Indirect Sizing Question

Laurelie

I am relocating my mod com boiler due to remodeling, and want to add an indirect hot water tank. I've been depending on a tankless for years, which works ok, but we want to become more efficient, PLUS add a circulator so we have more instant hot water at the far reaches of the house. (Tankless is at the end of its life, but takes forever to get warm water to the hand washing sinks).

Question:

I see varying ideas about how to size this. My heat load is small (less than 80btu on a 100btu mod con) and we run it low temps (130-140). I have a large family, and the adults like to bathe. I have done the calcs, and it seems we should go for a 80 gallon or even 100 gallon indirect heater, but other articles I've read says, no, no, no, too much water, too wasteful, not worth the money. "Stick to a 40 gallon indirect and you will be fine" says a plumber buddy. Other buddies say my mod con is too small for an indirect tank, but this surely seems incorrect.

Most of the kids take short showers. We do have laundry, but it's not at the same times we are bathing. We bathe at night; kids shower in the AM.

So which is it? Any help appreciated.

gschallert

Laurelie said:

I have done the calcs, and it seems we should go for a 80 gallon or even 100 gallon

Show your math. ;-) if by "large family" you mean half a dozen kids all needing to shower in the same timeframe then yeah maybe you need that large of a tank. Can't say until we see the demand.

Lay out your DHW demand.

How many fixtures and what flow rates? What is hourly peak flow demand in GPM? How many bathrooms? Kids taking concurrent showers? Showerhead flow rates?

Laurelie

We have five kids, two bath-loving adults, and one very old person who needs a shower almost all the time.

Ha ha.

Seriously.

Plus, two kitchens (two dishwashers), one washer dryer, two showers, and one 75 gallon bathtub. It's the bathtub that messes up the math for us (40 gallon vs 80 gallon), but we've figured we use 40 gallons of 140 degree hot plus 10-15 gallons of cold each use.

We each take baths at night, and it doesn't conflict with other water use.

Since we have a tankless, we are tapped out at 2-3 fixtures anyway, but we have never noticed an issue and have never (in the real world) tapped out. Like for example, doing the clothes wash, dishwasher, and showers all at once. Never.

The constant rate of flow (gpm) after the first hour is what concerns me, regarding the 40 gallon unit. I figure we'd get about 1.8 gpm, which would be enough for us.

I have plenty of room and money for the 80, but it seems keeping all that hot water hot would cost more over the long term.

I live in a cool coastal climate where the floor heat is "on" almost the entire year.

Looking at Weil McClain and Amtrol.

Robert O'Brien

What calcs have you done and why do you distrust them?

gschallert

So is it 5 (kids) or 6 (kids +old person) showers every morning? Are they back to back or concurrent since you have two showers?

Two 65 to 70 gallon baths each night? Just trying to get a handle on peak hourly demand....

Laurelie said:

Since we have a tankless, we are tapped out at 2-3 fixtures anyway, but we have never noticed an issue and have never (in the real world) tapped out. Like for example, doing the clothes wash, dishwasher, and showers all at once. Never.

That's because with a tankless what matters is concurrent draw, there is no recovery rate to factor in. Tank metrics that matter are first hour delivery & recovery rate in GPM. What is you avg inlet temp? Cool coastal climate doesn't give me a good starting point.

newagedawn

how many btu's is the boiler? gross or net?, how many zones with out domestic, what size main pipe comes off the boiler and what coastal area do you live? need data, data data,...lol

Bob Gagnon plumbing and heating

I would go with a 60 gallon SuperStor, run the tank real hot and with a mixing valve you should be able to fill the tub no problem, check out how many gallons of hot water the indirect will make per hour, it'll surprise you...

Thanks, Bob Gagnon

hot_rod

I tend to prefer larger tanks at lower 60C, 140F or so operating temperature. 140° is adequate for bacteria prevention. With the control thermostat near the tank bottom, you will stack 10- 20 degrees hotter water in the upper portion of the tank.

If you have a recirculation pump, piped to the tank, the tank will pretty much mix to one temperature.

Corrosion doubles for ever 20° increase in temperature, so elevating the temperature of a tank will precipitate minerals faster and shorten the tanks life expectancy.

Also note the water quality spec that the manufacturer states, warranty is dependent on maintaining water quality.

Zman

Any of the newer design indirects will have enough surface area in the heat exchanger to max out your boiler capacity (Many of the amtrols are still from the dark ages and will not) . That being said, as long as it is set up for priority, the continuous capacity will be the same with all the options. It will equal the boiler max output.

The size of the tank will just be a buffer for times when your usage exceeds that capacity. It sounds like a 60 or 80 gallon would work well for you.

The majority of the heat loss you encounter will be in the recirculated line. The larger tank will have little effect.