Need guidance selecting a DHW system

samnew

I'm doing a full gut remodel on a house in the desert southwest. I've gotten pretty far designing a simple radiant baseboard system that I plan to install myself, using a Weil-McLain Aquabalance 80. I'm getting a bit confused, however, deciding what I should do about domestic hot water. The combi units look like they have lowish flows compared to, say, a tankless unit: with a 70 degree rise, the AB-80 shows a 2gpm flow rate, and I'm afraid that using this as a boiler and DHW heater will result in not enough hot water. This house will have at most 4 occupants using 2 bathrooms and 1 kitchen.

So, I was looking at either an indirect system or a totally separate DHW system. I'm competent enough to install this simple radiant system with some local guidance, and I've installed tanked and tankless water heaters. The indirect option seems perhaps a little more efficient and roughly comparable in cost to a nice tankless unit, but seems like I might want to size up to the AB-120. I'm not really clear how this would affect the overall efficiency of the system if I increase my firepower for a smallish radiant baseboard system.

Should I, as an owner-builder, stay away from trying to figure out the indirect system and keep it separated and simple with a tankless or tanked water heater? Or am I making a mountain out of a molehill?
Thanks,
Sam

hot_rod

The choice between a tank or tankless or combi really comes down to how your family uses DHW. If you run multiple taps at one time or want a fast, dump load for a tub fill, a properly sized tank may be your best option.

The tankless or combis need to heat the water from incoming temperature to the useable temperature, around 103- 105 maybe for a shower. So they usually require a higher BTU size to be able to generate that "instant" temperature lift.

So the choice is to heat and store, or generate on a continuous basis.

I feel 120,000 is probably on the low side of a combi, capable of 2.5- 3 gpm depending on incoming temperatures.

Do you have an idea on the heat load?

Desert SW cries out for solar thermal

samnew

I had a manual J done which shows house heating loab at 36k Btuh. I'm beefing up the insulation quite a bit with double-studded walls and insulating my basement, and the footprint of the house is just a rectangle.

You probably are already familiar with the products but there is also a non-combi model available, which seems like the way to go if I chose to use indirect heating, yes?

hot_rod

samnew said:

I had a manual J done which shows house heating loab at 36k Btuh. I'm beefing up the insulation quite a bit with double-studded walls and insulating my basement, and the footprint of the house is just a rectangle.

You probably are already familiar with the products but there is also a non-combi model available, which seems like the way to go if I chose to use indirect heating, yes?

Yes there are plenty of non combi boilers available. So an indirect tank, stand alone fired tank, separate tankless, or plate HX are other DHW options. There are some boilers available in the 50,000 range they seems like a good choice for that heat load.
The indirect tank size would again depend on how much and how quickly you want or use DHW. With a 50K boiler you may want to go a bit larger on the indirect to store more, but recovery is slower also.

With a combi or boiler/ indirect you have all eggs in one basket. If the unit goes down, no heat or hot water. If that is a concern.

Zman

What are your hot water needs?
Do you have low flow showerheads? Any large tubs to fill?
What do you presently have and how does it keep up?

Larry Weingarten

Hi @samnew , It sounds like you'll be doing all new piping, so you would likely be well served to use low volume piping in concert with low flow fixtures and pressure compensating aerators. This can substantially cut water usage, waste and waiting time. Here's a link: https://www.garykleinassociates.com/writings.html to articles written by Gary Klein, who is a serious hot water nerd and a friend.

Yours, Larry

samnew

Zman said:

What are your hot water needs?
Do you have low flow showerheads? Any large tubs to fill?
What do you presently have and how does it keep up?

I've gutted the house, and am replacing everything. Water here is precious, so I'll be using low-flow heads and faucets wherever I can. I'm reusing the two cast-iron console tubs that were in the house when I bought it, they take maybe 20 gallons to fill. I'll have a fairly average house, two full baths, washing machine, kitchen, dishwasher.

Based on some average numbers for fixture flows, i'd guess I could need 8 or 9 gpm of hot water, worst case scenario (laundry, DW, shower and kitchen sink simultaneously).

Down the road I may convert my basement into a separate apartment, but I can see justifying the addition of a separate tankless unit for that load.

@hot_rod now that you mention it, i am concerned that the hot water and baseboard heat could go down together, it seems unlikely but then again i'm an inexperienced installer so maybe keeping things separated and simpler is the way to go.

hot_rod

Based on some average numbers for fixture flows, i'd guess I could need 8 or 9 gpm of hot water, worst case scenario (laundry, DW, shower and kitchen sink simultaneously).


If you see this happening, all loads calling for hot water at once at once, then yes you need to provide that amount of gpm somehow. Typically you are blending some cold at faucets and showers.

So if you maintain or mix the DHW to 120F, you would add some cold at the faucet to get it to a comfortable shower temperature, for example. A dishwasher will, a clothes washer could, take 100% hot water.

The tub loads are the big ones. IF you want to fill two tubs with 50 gallons of hot water at the same time, flowing maybe 2-3 gpm each, then the tank or tankless need to keep up with that.

