faster using a separate, supplementary line from a small tankless water heater. You're talking about getting bigger propane tanks, building sheds, moving in a new water heater tank, these are BIG EFFORTS that would cost a more money, take longer, be less aesthetically pleasing, and in some cases involve coordinating with contractors.

I can install a small, electric, tankless water heater and connect it to a faucet so that it dumps into the bathtub DIY in an afternoon for like 500 bucks.

So, again, just help me understand why you think this is such a bad or convoluted idea. To me it seems very simple - so simple in fact that the entire reason I made the post is for someone to explain to me why it would not work. You've offered some other options, and yes, some of them are more optimal, which I really appreciate, but you've not told me why my idea is bad. That's the point here, to educate myself and understand why this might not work and what kind of complications I should expect to encounter if I do it.

 

[Twowaxhack]

you’ve shot down every normal solution.

That leaves your solutions. So go ahead with your ideas. It doesn’t meet code because it’s not pressure balanced but it’s your house, you can do what you want.

I think you’ll find out that when your cold water temps are around 50 degrees and below that your small electric tankless isn’t going to bring you joy. By small I mean the breaker/wire size that’s required, they’re all physically small to me.

 

[Luke Maddux]

you’ve shot down every normal solution.

That leaves your solutions. So go ahead with your ideas. It doesn’t meet code because it’s not pressure balanced but it’s your house, you can do what you want.

I think you’ll find out that when your cold water temps are around 50 degrees and below that your small electric tankless is going to bring you joy. By small I mean the breaker/wire size that’s required, they’re all physically small to me.

Ok now we're getting somewhere, you've given me an actual reason why this is bad. What does it mean to not be pressure balanced? I'm basically just talking about a hot water hose bib (that's an oversimplification but it's pretty close). Is that not code-worthy? Does all hot water have to be fed through a mixing valve like a sink or shower valve in order to be code worthy?

Edit: I reviewed my local code. It sounds like it's not code-worthy to have a source that can supply water hotter than 120F. So if I set the tankless unit to deliver 120F water, then this is now within code, right?

 

[Twowaxhack]

No, it still wouldn’t be pressure balanced. Temp limiting isn’t pressure balancing.

You’d need a 24KW electric tankless heater that pulls 100 amps and requires 3- 240v double pole breakers to get 3gpm of flow at 105 degrees when your incoming cold water is 50 degrees.