I have a combi Navien tankless water heater. It heats my home as well as the water. I keep getting lukewarm water in the shower even after the lever is turned completely to hot. The Navien service tech is saying the machine is working as it should.

What I’ve noticed is during the summer when I’m obviously not heating the house the hot water at the sink is extremely hot and during the winter when the house is being heated the water is not getting hot.

When the hot water was completely shut off and we went to any of the faucets and put it to hot, we got ice cold water. I’ve had a Navien specialist out and a plumber and no one seems to be able to figure it out.

What else can I check?


3 Answers 3


I think we need more information to be able to know what's going on. Where are you located, and how are your water pipes run? If you're in an extreme climate and your water pipes are being run in un-conditioned space, that could be where a lot of your hot water energy is going in the wintertime. Then in the summer it's picking up extra heat from the outside.

Also, is your system sized properly to be able to provide both home and domestic water heating at the same time? If it's under-sized and is working hard to keep your home warm, it may not have enough capacity to also heat your domestic hot water needs.

One thing you can do is measure the temperature coming out of your water heater and compare that to the temperature of the water that you're saying is not as hot as you'd expect. If you have a large difference in temperature, I'd suggest looking at how the pipes are run, and trying to insulate them from extreme temperatures. If there isn't a large difference, then look elsewhere.


Your unit is Tankless, meaning it heats as the demand for hot water is requested - by opening a hot water faucet or by a heat exchanger for household heating.

I suspect that your unit is undersized for the load it has during the winter when you are heating your home. Read further for details..

I do not know how your heating system is configured, baseboard, floor mounted radiators per room, under slab unit or an aquatherm in the air handler .. you did not say, however; all of these units whatever one you have for heating are calling for hot water and probably recirculating it in a loop type system, they have a flow rate and it might be on demand or continuous. The number and types present to you the equation for your radiated heat requirements (demand) and in order to obtain that your Tankless water heater will need to provide sufficient hotwater to keep up with that demand. Again the demand equation is based on the heat exchanger types and quantity (total volume) as well as the temperature setting.

I am thinking that your heater continually runs but at a low flow rate in order to maintain household temperature - I do not know this for sure as again I do not know which type of heat exchanging system you have. If we knew this we would have a more accurate answer.

One more thing to add The unit should really have a method of controlling the water temp depending on load and where that water is going. For example max temp to the heat exchanger but a limiter on the household supply line If it was plumbed properly for that functionality.

I have an Aquatherm and I typically adjust my hotwater heater up about 15 to 20 degrees in the winter to accommodate for the load (it is a storage tank).

  • There's never a need to post a comment to tell the OP to check out a new answer. They're always automatically notified about answers and comments. Of course, they may choose to not check them...
    – FreeMan
    Aug 16, 2022 at 14:24

When the hot water was completely shutoff and we went to any of the faucets and put it to hot, we got ice cold water.

This might be a hint for a problem in one or more shower thermostatic faucets, since those faucets are normally the only devices in private homes which - if defective - allow cold water getting into the warm water net. This will happen if the back flow preventer for the warm water connection inside the thermostatic shower faucet is leaking, i.e. if it no longer prevents cold water from invading the warm water pipes.

Also the lukewarm water is a hint for one or both back flow preventer(s) being defective. The leakage could be intermittent, f.e. the back flow preventer(s) could work by chance in the summer due to other pressure levels both on the cold and warm water net.

Some thermostatic shower faucets do have 2 small shut off valves for the incoming warm and cold water, which are only accessible after removing the trim. They can be closed by using a screwdriver. If both are closed (of each and every thermostatic faucet in the net), the anomalies should disappear.

See this similar problem: Why does my mixer shower run intermittently cold?

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