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Old 11-17-11, 10:02 AM   #1
Daox
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Default Calculating / sizing a replacement boiler/furnace

Orange's post on using a condensing tank water heater for heating made me start thinking about my own setup and how to go about sizing the boiler (or water heater or furnace) that I will someday install as the backup to my soon to be solar hot water setup.

So, what is the best way to go about sizing a new heating appliance? Thankfully for me, I have three years of data to look back on. An alternative would be to get the utility info from the utility company for previous owner's usage.



In 2009 I had my cousin's family in the house so there was someone at home all the time. Since then, we've been able to reduce the temperature during the day and at night which you can see has led to significant savings. So, it looks like I'm using 4 to 4.5 therms of gas per day. That is 450,000 BTU per day, or 18,750 BTU/hr. We'll stick with BTU/hr since that is how heating appliances are rated.

So, I know I need 18,750 BTU/hr on average. There will be times where I know I need more. For instance, if this boiler is to support domestic hot water it'll need to do that. Or, if its a real cold day out it'll need to have a bit more capacity for that. At this point all I know is on average I need 18,750 BTU/hr.

To throw another thing into the mix, I'll also soon be installing a solar hot water setup which will assist with DHW and space heating. This system is estimated to bring in about 100,000 BTU per day in winter, or about 4,000 BTU/hr. Obviously, since it is solar, the output will vary from day to day.

Also, I still have some major renovations to do. My entire upstairs and still some of my downstairs doesn't even have insulation in the walls! So, when the renovations do happen to fix that, I'll have way less heat loss as I plan to not only install insulation, but also thicken the walls to really get some good insulation. So, in the future, I'll need MUCH less heat than I do right now.

Up until now, we've just looked at the worst case scenario. The majority of winter will require much less heat than this. As we get away from the worst months of winter it will not only be warmer, but the solar panels will provide more and more heat until the gas backup will not be needed at all.

So, with all of that I can say on average, I'll still need 18,750 BTU/hr for the worst of winter. However, there will be LOTS of variation and in the future I will need a fair amount less. So, I'm looking for a boiler that is capable of modulating down well below 18,750 BTU/hr (the lower the better), and still capable of putting out more than 18,750 when needed. How much more I am unsure, but I don't think it'll be much. If all else fails, I do have an on demand electric water heater that can kick up the temperature easily enough. So, I don't think I should need a boiler with too much more than say... 25k BTU/hr?

Looking at boilers, I have found that it is very rare to find boilers that put out heat in this range. Munchkin boilers have a 50k btu/hr unit that modulates down to 18k, or a 80k unit that modulates down to 14k. It would be great if I could find something that modulates down to 5k or even 10k. However, I have been unable to find something like this. I find this a little crazy as my house isn't even that efficient!

So, I'm looking for ideas, things I missed, any helpful info really. I don't think I covered everything here, but its a good start for me and anyone else looking to do the same/similar thing.

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