[Greenbuilding] Radiant heat boiler

[George J. Nesbitt]

1. With a tankless you will at some point draw more gpm than the unit 
can put out. It depends on several factors, incoming cold water 
temperature, outgoing water temperature, flow rate, pressure drop, 
exceed the capacity and you have cold water. With a tank water heater 
you are less confined by flow rate, but you can only draw so much volume 
before you have cold water (standard capacity tanks), unless you have a 
90% plus condensing water heater with a input of 100,000 Btu's or more, 
then you have almost limitless capacity (not necessarily a good thing). 
Not a question that can be answered easily and without some uncertainty.
    The baxi will shut down the heating loop when there is a domestic 
hot water draw. A buffer tank will increase the capacity significantly, 
but with some extra standby loss.
    The baxi is not the best of this style combined hydronic system, the 
quietside is the best I know of. The baxi is not that efficient because 
it doesn't control excess air.
   
2. No, I don't think separate systems are the best way to go, why buy 
expensive equipment to have it sit unused most of the year. And I don't 
think multiple systems for domestic hot water is a cost effective or 
efficient set up. Gary Klien will hopefully convince you of this 
tomorrow night.

    I doubt that the heat load is more than 50,000 Btu, and I don't 
think radiant floors are the best solution for most "green" houses. 
Insulate under the radiant floor or waste lots of energy. Radiant is not 
more efficient that forced air despite the false claims, EBN did a great 
article on this a few years back.

    What you need is a solar hot water system with a Phoenix storage 
tank with built in boiler for backup, $2000 tax credit. The big question 
is do you have to have a closed loop on the radiant floor, closed loop 
more complicated, expensive, less efficient.

    I am currently installing a Phoenix 55 gallon 130,000 Btu tank with 
2 hydronic air handlers and a Metlund on demand recirculation pump on 
the domestic hot water loop in El Sobronte. The heat load of the 5 
bedroom, 3 bath, 2 story house is only 30,000 Btu.

    I would love to never put in another furnace or water heater that is 
less than 90% thermal efficient, with sealed combustion.The Baxi doesn't 
meet this requirement, 85% with power vent.

    Didn't the architect design the heating system? Or is it left up to 
what ever HVAC and Plumbing contractor(s) the General Contractor hires? 
Or is the General Contractor trying to design the system?

    I gave a great 2 hr presentation on HVAC back in November at the 
Green Remodelers Guild, it would have given you a good understanding of 
some of the issues.

   
David Seth
 wrote:
> We have started a large residential remodel in Orinda, CA. The 2,800 sf 
> house will be heated with radiant floor heat. We were planning to use a 
> Baxi Luna 330 instant heater for combined radiant and domestic hot 
> water. I really love the Baxi Luna for its compact design and simplicity 
> of installation, not to mention its general high efficiency.
>
> However we are concerned about its capacity to serve the demands of a 3 
> bath house. The homeowner consulted with a supplier who suggested that 5 
> gpm, delta 70 degrees is the minimum required, more than the 144,000 
> BTU's the Baxi Luna provides. Baxi has a new Modula supplemental tank 
> that holds 21 gallons and serves as a kind of buffer for temporary high 
> demands. My homeowner is concerned this will not cut it. They are edging 
> towards having separate heat sources, such as a Noritz or an efficient 
> large tank for DHW.
>
> Here are my questions: how many gpm should we figure for hot water needs 
> at any one time, and how often might a family of four run into 
> insufficient hot water using the Baxi? Would you agree that providing 
> separate heat sources for radiant and DHW would be the best path?
>
> Thanks
>
> Seth Melchert
> Oakland CA
> _______________________________________________
> Greenbuilding email list
> List info: http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org
> List email: Greenbuilding at listserv.repp.org
> Managed by BuildingGreen, Inc. http://www.buildinggreen.com
>       publisher of Environmental Building News and GreenSpec(r)
> Hosted and archived by REPP / CREST http://www.crest.org
>
>
>