Most families get by fine with 40- 50 gallon water heaters, if you want or like taking tubs, and want two filling at the same time, a pair of water heaters may be needed, or a tankless/ combi's with that capacity.

What size water heater have you lived with in the past?
I think most households learn to live within the limits of what the heaters they have provide

Don't know that I have ever been accused of providing too large of a dhw system. But I have under-estimated a few customers needs, usually the large jetted tub homeowners with 10 gpm faucets, or carwash style body spray showers.

As families grow and want more dhw, you can upsize the tanks, or crank them up to a higher temperature and add a mixing valve to keep a safe 120F going to the fixtures.

I would not suggest a 199,000 combi for 36,000 design day heat load, time to split the system into heating and DHW, separate components or an indirect sized to wherever your desired DHW is. Your suggested DHW wants is driving the load in this case.

Larry Weingarten

Hi, I'll add that research has shown usage almost never exceeds 3 gpm. The downside to oversizing piping is longer waiting time for hot water and more water waste. I like HRs suggestion of splitting the systems. Also it could be interesting to clock your current usage, to see just what your highest use is.

Yours, Larry

samnew

Thanks, y'all!

Zman

I would use a 40 or 60 gallon indirect with the 80k boiler.
40 is probably plenty, 60 will give you peace of mind if you do have those occasional spikes.

Jamie Hall

Domestic hot water usage is one of those really vexing questions, to which there are no simple answers -- unlike heat.

If you can arrange your living so that you never run your big draws -- dishwasher or laundry machines, that sort of thing -- at the same time that there is any chance that someone will want a shower, and you never run more than one shower or try to fill one tub at a time (and aren't trying to shave while the lady is showering), etc., you can probably get by with 2 gpm, if you can tolerate low flow fixtures (particularly shower heads).

And @hot_rod is right -- most families learn to live with and adjust to whatever they have. After all, my great grandparents learned to live with five gallon pails of hot water from the stove on Saturday night...

hot_rod

In my early days I spent a few years ski bumming. We had anywhere from 4 to 12 people in the various rentals over the season. Folks that I didn't even know were related showed up for weeks
Laundry and showers were almost non stop at night. Never had more than a 50 gallon gas fired water heater in any of those homes.

Jamie Hall

hot_rod said:

In my early days I spent a few years ski bumming. We had anywhere from 4 to 12 people in the various rentals over the season. Folks that I didn't even know were related showed up for weeks
Laundry and showers were almost non stop at night. Never had more than a 50 gallon gas fired water heater in any of those homes.

You too? Mostly at Mt. Snow. When Walt was still running it. Also had a friend whose family owned a lodge in Aspen...

But to keep on track: that's the advantage of the independent fuel fired units. Even a relatively small one will cheerfully run 4 gpm until you run out of fuel. And they come bigger

hot_rod

You too? Mostly at Mt. Snow. When Walt was still running it. Also had a friend whose family owned a lodge in Aspen...

I went west, young man. Mountains of Utah.

Jamie Hall

hot_rod said:

You too? Mostly at Mt. Snow. When Walt was still running it. Also had a friend whose family owned a lodge in Aspen...

I went west, young man. Mountains of Utah.

Much nicer snow out there! I never got that far.

hot_rod

Greatest snow on earth was the Alta slogan, champagne powder, snorkel skiing.
At Grand Targhee, Wyoming we had stickers "Snow from heaven, not hoses"

Jamie Hall

I was active before we had much snowmaking. We had New England powder -- more or less the consistency of cottage cheese.

I still have my giant slalom skis -- 215 cm Rossignols (wooden!) hung up on the wall. Wouldn't dare use them now! Safety bindings? whazzat? But those were the days, my friend!

hot_rod

Interesting you say that, I have a pair of 205's on the wall. Thinking of getting some modern day bindings and taking them out on a cruiser day.
180 cm are what I ski nowadays, actually a good idea for "senior" skiers maybe.
Worked in a ski shop one winter, took out a pair of "leaded" 210 downhill skis, back in the run away strap days, have the scar on my nose to remind me of that experience.

Helmets, 6 person high speed chairs, digital passes, my how times have changed.

Sorry to the OP for hijacking a bit.

samnew

No problem, glad to provide a place for adventure memories. I live out here for similar reasons, although i’m more interested in rocks than snow.

hot_rod

samnew said:

No problem, glad to provide a place for adventure memories. I live out here for similar reasons, although i’m more interested in rocks than snow.

Climbing, or hounding rocks?

samnew

Both, but mostly climbing and biking and walking on them. Hard not to pick ‘em up when they look strange, though.

ethicalpaul

See if your state or utility offers incentives for one of these. The cheapest hot water you’ll ever make. They have other sizes too.

ProTerra 80 Gal. 10-Year Hybrid High Efficiency Smart Tank Electric Water Heater with Leak Detection & Auto Shutoff

https://www.homedepot.com/p/312741448