30 gallon indirect water heater too small?

seized123

There are lots of places to calculate this online but I get a wide range of results so thought I'd throw this out there.

Three bedroom, two bath house but only two retired people here now, we have no dishwasher and never use hot water to wash the clothes. Sometimes but not always we take showers in the same hour, and would like to keep that capability. I like a long shower now and then (10-15 mins). We occasionally have guests stay over so every once in a millenium maybe four people would want to take showers in the same hour (like after hiking or something) so really maybe once a year that might happen, easy to work around if that strains the system.

Current defunct water heater is 40 gals and that's been fine, but the heater I'm looking at (Burnham Alliance LT, plastic, I've decided on plastic due to chlorides) comes in only 30 and 55 gals. The equivalent Vaughn Featherweight (also plastic) comes in 45 but I can't find Featherweights for a homeowner to buy - actually I can hardly find them for anyone to buy.

I figure 55 gals might be foolish because it would waste oil energy keeping it hot and we would almost never need the volume?

kcopp

The 30 w/ a mix valve would be fine. Set tank at 140. mix down to 125F.

seized123

kcopp said:

The 30 w/ a mix valve would be fine. Set tank at 140. mix down to 125F.

Interesting. Was definitely planning on a mixing valve (order one first, learn what the heck it is later). Thanks.

Hot_water_fan

Standby losses are generally nothing to worry about. Maybe a 1 or 2% per year difference between the two sizes? You’re talking a few bucks. 

seized123

Hot_water_fan said:

Standby losses are generally nothing to worry about. Maybe a 1 or 2% per year difference between the two sizes? You’re talking a few bucks. 

Interesting. If that means we would pay only 1%-2% more for oil to maintain a 55 gal than a 30 gal then there's no real downside to the 55? (Initial cost is not that much more than the 30.)

Hot_water_fan

https://s3-us-west-2.amazonaws.com/catsy.782/Alliance+LT++SL+Product+Data+Sheet.pdf

the different between the 30 and 55 gallon versions of these is about 4 gallons a year. So ~$1/month. 

seized123

Hot_water_fan said:

https://s3-us-west-2.amazonaws.com/catsy.782/Alliance+LT++SL+Product+Data+Sheet.pdf

the different between the 30 and 55 gallon versions of these is about 4 gallons a year. So ~$1/month. 

Yup I think we'll definitely go for 55 instead of 30 (40 or 45 would be fine but so far I'm not finding it in thermoplastic). What you said, plus lazy roaming over the internet (ignoring formulas or other brainwork) I get the impression that with indirect water heaters (unlike boilers) oversizing a bit is not a bad thing.

So I think that's settled, but if you feel like it I'd be interested in how you got the 4 gals a year difference from those specs in the data sheet, so I can explain it to myself.

Hot_water_fan

Yup no problem! Just some rough math here. 

For the 30 gallon one, you lose .8f/hr (they didn’t specify at which temp the water is held at, which they should have), which is 30 * 8.34 (weight of 1 gallon in lbs) * .8= 200 Btus/hr. That’s about 1,750,000 Btu/year, which divided by 138,000 gives you output gallons of oil. Assuming 80% efficiency, divide again by .8 and you get about 16 gallons of oil per year. The 55 gallon calculation is about 22 gallons per year, so a difference of 6ish gallons. 

hot_rod

What size boiler is it connected to. With enough boiler the indirect can keep up with a single load.
The indirect tank data sheet should give you output charts with various btu input.

As @kcopp mentioned, raise the tank temperature and use a mix valve set to 120. With a 150 tank, you may get more DHW than you did with the 40 gallon

seized123

@Hot_water_fan, wow, I feel about 4x smarter than I did before I read that. I guess if I’m understanding it, that means if we didn’t use any hot water for a year it would take 22 gallons of heating oil to keep that 55 gallon tank at temp, which is much less than I would have guessed, not having had any idea.

JUGHNE

Also an advantage with the larger tank is the extra reserve of hot water available.

If power or the boiler is down you have a little more hot water in your basement.