Adjustable-pressure tub/shower valve

All,
I currently have problems giving the kids a warm bath, because our bathtub draws more water than our tankless indirect water heater can produce. If I could just ask the tub to draw less water, that would fix the problem. Are there tub/shower valves that will restrict the water flow when the water is hot, adjustable between 0% and 100% of the available water? Thanks,
David Saff
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I thought all valves vary water output. If its NG your tankless gas supply is probably to low due to to small a supply pipe, have it tested, its a common installer error.
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Mark,

As Smarty says, my valve only adjusts temperature, and keeps the pressure constant.

It's an indirect coil in a oil-fired boiler.
Thanks for your suggestions,
David
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David,
My valves which are modern, Kohler, temperature balancing valves for tubs, do not allow the volume of water to be restricted, They only allow the temperature to be adjusted. I can therefore see how you could have a flow which exceeds your tankless heater's capacity to maintain hot water.
Can you restrict the flow in a simper manner, such as installing a threaded-in piece at the discharge of the filling spout? Some spouts have a threaded discharge opening where an optional spray or other piece can be installed. A restricted flow orifice would slow down the demand on your water heater. Alternately, can you increase the water heater discharge temperature so that you will need less hot water to mix to the blended bathing temperature?
Smarty
Smarty

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install ball valves, or regular shut off valves, they may already exist, just turn partially off........ throttle as needed.
do tell do you like your tankless?
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This became common in valve designs a few years ago and I could never figure out why. I assume the design assumption was that you would never need the tub fill less than full on and the shower flow would be adjust by a separate valve on the shower head. What you end up with is a screw type valve that forces you through full cold before it gradually opens hot.
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David Saff wrote:

Install a hand-held shower head with a 6' hose. The flow through that will likely be restricted enough to let the heater keep up. If the heater's still having trouble, install a shower volume control: http://www.clawfootsupply.com/class268
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