My wife and I just had a new boiler installed with on demand hot water. The thermostat for the water is turned all the way up. You will almost burn your hand while using one of the sinks, but when you use the shower, it is just comfortable. If you try taking a bath, forget it. it is semi-warm at best. I understand that this may be cause because the water is coming into the tub faster than the water can be heated. We have two bathrooms in our house and both have single valve shower faucets. They are the type with the single dial that you have to turn all the way counter clockwise to get hot water. So there is no way to have hot water come out slower so it can heat up more. I was wondering if there is a work around for this or if we eventually have to replace our single valve shower faucets with a two valve system?

You might have to adjust the shower valves because they could be set to limit the temperature of the hot water, regardless of how hot it really is.

Thank you for the information. I was an electrician for a long time, but plumbing is new to me. I will look online to see how to do that. thank you again for the reply.



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if your temp is different at the tub vs the sink its pretty much a 100% chance the hot limiter is set low....i set them all the way up to avoid call backs for the issue

quote..
" The thermostat for the water is turned all the way up. You will almost burn your hand while using one of the sinks"..
maximum allowable hot water temperature by law is 125 deg.
if your showers worked well before the new heater was installed then the heater is too small.
if the installing plumber turned the temperature control all the way up, I suspect he knew the heater was undersized and was trying to compensate by increase the output temperature even if it meant violating plumbing regulations.

What brand and model of water heater was installed ?

On a side note, the shower valve you are describing is a 'single dimension' valve, i.e., when you turn it on, regardless of where you position the handle, the water volume is full open and non-adjustable. A 'two dimensional' valve would allow you to adjust temperature AND volume.

Thanks for all the feedback and tips. For those who want to know the boiler is a Lockinvar cadet ccn120. I was going to try adjusting the shower valve this weekend and see if it helps. Otherwise, I'd like to see what other options we have to fix this problem.

That combi unit will only give 3 gallons per minute @ a 70 degree rise. I would bet your water is coming into the house around 35-40 degrees right now giving you only around 105. I would try putting in a low lower flow shower head.