replace undersized radiant demand with boiler

jsand

7 year old 3000 sq ft house Seattle area- natural gas

in slab radiant on 1500sq ft  1st floor -one thermostat now(plumbed but not valved yet & wired but no thermostats  for 4 zones but not set up)



2'd floor  1500 sq ft  (4 rooms) stubbed for radiant baseboards and zoned for valves but not set up.

replacing undersized Tagaki demand with boiler.



want boiler to be sized for whole house (2 temps- slab and baseboard) but will do 2'd floor as additional cash allows



We really don't use the 2'd floor but want the house set up so if we sell in 7 years or so,

we can finish heat on 2'd floor or at least have it close.



In the intervening 7 years (guessing on this)

1. are we crazy to do this in 2 steps rather than bite the bullet on eventual sales price of house

2. will the oversizing of the boiler for what we need now vs what would be needed to heat the additional 1500 sq ft cost more than it's worth in boiler cost and fuel in the next 7-10 years say.

Looking initially at a Burnham ES24NI

Zman

Questions

Is the existing demand heater also doing domestic hot water (open system). If not how are you heating the DHW? A modcon boiler is best for in slab radiant. I would use low temp radiators upstairs when the time comes. I would not be to concerned about being oversized prior to expanding. Modcons can turn down to 20% and are most efficient under light loads. Don't do a conventional boiler with radiant, you end up heating water just to mix it down. you will see 20-30% better efficiency with condensing.

SWEI

sizing

is critical.  You need a proper heat loss calc for both the existing single story house and the planned 2-story version.  If the difference is significant, I'd size for the existing house alone, since that 5:1 turndown is important to both your comfort and cost of ownership.  7 years is long enough to consider adding another small boiler or even replacing the first one.  By that time, micro CHP could well be a viable option here.