indirect as buffer

Here's the situation: If you wish, just scroll down to the BIG QUESTION below

So I have a customer with no backup heat source.  the entire house (1200 dn, 600 up) is taken care of by a solar high mass in floor system (read: solar heated 2'deep sandbox)

and a wood stove masonry stove.

the house will usually self maintain at about 50 degrees with no wood fire. 

the customer is re-fitting the upstairs as an apartment, and wants backup heat for the all house.

 the plans are as follows: upstairs: 3 panel radiators, one in main room (5000 btu), one in bathroom (possibly towell warmer 2000 btu) and one in the bedroom(3000 btu).  She would like each one upstairs zoned separately. 

Downstairs, there will be a fan convector (10,000) for space heating in the open middle of the house.

I see a few options that could work well for this system, and am looking for feedback, as in "throw it at the wall and see what sticks. "

the other caveat: we have really restricted space to work in.  3'-4' wide, 8' high.



1: size everything with low temperature water and use an AOSmith vertex tank style WH, with a flatplate HX to go to the heating system.  run the WH at 140 and reset down to whatever's necessary.  I have done this successfully with a couple of systems with really low water temps. 

2: TTP prestige excel all-in-one, and hope the mass of the internal tank will prevent short cycling. 



The BIG QUESTION

Here's the one I'm wondering about, from an academic standpoint as much as any other:  can i run a single primary loop thru a tank-in-tank indirect heater (think weil Mclean or triangle tube smart) and set the boiler reset curve between 130 and 160 for the radiators, activate the circulator on either the DHW call or heating call, and before the flow returns to the boiler, put closely spaced tees on the line back to the boiler, with a GF1555 alpha and uponor manifold with loop actuators.  I'll try to post a schematic shortly.  The goal is to buffer and produce adequate dhw in a tiny tiny footprint.



This seems like a best of several worlds option.  I'd probably be using a MC-50 or MC-90 if necessary for DHW load.

cheers,

Karl