Domestic hot water

raricky

Hi

I am looking for any advice which could resolve the issue I am facing with our combi boiler unit.

We got a tankless water heater connected with a hydronic air handling unit . We started using our home heating system yesterday and since then we are getting mild hot water from the faucets. The strange part is every time the heat turns ON faucets give ideal temp hot water as they use to before. But when the heating system is not in use its again average temp hot water.

Note- tankless heater was descaled recently.

What could be the potential reason for this.

GGross

take a pic of the install

Snowmelt

One of your fixtures (tub, bathroom sink, kitchen sink. Laundry room …….. something somewhere has the cold water tossing in hot

raricky

@GGross

raricky

https://forum.heatinghelp.com/discussion/comment/1866913#Comment_1866913

but this started only yesterday . It was completely fine until the home heating system was turned ON.

Jamie Hall

I guess maybe I'm a bit confused. In one place you mention a combi boiler. In another place you mention a tankless water heater. How are these interconnected? And what is the control strategy which determines which (or both?) is supplying hot water? Is the combi big enough to function as a tankless water heater (make and model and size would help here) or is there also an indirect in the mix somewhere?

EdTheHeaterMan

@raricky said: "I am looking for any advice which could resolve the issue I am facing with our combi boiler unit."

I do not believe that this is a combi boiler. This is a water heater and is not designed for space heating. My guess is that your air handler coil has potable water (from the open system) sitting in the coil all summer when there is no need for heat. That water can get microbial pathogens to grow in the stagnant water over the summer. Once you start to circulate that water back into the water heater system there is a very strong possibility that those microorganisms will end up in your cooking water or your dishwasher. That can't be healthy.

As far as the warm water when the heat is off and the hot water when the heat is on, I would not even worry about that until you get the health concern resolved.  

I don’t want to be that guy that says “How can you live here with such a dangerous condition” BUT…  Legionella pneumophila is one of the bacteria that can grow in the stagnant water like your heating coil is, over the summer. 

This is an approved use by IBC.

raricky

https://forum.heatinghelp.com/discussion/comment/1866922#Comment_1866922

Its a Ibc 199 sft tankless water heater which feeds a IbC Air handler as well.

EdTheHeaterMan

"Well blow me down". Quote from Popeye the Sailor Man

I appears that IBC does offer an air handler that can be connected to their tankless water heater (does not need to be the Combi)

I am personally not a fan, but is an approved use by the manufacturer. So I will look into the manual to see what may be causing the lower DHW temperature during the winter season while the air handler is not calling for heat.

Jamie Hall

Aagh. Well, like @EdTheHeaterMan , I'm not at all keen on that type of system. However, it's what you have.

My bet is that this is a control issue and that somewhere — probably buried deep in a setup menu — there is a parameter that determines whether the domestic hot water can command the water heater when it is configured for providing heat for that coil. If that is the case and it is set to shut off the domestic hot water sensing when the system is configured for heat, you would get exactly what you describe.

mattmia2

https://forum.heatinghelp.com/discussion/comment/1866941#Comment_1866941

or there is a diverter valve that got stuck after the heat call.

Snowmelt

all of these tankless have a page in there install manual for a side take off for a heating coil on an air handler. Is that what I like to do , the answer is no but there out there ………. With out being there it’s hard to determine what’s going on . So let me ask, when you’re using the heat and the domestic water, is the water hot enough at your faucets ?

raricky

https://forum.heatinghelp.com/discussion/comment/1866955#Comment_1866955

As long as the heat is running the domestic hot water remains hot enough. After maybe 5 minutes the water temp drops again.

hot_rod

I think you need to get into the control and set up parameters

HVACNUT

https://forum.heatinghelp.com/discussion/comment/1866933#Comment_1866933

Popeye likes his spinach. Lots of IRON.

Kaos

These systems have a check valve between space heat and DHW. This is either an itegrated check inside the pump or a stand alone one. If this check is stuck or leaks, it creates the symptoms you see as cold water can bypass the tankless when the pump is off.

A quick check is to see if the inlet of the pump gets cold when you have a DHW draw. If it does, you need to clean/replace the check valve.

EdTheHeaterMan

I might check the check. as @Kaos mentioned. when you started to heat for the first time this season the check valve may have not completely closed on the space heat side of the system. An easy to check it is if the proper valves were installed you can shut off a valve to the space heat loop to the AHU and see if the problem goes away.

The following diagram illustrates how the water can flow backwards when the circulator pump is off and the check valve is passing.

When the circulator pump is operating, the reverse flow can't happen so you get HOT water. When the circulator is not operating the reverse flow will allow lower temperature from the AHU coil to mix with the Hot water and you end up with Warm Water

GGross

I don't think you are going to have separate settings in there for space heating that's an SFT model not the SFC/SFB boiler models. this is almost certainly a piping/flow issue

raricky

https://forum.heatinghelp.com/discussion/comment/1866967#Comment_1866967

yeah i did changed the temp but no luck.

raricky

@EdTheHeaterMan @GGross @Kaos

As mentioned by some contributors it could be the check valve in circulation pump causing the problem.
I had this similar issue when i descaled this unit back in summers but that time I flushed water out from the plumbing system and it worked fine after that.
I am probably going to call a professional to take a look what exactly is going on.
Thanks everyone!