Hello, my family and I recently bought a 1800 sq. ft. house in Minneapolis that was built in 1900. The house is currently heated by a Weil Mclain HE-6 cast iron gas boiler but it has not been well maintained and needs replacing. The plan is to replace it with a mod-con boiler with an indirect water heater this summer. The radiation in our house is via cast iron radiators.
In order to properly size the new boiler it seems to me that the most accurate way to do so is to use the equation mentioned many times on this forum that uses therms, HDD and the 99% outside design temp. If I have understood the previous posts correctly the formula is as follows:
BTU/hr = (therms used over a certain period * 100,000 / HDD over the same period / 24 hrs. * Estimated efficiency of current boiler) * (65 - 99% outside design temp)
If I plug in my numbers:
Therms (Sept '14 - Feb '15): 1183
HDD (Sept '14 - Feb '15): 5958
Estimated boiler efficiency: 75% (which is probably high)
99% outdoor design temp for Minneapolis: -11
47,157 BTU/Hr = (1183 * 100,000 / 5958 / 24 * .75) * (65 - (-11))
With this in mind, my questions are as follows:
1. Is my math above correct?
2. If so, can someone confirm that even if an indirect water heater is added to the mix, there is no need to increase the BTU/hr figure. If I understand correctly, unless it is a 99% outdoor day and someone needs a hot shower during the coldest part of the day, the boiler will be more than adequate. And, even in this extreme circumstance the indirect will be on a priority loop so it will ensure we have hot water even on the coldest days.
3. We are also hoping to insulate and add heating to our mud room/entry porch this summer which is 90 sq. ft. If so, how much would this addition affect our BTU/hr number. Assuming the space is insulated to a level that is at least on par with the rest of the heated space would the equation below be an approximation of the additional load?
Updated BTU/hr = (current sq. ft. + additional sq. ft.) / current sq. ft. * current BTU/Hr
49,515 BTU/hr = (1800 + 90) / 1800 * 47,157
If not, are there any recommendations as to how to calculate the additional load?
4. When searching for a new boiler, I should be looking at the DOE rating rather than the Max input or IBR rating? (assuming the boiler will be in a basement not exposed to outside air)
I think that sums all of my queries for now. Thank you in advance for any and all assistance.
In order to properly size the new boiler it seems to me that the most accurate way to do so is to use the equation mentioned many times on this forum that uses therms, HDD and the 99% outside design temp. If I have understood the previous posts correctly the formula is as follows:
BTU/hr = (therms used over a certain period * 100,000 / HDD over the same period / 24 hrs. * Estimated efficiency of current boiler) * (65 - 99% outside design temp)
If I plug in my numbers:
Therms (Sept '14 - Feb '15): 1183
HDD (Sept '14 - Feb '15): 5958
Estimated boiler efficiency: 75% (which is probably high)
99% outdoor design temp for Minneapolis: -11
47,157 BTU/Hr = (1183 * 100,000 / 5958 / 24 * .75) * (65 - (-11))
With this in mind, my questions are as follows:
1. Is my math above correct?
2. If so, can someone confirm that even if an indirect water heater is added to the mix, there is no need to increase the BTU/hr figure. If I understand correctly, unless it is a 99% outdoor day and someone needs a hot shower during the coldest part of the day, the boiler will be more than adequate. And, even in this extreme circumstance the indirect will be on a priority loop so it will ensure we have hot water even on the coldest days.
3. We are also hoping to insulate and add heating to our mud room/entry porch this summer which is 90 sq. ft. If so, how much would this addition affect our BTU/hr number. Assuming the space is insulated to a level that is at least on par with the rest of the heated space would the equation below be an approximation of the additional load?
Updated BTU/hr = (current sq. ft. + additional sq. ft.) / current sq. ft. * current BTU/Hr
49,515 BTU/hr = (1800 + 90) / 1800 * 47,157
If not, are there any recommendations as to how to calculate the additional load?
4. When searching for a new boiler, I should be looking at the DOE rating rather than the Max input or IBR rating? (assuming the boiler will be in a basement not exposed to outside air)
I think that sums all of my queries for now. Thank you in advance for any and all assistance.