Tankless water heater - DHW/Space Heating sizing 1

[disruptoergosum]

Question du jour about sizing tankless water heaters (instantaneous type):

Tankless water heater installed in conjunction with a hydronic heating coil in an air handler.

-Tankless input: 160,000 BTU/h
-Tankless output: 157,670 BTU/h
-Space heating capacity (Hydronic coil with air handler): 25,025 BTU/h (based on water temperature coil inlet 120F, coil outlet 94F, ΔT= 26F @ 2.0 GPM)
-Ground water temperature = 42 F
-DHW use prioritized over space heating and tankless modulates output

Temp rise & GPM for 160 kBTU/h unit

Temp Rise (°F) Flow Rate (GPM)
35 9.0
45 7.0
55 5.8
75 4.2
90 3.5

So 157,608 less 25,025 BTU/h=132,583 btu/h for DHW use, which if one uses a 78 °F ΔT [120°F-42°F], this works out to 3.4 GPM available for DHW (based on ..so two showers at 10 mins and 1.5 GPM draw from each results in 3.0 GPM and 0.4 GPM left for everything else that may be short & intermittent.

This in contrast to:

Other method I've seen estimates 99th percentile of peak DHW load based on Buchberger et al (referenced in ASHRAE Handbook- HVAC Applications, Chp 51 Service Water Heating, IP version) where one gets 5.8 GPM as the 99th percentile (exhaustive enumeration) with 8 fixtures.

Any suggestions or tips to improve analyses? Missing anything? Looking to see if a larger unit is more suitable for dual space heating/DHW application.

The profile of the 199 kBTU/h unit is:
Temp Rise (°F) Flow Rate (GPM)
35 11.2
45 8.7
55 7.2
75 5.2
90 4.4

 

[disruptoergosum]

@PEDARRIN2: I'm stuck with 1 x single unit of tankless (condensing type). Not boiler.
The heat exchange life diminishes to 3 years from 15 years if used in combo DHW + space heating (vs standalone DHW).

 

[HVAC-Novice]

I assume the 3/15 years come from manufacturer? They aren't build to run multiple hours a day during all winter.

Low-mass boilers in general don't work well. Depending on turndown and control and system volume, they require a buffertank. Your single coil looks like a very low volume system. Even if it was build to boiler standards, it wouldn't be great without a buffer tank.

There are reasons a proper heating system is much more involved than a cheap water heater and a loop added.

 

[disruptoergosum]

@HVAC_Novice: yes it's the detail from the manufacturer. As noted earlier, it's a new build (4 years roughly). The plans approved by the local municipality showed a 199 kBTU/h unit whereas they installed 160 kBTU/h. They don't have the same capacity obviously. Hydronic coil draws 2 GPM steady when heat called. Otherwise when heat not called it's in the re-circ loop that's mixed with the DCW supply that you see running here back into the tankless unit. To no surprise, once you start using several DHW loads (as it's prioritized), the air handler water inlet temperature drops, blows ambient temperature (or cooler) air through the supply trunk (from blower fan).

 

[IRstuff]

Otherwise when heat not called it's in the re-circ loop that's mixed with the DCW supply that you see running here back into the tankless unit.

Do you have a piping diagram? It sounds like the heating and DHW is one and the same? Otherwise, how could it mix with the cold, since that sounds like a pre-heat loop for the DHW when a dedicated circulation loop wasn't built-in.

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[LittleInch]

Sounds like the blower fan needs to be part of the control loop when it isn't getting any hot water.

I assume the recirc pump just turns off once the unit detects a lot of DHW demand?

Or just connect the heating pump to a different supply and keep it running.

After all 25,000 btu versus 160,000 isn't a big amount for the system to provide.

IR stuff - Yes 6l/min will give yu a shower head with probably acceptable flow, but I like the waterfall type ones which are in the 10-12l/min mark. But then I can shower in less time. Each to their own and I see the US has a max of 2.5 GPM since 1992 and in California its apparently 1.8 GPM max flow, so fair enough for the flow mentioned previously. The issue then is if there is enough water pressure to drive that flow to two showers at the same time.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HVAC-Novice]

Besides all the other issues.... you need a control that will stop the fan for the coil or you risk blowing cold air. A combi boiler designed for space/DHW heating has such control.

i didn't read the Navien control... but I wouldn't buy a product from a company that is so flaky with documentation and makes it look like all is easy-peasy as long as you buy their product. Look at the boilers I linked above and their manual. quite a bit different (and has a chance to be code compliant).

 

[disruptoergosum]

@ LittleInch: Yes the pump (Grundfos non-submersible circulation pump) only active when heat called. I believe there is intermittent operation to prevent Legionella.

Schematic here (not entire property)- not showing the DHW loads (eg. shower, faucets, etc)

 

[LittleInch]

That's not what I asked- I said does the circulation pump turn off when there is DHW demand? If so maybe just disable that part of the operation. or set the turn off level a lot higher. It's not clear how the system knows to prioritise the hot water system though - Is there a flow meter somewhere?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[disruptoergosum]

Apologies LittleInch.It must when there is DHW demand (presumably)?. All I've measured are with temperature clamps a fairly rapid decay in the inlet temperature of ~ 2.5 °F/min to the air handler coil and the temperature in the outlet increasing (which presumably means it's not moving through the coil here) and through the pump into the re-circ loop. These inlet and outlet temperatures were measured with contact thermocouples (k-type). Testo 115i.

 

[PEDARRIN2]

Back to the plumbing side. As long as there is a backflow preventor installed on the make up water connection from the DCW and the heat transfer of the two systems are separate - there is no code issue. Double wall heat exchanger is not required as the heat transfer is from gas combustion and the backflow maintains potability.

 

[LittleInch]

My guess is that there is somewhere a flow meter that either recognises the increased flow due to DHW or is on the DHW line itself.

But the solution is still the same and that is to decouple the circulation pump and either just run it all the time and let the fan do the room temperature control / on / off fan or couple it together with the fan, but then somehow you need to run it occasionally during summer, with no fan to avoid killing yourself with legionnaires disease....

Or just turn the fan off when the pump goes off so that at least it doesn't blow what you perceive as cold air (actually its the same temp as the room but blowing it makes it seem "cold") although there could be some fresh air being let in as well?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.