Typical Tankless coil not keeping up with hot water demand after shower remodel.

Brianosaur

Should ONLY cold water flow into a POU Tankless hotwater heater?

I knew I would be pushing it with my bath remodel and new shower. The GPM flow on new rain shower head the DW had to have is too much demand for tankless coil to keep up with. The anti-scald thermostatic mixing on the new shower valve doesnt help either.

Before the remodel, with the old dual handle hot/cold knobs, I could restrict the flow and tweak the water so I would have plenty of hot water at a lower pressure. The new valve makes it near impossible.

I put a $5 adjustable restrictor on the shower head to slow the flow but it doesn't really work well. If I switch to water direction to use the hand held sprayer in the shower I will get hot water for a long time.

I have zero room to install a indirect water heater tank next to the boiler. It is pretty much in a sealed closet, in a finished basement, with a 'fan-in-a-can' bringing in outdoor combustion air.

I thought I could install a POU tankless water heater next to just this shower for additional hot water, but I am uncertain if it is acceptable to have pre-heated hot water from the boiler flow into it. It would be mostly hot water on the inlet supply, until the coil doesnt keep up, and then it would be luke warm water. Should ONLY cold water flow into a POU Tankless hotwater heater?

My other thought would be a hot water booster, but since there is no tank dont think this is possible either.

What are my options here without being able to add a indirect tank?

In line flow restrictor? Thermostatic mixer at the boiler?
Tankless POU? HW Booster? Ditch the rain head? Something else?

Thanks so much for the help

Jamie Hall

The problem with the rain shower heads is.. as you have discovered, they take a lot of water. And the problem with that is you either have to store that water or heat it as it comes in.

That said -- first thing I'd do before I spent much money is make sure that the tankless coil in your boiler is clean and not scaled. There are descaling compounds one can use, but one can try white vinegar as well. It may just need to be cleaned to be able to keep up.

Then, given your tight space, the alternative is a point of use heater. However, whether or not it is fed warm or hot water (check with the manufacturer on whether it is suitable for that), sooner or later in the shower you are going to be heating a lot of water with a good temperature rise -- and that takes a lot of power. So... consider: do you have the ability to run a gas line or a big enough electrical circuit to the unit?

Brianosaur

Pulling a 3 wire 10 awg cable wont be an issue. My circuit panel is right below the bathroom and I have room for a 2 pole 30a breaker. 

  As far as temp rise it wont be cold fed city water from the street.  I know there are maps with ground water temps and the unit size that should be installed. At most I am hoping for luke warm water made hot for less the ten minutes for 3 showers a day. Prob even less than 10 because the first few minutes are hot off the coil. 

So it is not unheard of the have a POU tankless inlet fed with hot water?   Depends on make/model?   It HAS been accomplished successfully by others? 

Perhaps I will start with a new shower head and see how we do. Wife will sacrifice smaller shower head for hot water. 

Jamie Hall

Optimist. 30 amps isn't even close -- for 2.5 gpm you need 100 amps/240 VAC -- or more.

mattmia2

The first thing to do would be to figure out the flow of that shower head and your worst case incoming water temp. It may be the case that you need the tankless coil to preheat it from say 35 degrees to 60 degrees then you need a 200,000 btu/hr gas tankless water heater to bring it up the rest of the way.

Brianosaur

Jamie Hall said:

Optimist. 30 amps isn't even close -- for 2.5 gpm you need 100 amps/240 VAC -- or more.

Oh, okay.
But Is 100 amp overkill?
I didnt think I would need a full 2.5 gpm POU since the inlet water wont be cold ground water (as per my op).
I did see some 2 gpm units on double pole 40a.
And some smaller 1.75s on 2x30s

Should I still go for a full 2.5 gpm when the water is *mostly* heated and just goes luke warm?

I would *guess* a 15 deg rise in temp is all I would need, but I could check water temps.

mattmia2

You need to figure out the actual flow and temperature rise from the the utility or well, you will then be able to calculate how much energy you need to add to the water to bring it up to the desired temperature. Those gmp ratings given in marketing literature are based on a specific flow and temp rise which is usually pretty optimistic for a cold climate. The boiler can add some of that energy and the tankless water heater can add some but once you know the total you can figure out how much has to come from each. A 40a circuit at 240v can provide about 32,000 btu/hr.

Jamie Hall

If you do go to lower flow, you might be able to find a unit with less current draw, but... the 40 amp ;circuit will give you at most a bit over 1 gpm at a 60 degree (60 to 120) rise. And I'm not at all sure I've seen any "instantaneous" (point of use) water heaters even rated that low.

That's the joker in the pack on point of use water heaters.

Brianosaur

Okay,  thanks for the help. 
Will try to get in there this weekend with  5 gal pail,  a 1 gal marked pitcher,  my phones stopwatch and a digital thermometer.

Its just all guess work until I have hard numbers, as suggested. 

mattmia2

If it is municipal water you will have to get the worst case min temp from them. Where it comes from and where it is stored will have a big effect on the winter temp.

HomerJSmith

Does your rain shower head have LED lites in it?

motoguy128

Might need to remodel the basement to add an indirect. Surely there’s another 6-8 square feet you can give up adjacent to the boiler.

mattmia2

motoguy128 said:

Might need to remodel the basement to add an indirect. Surely there’s another 6-8 square feet you can give up adjacent to the boiler.

It doesn't even have to be near the boiler