Is my figuring right?

[Armaton]

If my storage is 840 gallons, and all 840 are at 180 degrees, thats 1,261,008 btu's stored, (840 x 8.34 x 180). If I use storage down to 120 degrees that would be 420,300 btu's, (840 x 8.34 x 60), and at 35 kbtu p/h design load that would be 12 hours useage from storage, right? Doesn't seem that long when I'm reading 16-20 hours from others, and I don't feel my house heats hard. Or because I'm using "Design" load, that would be a minimum time, since design is supposed to be worse case?

 

[tom in maine]

The calculated heat load is based on the indoor temperature and the design temperature for outdoors. ( -10F in your area ?)
Any time it is not that cold, your heat load is going to be a fair bit less and the storage run time would be longer.
As you mention in your post, it is the minimum time.

 

[Floydian]

If you can get by with 120 supply at design temp, you should or could get by with a bit lower supply temps during avg outdoor temps. This is where outdoor reset control of supply temps is really nice.
I routinely run my storage down to the mid 90's during shoulder season and milder temps. A dumb valve and bang bang controls will certainly get the job done though.

Noah