boosting Tankless output

kcopp

Is there a way to boost the output on a Tankless?
The Rinnai lsi75 (180K) unit works fine for all the fixtures in the home w/ the exception of a walk-in style tub. Customer complains that it takes too long to fill the tub (which is huge.)
You can see as the tub is filling that when you open up the hot side fully that it does restrict the flow for in order to maintain the 120F output.
If I were to add a electric water heater before the tank would that help in the flow/ output.?
Or do I have her bite the bullet and get a larger unit? 199k btu.

gschallert

Either turn the temp down (105 -110) and draw only hot water or turn the temp up and use the tub fixture as mixing valve filling from both taps. Raising temp you have to keep scald protection in mind for other fixtures. Kind of depends on volume of tub and desired bath temp. That unit can deliver 4.3 gpm @70 degree rise or 6 gpm @50 degree rise. What's the inlet temp? How many gallons needed for tub? Desired bath temp?

If I couldn't make the existing tankless work with high volume tub I'd probably stick an 30-40 gallon electric tank in the design as storage only (not using the elements) using the tankless to heat it. Seems like that would be the cheapest route.

kcopp

70 gallon walk-in tub. It municipal water so the incoming temp here in NH is around 50F.. good water pressure too. 3/4" water main and 3/4" to the tub.
I will do some homework. Thanks

Leonard

Here in southern NH I haven't measured city water temp in winter, but it's pretty cold. Min temp seems to be about February. City water comes from a LARGE lake several miles across.

kcopp

This is in Durham... from a well that is in Lee.

Jack

I would set the temp on the unit at 105, as noted, and see how it does. It will not really out produce the 120* as a btu is a btu. Every system has a personality and this ones is that it will heat the tub at about 5 gpm, so a 14 min fill for a 70 gal tub. I would counter the homeowners dissatisfaction with the fact that should the owner choose to have an extended bath that the tankless will always be available to peak the tub. I have found that to be a big deal with tubs. Once you get a tub up to temp, you are immediately loosing heat to the tub body, atmosphere and you in the tub. Many people take a lot of short baths because their tank hasn't recovered.
In order to fill that tub in 7 min vs 14 you have a few options. I guess you could put an electric ahead of the unit, which could get the RL-75 up to its max flow of 7.5 gpm. Going to a 199 unit would get you up to about 4.8 gpm on the .82EF and about 5.3 or so on a .95 EF. Those are at a 70* rise. Another option, but very costly would be to use the tankless as the engine and add a 50or 80 gal storage tank. You would need a circ capable of 5gpm at about 25-30' head. I had good luck with those type systems in apt buildings and restaurants.
I would appeal to the customer that having to wait a few more minutes to fill the tub is not the end of the world. Easy for me to say, huh? The water will be just as hot. What is different is it will not come out of that 3/4" supply the same.

kcopp

Jack thanks... This Rinnai is 2-3 years old. The tub is new. I think this et up is fine but some people cant be patient enough to wait a few minutes longer.....
The other feature of this tub in that it has a electric heater in the whirlpool so the tub will stay hot longer than a standard soaking tub. I am going by today to get some more info.

kcopp

I stopped in today. Rinnai is working fine. 4-5 gallons / min. 115 degree water all day long.
50* incoming water.
Here is the issue. It takes 15-20 min to fill. Its a walk in tub.
She has to sit in the tub for this to fill as the door latches.... bit of a wait and she gets cold.
I made a couple adjustments to the temp at the heater and hopefully she can add some cold water to the hot and cut the fill time in half.
thanks.

bob eck

What about adding a second tankless water heater to work with the first one?
Or install a 50 or 80 gallon electric water heater after the tankless gas water heater hook up the elements on the water heater to maintain the tank at 120 - 125 degree water temp.

kcopp

If this does not work I will do something like that w a tank.

Jack

I don't think increasing the temp will work. You will get a higher temp at a lower flow. Once you are max input, which you are, the output is the output. if you have the capacity gas wise, can you get a second unit in. Maintaining an electric tank for an occasional tub isn't the best but you are in a tough spot.

kcopp

When I tried to fill the tub w only hot water I got 5 gal/ min. When I boosted the temp and added just a little cold to the boosted temp the GPM doubled.
Hopefully that will work. She will let me know.

Jack

I just checked the 75 specs. At 55* temp rise which gets to 105, max tub temp the unit will produce is 5.5 gpm. 60* rise=5, 70=4.3, 80=3.8, 90=3.2. My fingers are crossed for you;)

rick in Alaska

This is precisely why I would not recommend a walk in tub. Not only do you have to sit there while it fills, you have to sit there while it drains.

Canucker

I agree with you @rick in Alaska , if you're able bodied. If not, they can help preserve some independence as you age.

kcopp

I agree...
Not my choice. She wanted it and it was ordered before I had any say in the matter....
Contractor just told me to hook it up.
I did as I was told.
Now I have to make it work.
I love my job.... really.

JUGHNE

Our nursing home has a walk in whirlpool tub that has an insulated reservoir tank that will auto fill and then can be dumped into the tub in less than maybe a minute. This is designed for an attendant to supervise the operation. But maybe there is a residential version?

You don't really have to wait for the tub to drain to open the door.....that is why there is a floor drain in the room. It happens about once a year with the turn over of help.