confusion over sizing a boiler for radiant/domestic

[dfinn369]

My 1964 house in Marin County, CA is 2500sf and we are re-insulating the house and installing all new double-glazed lo-E windows. I have also installed all new roofing insulation. We have 4 bathrooms, 2 kids (5&2) and I have a wife who is always cold.

I am changing the forced air system to radiant and am having a conflict between my plumber (bidder #1) and another radiant heat sub (bidder #2). We are looking to install a natural gas high efficiency wall mounted boiler along with a 40 gallon indirect fired water heater.

Bidder #2 (much less than bidder #1) has suggested a Prestige Solo 110 (30k-110k Input modulation MBH & a net I=B=R MBH of 88k). He is basing his calc on approx. 20BTU's per sf for radiant.

Bidder #1 (very expensive - $12k more) is saying that the 88k BTU boiler is half the size necessary and that I need to install a Buderus GB-142 with approx. 180k BTU. He is using a calc. of 40-50BTU's per sf. He is saying that I will be able to take a 3 second shower with the Prestige unit when the radiant is used at peak.

Is anyone right in this situation?

Need help - I am a layman when it comes to this stuff!

[craig1]

i don't know which is the right size for your house, but i can tell you that the size of the boiler won't affect your shower like bidder #1 suggested. the reason is that the boiler does not heat the radiant and the indirect tank at the same time. when the indirect calls for heat, the boiler shuts down the space heating and sends everything to the indirect tank. of cousre a larger boiler will heat the indirect tank faster, meaning the boiler can switch back to space heating sooner.