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Tankless water heater - DHW/Space Heating sizing 1

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disruptoergosum

disruptoergosum

Aerospace
Jun 17, 2021
17
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
 
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contact you plumber and JHA about the requirements to separate domestic hot water from a boiler loop. Need double-wall heat exchanger. No one wants to accidentally drink year old chemically treated heating loop water.
 
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It is separate.
Screenshot_2023-08-21_160026_bjdi5f.jpg
 
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Seems like this would depend somewhat on the size of the dwelling as well as its location. in SF, a 1000-sf condo with single shower, dishwasher, and one washer/dryer, my son's, has the Navien NCB-180E installed IF TLDR, the heating is 80 kBTUh and water is 150 kBTUh

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I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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I don't see an HX (and not and it needs to be double-wall). The very same water that goes into the coil, also will go into the domestic hot water. I don't know where you live, but that should be illegal in all areas that have decent plumbing codes. Once you make it legal, a separate boiler and water heater usually is better.

As for sizing, that depends on construction and the enclosure surface areas. Just floorspace is meaningless. Is the house a semi-sphere, or a complex zig-zag? Could be surface difference of 3 times. All window or wall? Extremely well insulated and low infiltration, or built like a card board box?

Probably also want a control to prioritize domestic hot water over heating for the time everyone has a shower.

You also need extremely good water treatment since those instant water heaters have very thin channels for the water. any tiny scaling, dirt or otherwise can clog them. They are supposed to be cleaned annually.
 
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The HX is a hydronic air handler. Coil sits above the blower fan. Water comes in at 120 F exists 94 F (delta T = 26 F), which leads to 25,000 btu/h of supply at 2 GPM.

Heat loss ~ 19,000 btu/h

There is prioritization for DHW.
 
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The combi-boiler has a isolated loop for the HX, at least, that's the way the Navien is configured. You can flush the HX loop and refill from the main and then close it it off again.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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The normal issue is that in order to supply two showers AT THE SAME TIME, you need a huge heater of about 70-80kW (250,000 BTU/hr) at the delta T you describe which seems reasonable and also maintain enough flow for a shower. Most mains fed showers work at about 12l/min / 3 GPM. 1.5 GPM is a dribble....

But your load which runs the vast majority of the time (your heating) then only needs 10% of that.

Most single boilers / furnaces simply won't turn down efficiently to that level so you end up with constant cycling.

And your heat load won't always be flat out either.

So a compromise is required - either buy two heaters - one for the heating and one for HW or accept you can't run two showers at the same time.
My house the boiler is sized at 35kW / 110000 BTU and for hot water / showers / baths etc works just fine, but you can only have one at a time and that's mainly water pressure issues.

Heating we have radiators so it takes about 25 to 30 minutes at full power to fully warm up the whole system then modulates down to about 5 to 6 kW (20-25,000 BTU/hr) so 15 to 20% of max.

So its up to you, but when you talk to heating engineers, they usually say people buy units which are too big because they want to cater for a event which happens for 10 minutes a day instead of buying a more efficient and cost effective unit for things which happen for > 12 hours a day.
Most don't have enough space to buy two, but I guess it depend son the number of people and the number of showers etc happening at EXACTLY the same time.

My system is two separate ones for DHW and heating within the same package (DHW is a plate HX and you can hear the divertor valves switch boiler flow from heating to the HX) but if the loop is small enough there is no big reason why you can't run it as shown with a single system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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IME, most loack of heating is due to the space-unit (coil, radiator etc.) being too small. the boiler size never is the issue when we investigate " too cold" issues.
instant water heaters suck in general. Look at very low flow situation and you see they can't maintain stable temperature. Some add a 1 gl tank to help with that... why not just use a tank heater?

What you can do is use a boiler and storage tank and HX for domestics hot water. Basically an externally fired water heater. There are boilers actually build for that. With the storage tank, you don't have to worry about the peak demand. That is how 99.9% European homes (the ones that use gas or oil) are heated and get hot water. inc. apartment building central heating.
 
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It might have been 99.9% 30 years ago, but the rise of the "combi" boiler shows no limits.

Many houses now don't have room for a HW tank which often just leaks heat away over time instead of only creating hot water when you need it.

Basically anyone changing a gas boiler in the last 30 years has ripped out the HW tank and just installed a combi boiler. so my guess is that now it's probably about 50% or more of gas fired homes have a combi boiler.

Apartment block or similar centrally provided hot water is a completely different animal.

how many times do you want or need a "very low flow situation"?

I think they are great and don't suck at all. Just my opinion.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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I believe the building code here has showers fixtures rated a minimum 1.50 w.s.f.u. (or 1.5 gpm) minimum. Two 2x showers would be 3 gpm simultaneously.

Space is the issue. Can't fit a storage tank. Mech designer used a method to size conventional storage type tanks (using FHR-first hour rating) to justify his decision.

I know combi-boilers would be ideal, but stuck with a condensing type tankless. They had specified 199 kBTU/h which has a higher output (in GPM) at all temperature rises, but installed a lower capacity unit 160 kBTU/h.
 
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A "condensing type tankless" is a combi boiler here in the UK....

1.5GPM (6l/min) for a shower is a dribble (hence "minimum"). Just wait 5 minutes for the other person to finish FGS. Truly a first world problem.

I think your 160,000 BTU/hr is more than sufficient for your needs. Another 25% wouldn't make any difference on the HW front but would struggle to perform effectively when heating your house operating as such a low turn down ratio.

All IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Not only do you have to maintain the potability of the water (during summer months when there is very little if any heating hot water demand), you may also have to comply with Lead Free requirements of all pipe/fittings/valves/equipment in contact with the water.
 
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Correct, in summer the heating coil loop basically is a dead leg that will have NO chlorine or other disinfection.

You also introduce oxygen to the HVAC portion all the time. This is the opposite you want in an HVAC system.

That heater also has to be labelled as a "boiler" in a way that is acceptable to JHA. So many other red flags.....

Here a product actually designed for what you want to do, it has an HX (duh!).

In my jurisdiction a double-wall HX would be needed, so you should talk to your JHA if a single wall HX is acceptable. If they require one, you may talk to the manufacturer if they can fit it with one or if you need an external HX. A double-wall HX will be larger, so external may be required.

Please have it professionally designed and installed and approved by JHA. No one wants to drink boiler water and when the damage is done, non of the "people from the internet" that told you a regular water heater is fine will take the blame for you. When something goes wrong, you will be all alone....
 
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Probably best to just show a typical installation; the only thing that the Navien combi-boiler heating loop shares with its DHW supply is the burner assembly and control circuitry. They have separate heat exchangers and the only potential issue is if you leave the makeup valve open, but that would be the same issue if you leave the makeup valve open in any hydronic system. The only way to get air into the heating loop is if you do a bad job of refilling after flushing.

Navien_bejq5h.jpg


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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There is oxygen in domestic water. and Chlorine. And I bet that Navien manual says somewhere that YOU have to consult local codes.

If you don't want to install per code, and just trust what you saw "Cousin Eddy" do 30 years ago, Go ahead.
 
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If you want, just put two Navien heaters and a RPZ backflow upstream of the space heating unit to make that water non potable. Since it will be based on flow rate, it will be accurately sized and when the space heating is not used, the water heater will not be operating. Since it will be non potable, eventually all the oxygen and chlorine will not be an issue.
 
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