Sizing a Gas-Fired Combi Boiler
My house is 5 year old single story 2250 SQFT home with a Superior Wall insulated basement located in Warren Pa Zip Code 16365.
Currently my Heat/AC is provided with a Closed Loop Geothermal Heat pump (5 ton) connected to a mix of radiant tube and Forced Air Central Air system (See Attached Diagram). I have recently been given access to Lease Gas and want to install a Gas fired Combi unit (into my existing system) to provide my DHW and home heating, which will significantly reduce my electric bill.
Initially I was told to just install a Weil McLain Aqua Balance 150 and it would do everything I will need, but after reading a lot of the posts on here about sizing boilers and combi units I think an AB 150 is oversized and would have short cycle issues.
I do not have a Manual J for the home and have not been able to find anyone to perform one, but I did find a heat loss calculator on the web and was able to generate a Heat loss calculation (See Attached Document). Unfortunately my blower door test did not include the garage so theACH50 infiltration rate does not include the garage so that skews the Infiltration data on the Heat Loss calculation.
Blower Door Test results for living area are: CFM50 1658, ACH@50 = 2.38, ACH Nat = .1575 (I know I need to install an HRV to provide ventilation, which I am currently researching)
As I sifted through all of the data provided and reviewed all of the boiler sizing posts I am confused on a couple of points:
When sizing a combi unit how do you select a the proper size? Is it the Heating load or the DHW requirement that dictates the size? (An AB150 seem oversized)
Given my need for 5 GPM of DHW and the BTUh of heating given in the Heat loss calculations there doesn’t seem to be a combi unit that fits the requirement.
How does a units Turn Down capability fit into the equation?
How does the outdoor reset effect the decision?
Would it make more sense to install a dual use hot water heater (like an HTP Versa Hydro)?
Thank You for any info and guidance.
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