Sizing W-M Ultra to balance heating efficiency & DHW demand

[JoelR]

Hi,
We live in Anchorage and are replacing our gas boiler (1973 model) and hot water heater (about 8 years old) with a W-M Ultra w/ sidearm W-M Ultra-Plus indirect hot water heater. I'm confused about sizing.

Some facts before the question(s):
- baseboard hot water;
- heat load of ~55,000 BTU/hr, but with planned upgrades (sealing of leaks, added insulation, etc.) expect heat load to go to ~48,000 BTU/hr;
- large two-person bathtub (overflow at 85 gallons, so lets say max demand ever is 55 gallons) used about once a week.
- two folks in the house,
- we DO NOT wash clothes, dishes, and take baths simultaneously.

Given the heat loads, my thought was to go for the Ultra 80 (80 MBH) w/ Ultra-Plus 60 gallon tank.

The contractor bids, however, all came in listing either the Ultra 105 (105MBH) or the 155.

When I called contractors to discuss this discrepancy, I was told that
(1) the larger sized boiler was required to guarantee meeting the (once a week) bathtub demand, and
(2) the modulating features could be programmed with different maximum 'rates' (my poor phrasing for the plumber's discussion of peak heat loads, etc.) separately for the baseboard vs the domestic hot water supply so that we could have both a lower 'ceiling' for the baseboard, thus avoiding cycling and improving efficiency, AND take a bath w/o boiling pasta-pots full of water on the stove.

Yet from all I've read, one of the crucial aspects of improving home heating efficiency is avoiding oversized units.

Advice for the 'sizing confused'?

Thanks. And thanks for all the helpful information in the previous 'high efficiency' threads.

Cheers,
Joel in Anchorage

[jadnashua]

Often, the DHW is put on a priority zone...this means that when it is calling for heat, the rest of the system does not get any heat (it usually doesn't take all that long to satisfy the indirect - plus, it automatically (normally) ramps the supply temp to max while doing this priority circuit then goes back to modulating to satisfy the house needs). It can circulate what heat's already there, but not add any. Given your situation, the smaller unit should work fine if your numbers are correct. The thing to look at on the indirect is the first hour volume which should be more than enough with a 60 gallon indirect...you could probably go smaller. See what the pros here have to say, but that is my experience. I've got a similar system using a Buderus boiler - it has the priority zone logic built-in, but if not in yours, it is easily added.