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on another thread about sizing a boiler for the heating load of the house. I got some E-mails asking questions about it. I always sinze to the heat load. I feel it's especially important for a mod con boiler. I often find myself at odds with supply house designers who recommend to their clients the contractors, to size it for the domestic hot water load. I want to throw this open for discussion.
My reason for sizing to the heating load is that during the Winter the boiler is mostly heating. That's it's job 99% of the time. With a mod con it is especially nice to target the capacity of the boiler so it can modulate up and down rather than shutting on and off. You can acheive the highest efficiency that way. If your boiler has a nice tight throttling range that matches the house's heat loss you can maximise your efficiency. It's just like the difference between stop and go city driving vs. Higway mileage in a car. Low firing rates and long cycles saves the most money on your fuel bill.
Domestic hot water is another animal altogether. Usually in the morning everyone wants a shower. You want a lot of hot water all at once in a short time period. This is totally at odds with your heating needs.
What do you want? Lots of power and fast recovery, or long steady minimal gas firing? You can't have both with one appliance.
I run into this conflict a lot. A lot of row houses downtown have very small heat losses. I often see a need for only 40,00 btuh on my heat load calcs. The row houses are often being renovated by young uban professionals who want, along with a shorter commute to work, a ginormaous soaking tub with a gazillion jets to unwind in after a hard days work. I am also seeing multi head shower stalls that would make a car wash envious. You can't support that with a 60,00 mod con boiler. No way!
I always go for more storage. A larger indirect tank. A lot of my customers like the option and I make sure they inderstand why it is needed. Sometimes to save space We put in a small indirect and run the hot water through a tankless water heater. That way, if you start running out of hot water you have a back up. You get the high efficiency boiler water from the mod con boiler and the longevity of the tankless. There are now condensing tankless water heaters so you can do away with the storage tank altogether if you want, and not lose the high efficiency you crave.
Another way to go is to up the boiler size and install a buffer tank. That's waht I have in my house. In my case though the buffer tank also doubles as my solar thermal storage tank. It helps my boiler to keep from short cycling a lot. In the Spring and Fall the solar collectors fills the tank with hot water and the boiler gets to rest. Yaah!
I understand the supply house designers are doing their best to sell what they think is right but I think they should be required to get some field experience too. How can they be put in a position of resonsibility as important as design and engineering and not be tempted to just throw a big capacity boiler at a job to CTA?
Sorry I ran on so long. I hope I didnt bore you. Please share how you appraoch this matter. WW