Combi or not?

josephny

Wow, I'm just checking in and the conversation has taken some interesting detours.

I downloaded the excel tool for hot water demand after watching the video (which is brilliant -- thank you Larry). I added one "other fixture" of 2gpm with a 5% chance of usage and I get a 99th percentile demand of 11gpm.

It sure sounds like a safe approach is to have a large indirect tank (80-100 gallons).

Anyone care to address either why it's wrong or unwise to use a modulating boiler that goes from 20-100kbtu so that when heat is called it can operate at 20 and when hot water is called it can operate at 100?

Thank you!

josephny

I was looking at the AO Smith tankless.

Am I correct that I would probably want to set the tankless to 125 degrees F. if my ground water is 40 degrees F, that's an 85 degree rise. According to the product's specs, it would max out at 8gpm.

Are my numbers correct?

I wonder what happens if the draw is 10gpm? Does the water temp drop?

I do like the idea of a tankless and have them in quite a few other places.

hot_rod

Without some hard data it is basically speculation. I feel Larry is closest with a 3-4 gallon max. draw with typical low flow fixtures. A large fast fill soaking tub would throw a wrench in the calculations. There are updated fixture unit sizing tables available, adjusted for today’s DHW loads. 
Whichever way you go you need to size to the largest load, period. If you want or think you need 8 gpm continuously then Jamie’s math is how you would determine required BTU.
combi and tankless work fine IF you work within their limitations.
A 150 combi with a 10-1 turndown covers must residential jobs.
Of all the choices the tank combi that Rich showed covers all bases.

Alan (California Radiant) Forbes

I once installed a combi boiler for a house here in Berkeley about the same size as yours. It wasn't at my suggestion; instead as the owner requested. He was committed to using less energy and water; installed water saving shower heads and scheduled showers and water usage for his large family.

There are those, however, that never want to think about running out of hot water, no matter what the demand is. That's what the large capacity, high recovery tanks or on-demand heaters are for.

First world problems.

Rich_49

Not only is it what I would recommend to a customer with your requirements , I would use it in my own home as well under those wants and needs .

mattmia2

Alan (California Radiant) Forbes said:

I once installed a combi boiler for a house here in Berkeley about the same size as yours. It wasn't at my suggestion; instead as the owner requested. He was committed to using less energy and water; installed water saving shower heads and scheduled showers and water usage for his large family.

There are those, however, that never want to think about running out of hot water, no matter what the demand is. That's what the large capacity, high recovery tanks or on-demand heaters are for.

First world problems.

The thing is if you use an indirect with a really well insulated tank and design it so that a modcon can condense while heating it, the difference in efficiency between the indirect and a combi or tankless is negligible.

Jamie Hall

May I humbly submit that the choice as to whether to run out of hot water... or schedule showers... or use low flow shower heads... or wear a hair shirt... is one which should be made by the individual, not forced upon them. I will readily grant that water is real short in some places -- southern California or Las Vergas are decent poster children for the problem -- but that is a result of errors and omissions in the application of available technology (and, to a certain extent, inappropriate pricing of a commodity), and that is a political problem, not a first world problem.

Jamie Hall

Just did some back of the envelope scribbling... if one combines treatment and recycling of grey and black water with pretty basic solar distillation, you'd need around a quarter section of desert devoted to your solar collection system to supply LA using seawater... if you did it intelligently.

Or the waste heat from San Onofre would do it very nicely, too... oh I forgot. The greens shut that down. OK.