Opinions on DHW options

Bernie_the_Brewer · March 19

In a few other posts, I have been gearing up to specifying a new hot-water boiler (likely a mod-con) for a large old duplex. I presently have two conventional tanked water heaters with natural, atmospheric draft. Once we get rid of the 70-year-old boiler, I want to also get rid of the natural-draft water heaters and decommission the flue. I am seeking opinions on replacement DHW equipment.

We use this duplex as a large, single-family home, so I probably will get rid of one water heater. Our hot-water loads are actually quite modest, as we are just 3 adults who don’t take lengthy showers.

As I see it, we could use:

Conventional direct-vent or powervent tanked water heater
Tankless
Indirect from the new mod-con
(I am not really considering a heat-pump at this time.)

I installed a tankless in my last house, and was pleased. I have installed a powervent in my in-law’s house, which seemed fine. I have no experience with an indirect, but it seems like an interesting option to me. In fact, I only really “know about” these from this forum. I wouldn’t mind NOT having to run more venting lines and punch more holes in the wall.

Is an indirect a no-brainer in my situation, where I am about to have a mod-con installed? Seems like the only downside is the added complexity of the boiler system and a bit higher expense.

Thanks for any thoughts.

hot_rod · March 19

If you lived with a tankless, then you could consider a mod con combi boiler. A 150K could supply 4 gpm or so depending on incoming water temperature.

Like a tankless they need ocassional descaling, so install it with service valves.

Will it ever be a duplex again? If so you may want more dhw capacity.

mattmia2 · March 19

the combi is easier to install, the manufacturer baked in the complexity but the parts tend to be more proprietary if it breaks. the indirect the install is more complex but the parts tend to be off the shelf parts.

with the combi you're replacing the dhw when you replace the boiler, with the indirect they could end up failing at different times and stagger the replacements.

check your incoming cold water temp in winter before deciding on a tankless or combi. in my area 200,000 btu/hr in is more like 2-2.5 gpm because the incoming water is nearly frozen.(i'm doing the math again and coming up with around 4gpm, i don't know why i was thinking about 2 gpm before, i may be missing something)

you do need a big gas supply for a boiler and a tankless

hot_rod · 1:29AM

https://forum.heatinghelp.com/discussion/comment/1892236#Comment_1892236

500xFx∆T

500 x 4 x 85=170,000

Assuming water at 35°- 120°

199,999 btu/hr X 87% = 173,000 btu/hr

mattmia2 · 1:34AM

maybe i was looking at something like rinnnai's table and it was doing something weird

4GenPlumber · 1:47AM

I prefer indirect bc its more consistent with a recirc line and I dont want my water heater to determine my flow. HOWEVER, if you already have the domestic hot water pipe separated since it was a duplex, you could use 2 tankless heaters and not really have to worry about it BUT, thats a lot of gas required to the locations even though you wont use it all the time, if ever. Sorry, my opinion probably only murkied the waters further.

Bernie_the_Brewer · 5:08PM

Thank you all for the feedback and thoughts. I apologize because my following thoughts are a bit disjointed, and perhaps even somewhat random! :)

I have not looked into combi's, for whatever reason. Somehow, I had the impression from reading this forum that they were the red-headed stepchild (i.e., not viewed very favorably by this group). Maybe I am wrong?

In my last house, I was an "early adopter" of tankless. I put in a Bosch 250SX in about 2003. It was okay, but LOTS of cold-water sandwiches, failures to ignite, and requests from my cold/naked wife in the shower to go reset the water heater. When that gave up the ghost in about 2020, I replaced it with a Rheem RTGH-95. That was a different kettle of fish — instant lighting, reliable, basically indistinguishable from having an infinite capacity water heater.

However, the gas supply piping is marginal for supporting a tankless in this house. The good news is that currently there are three gas meters (!): one for the boiler, and one for the DHW, dryer, and range for each of the units. The boiler gets 1" piping, but the other trunk lines are 3/4" pipe, and are over 40' (and some elbows) away from the meter. I think that means that those two could only handle ~130 to 170 kBTU/h?

Will this be used as a duplex in the future? Not sure. The neighborhood supports SFH this large, and many duplexes in the area have been converted. We have decided NOT to physically convert ours, but just use it as a SFH. My feeling is not to worry too much about potential future duplex needs, but to deal with that when the time comes (i.e., by adding, for example, a tanked powervent for one of the units if necessary). I'm even hypothesizing that the faster recovery time of an indirect would make using one shared tank more feasible.

There will NOT be a recirculating hot water line added. I really ought to replace the original galvanized on all the water lines, but that will be enough of a bear in this large, 3-story, 5 baths, 2 kitchens, 2 laundry rooms 1925 house. I am avoiding that for as long as possible!

ethicalpaul · 5:31PM

(I am not really considering a heat-pump at this time.)

Unfortunate!

mattmia2 · 6:00PM

The capacity for the tankless or combi comes from the capacity of the service too. Running a new line from the meter isn't a big deal, running a new service could be very expensive, or even just upsizing the meter can be expensive with some utilities.

GGross · 6:10PM

What worked out best for me and my family was a powervented gas water heater with a separate heat only boiler. I like indirects on a high efficient boiler, I sell them all the time, I could get one for free, but I went with the gas water heater that way if one breaks I don't lose both systems at once. and a tank water heater is really cheap to replace IMO. wake up to no heat? at least I can still shower! no hot water? at least its still warm!

hot_rod · 6:24PM

The biggest load determines the size of the boiler. In some cases the DHW is a larger load than the heating.

With a large indirect, the boiler goes to priority for maybe 20 minutes or more to recovery.

The math is the same regardless of the type of heater 500 X flow X delta T

The selling feature, and sometimes downside of the tankless is the amount of BTU/ gas supply to make a lot of DHW instantly.

The other option is to reduce DHW demand. Low flow shower heads, reduce water pressure to 45-50 psi are two way to lower the DHW load, and cost of DHW generation.

If costs are a concern, plug in you actual numbers here.

Hard to beat operating cost of a HPWH or tankless.

https://www.efficiencymaine.com/at-home/water-heating-cost-comparison/

Screenshot 2026-03-20 at 12.20.43 PM.png

Larry Weingarten · 6:34PM

Hi, I know you're looking to get away from tank-type water heaters, but have you considered a power vent heater? That would let you skip using the chimney and have good hot water without a bigger gas line or much added complexity. 😺

Yours, Larry

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