sizing hydro air coil to know how many btu's it will add to boiler size

Joseph_4

I have a customer where I'm replacing his boiler. all of house till now are sunrads. but he is extending his house the first floor extension will be 33x20 as well as a basement ( to mirror those dimensions) He tells me he plans to use a 3 ton unit to cool both and a hydro air coil for the heating . According to my heat loss Calculation he needs on the heating side for the whole extension around 26000 btu .( I do not want to get into the particulars of that number for this discussion, windows, Low E glass, first floor versus basement etc..) In the past where I have done jobs to heat a Hydro Air coil, the Hvac guy tells me its a 3 ton unit and then says its a 15 x 15 , 15 x20, or 20 x 20 coil.. When I ask the Btu output these sizes are minimum 35000 btu. It seems they just fit whatever coil fits the dimension of the cooling unit. Its usually way larger than needed for the heating side in terms of the heating btu output. If I size my boiler to their coils the boiler will be much larger. Yet I have seen a 175k boiler do fine with 5 ac units a one 3 ton, a two 4 tons and a 5 ton But according to coil size should be like 250 or more. I have seen some past threads asking the same question but haven't seen any good answers.
I have asked one of the biggest installers for 30 years in my neighborhood of ac units and have worked on jobs with him and he cant give me a straight ans. I know that gpm and water temp can affect the btu output but when you look at manufacturer specs their minimum is always way higher in BTU's than is needed.. Plus on other systems where I haven't beedone I dont see people using lower water temps. They are generally 180 input on all coils and the customers are happy with it.. I'd love if someone can explain this anomaly.
I'd like to size this job the correct way.
Thanks
Joe

Zman

There is a little more to sizing fan coils that just the rated output. One the air side you need to know the CFM and design delta T. The water temp and flow rate also need to be considered.
The coil manufactures usually have a calculator like this to help http://www.coilcompany.com/.
Also keep in mind that coils can have any number of rows, the depth of the coil effects output as well.

Joseph_4

very interesting thx