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this indicates the boiler pump turns off on dhw call

If it is in zone mode a primary loop pump would also run

The boiler needs to be sized to the biggest load. In todays energy efficient homes, the DHW ends up being the biggest load often

So knowing what a homeowners expectations are will help with tank selection.

If a tub is used frequently I suggest more storage capacity compared to a larger boiler able to give you 4 gpm or more

Tank temperature could be elevated and mixed down

Operation above 140 will shorten tank life and precipitate more minerals

What did you have for DHW in your previous home?

the tub is 66x34 free standing tub. It’s approximately 45 gallons at 3/4 full. Now writing down those numbers and knowing how large my indirect tank is, my tank doesn’t seem large enough at all.

i have a similar setup to theirs but with a 20,000 btu/hr larger boiler and i can run it out of hot water with a long, adequate shower all by myself. if they want to fill a big tub or use 2 or more showers at once their capacity is way too small. they would need a huge electric to do it with electric because of its essentially nonexistent recovery rate compared to their consumption. that boiler can recover an 80 gallon tank in less than half an hour. electric would take about 130 minutes to recover 80 gallons.

it is usually better to size the boiler to the heating load and get the needed dhw capacity with bigger indirect tank(s). if you get the capacity through boiler sizing then the boiler won't have sufficient turndown to match the heating load when it isn't at design conditions which will be most of the time.

Hi, Another option could be to add a 40 gallon (or bigger) tank in series with the existing 40 and pipe it so both are heated by the boiler. It could be a lower cost way of solving part of the problem. Still seems DHW priority isn't figured out yet.

Yours, Larry

a 40 gallon tank? with a 2 gpm recovery? for 3 and a half baths? nope.

You're asking an awful lot of that system. 80,000 BTUh for domestic hot water works out to about 2.5 to 3.gpm recovery rate — call it 3 litres — per minute. That's one shower — and pushing it for just one shower.

What's happening is that that one shower (or bath) takes all of that 40 gallon tank, or nearly so, and the water heater simply can't heat the water flowing through hot enough. So you need to wait for the boiler to catch up — which it will, but not that fast.

Solution? Bigger tank. That won't solve the problem completely, but with an 80 gallon tank — which is the largest you can get for a residence but is the smallest I'd recommend — you'll have a better chance.

Also set it to hold the water at 60 Celsius or so and use a thermostatic mixing valve to bring it down at the outlet to a save temperature.