Dana,

Thank you for replying. I was secretly hoping you would. I spoke with the local tech and I feel a bit better knowing he has a buderus modcon in his home and said we can definitely work to tweak our Alpine to attempt to get it into the max efficiency range if possible.

Now to the task you assigned me. I read your article (thanks). Honestly, much of it was over my head and brought to light how much I don't know because I don't have a heat loss calculation for my home nor a fuel use load analysis (our propane tank also is for cooking and clothes dryer). Some of the article information I do not understand. But here goes, my system is an existing 1950s closed loop 1 zone system. I measured and totaled ALL the cast iron baseboards in our home for a total of 155 feet. Here is where I start to get lost. You said the Alpine150 puts out 28,500 btu per hour in condensing range BUT I don't know how to calculate the needed btu's for my home to be comfortable because cast iron btus change based on the water temp flowing through the system. How do I calculate my homes needed btus? I don't understand how you determine the houses seasonal heat load. I know my "crappy" crystal replacement windows have a ufactor 0f 0.49 and I have been attempting to rudimentarily calculate a heat load based on the outside design temp of 13 degrees versus an indoor design temp of 70 degrees with a delta t of 57. But after this my eyes start to cross.

I'll keep plugging to get the numbers but I would appreciate any hints or tricks to get them figured out faster. (Just assume every post is ending with thank you, much thanks, thanks so much...)

To be clear, I'm assuming that is 155 square feet EDR (equivalent direct radiation, using the

__methods found here__), rather than linear feet of diverse types of radiation or something else.

Assuming 155' EDR, at minimum modulation of 28,500 BTU/hr that becomes a ratio of 184 BTU/hr per square foot EDR. If you look at the nomograph on p.2 of that radiator sizing document you'll see that the radiators will emit that much only at an average water temperature of about 180F (which usually means a boiler output temperature of 190F, a return water temperature of 170F). If the boiler is set below that temperature there is more heat going into the radiators than is being emitted, which causes the boiler to cycle on/off.

To actually run at a bare 88-90% condensing efficiency the return water temperature needs to be about 125F, which happens when the boiler is putting out ~135F water, with an average water temperature of about 130F. Looking at that same nomograph at an AWT of 130F the radiators are emitting about 70BTU/hr per square foot EDR. So with 155' of rad you're looking at 70 x 155= 10,850 BTU/hr of heat being delivered to the house, but the boiler is putting about 28,500 BTU/hr into the system (at minimum fire). With 28,500 - 10,850= 17,650 BTU/hr of excess heat going into the water, the temperature rises, the boiler senses that it's temperature output is going higher than the set point (whether outdoor reset setpoint, or programmed fixed temperature), and it turns the burner off, then turns it back on after enough heat has been emitted by the radiators to cool the water back into range. So under those conditions during an extended call for heat from the thermostat the burner will cycle on/off, operating at a 10,850/28,500 (= 38%) duty cycle.

As long as there is enough water + iron thermal mass in those radiators and pipes to keep the burn times to at least ~3 minutes and off times of ~5 minutes (3/(3+5)= ~38%), there will be 7-8 burns per hour, which isn't great, but it's not terrible. When it's 10+ burns/hour and 2 minute burn times it's taking a toll on efficiency, and putting excessive wear & tear on the boiler.

So, to decide whether it's worth adding the outdoor reset sensor and programming the boiler to ramp the temperatures up or down as the outdoor temperature changes (aka "outdoor reset"), you need to do some experimenting. Program the output temperature to 125F, turn up the thermostat to 80F or something to ensure that it takes a long time for the house to come up to temperature. Then, using a stopwatch or a watch with a second hand, time the first 4-5 burn cycles, and off cycles, measured to the nearest 10 seconds. If it's looking like it'll be fewer than 10 total cycles per hour with the average burn time being 3 minutes or more it'll definitely be worth setting it up for outdoor reset.

Whether or not 155' EDR of radiator is enough to keep you warm at +13F even at the maximum output temperature of the boiler can be determined by fuel use. The error from other uses of propane such as hot water or cooking/laundry etc is offset by the error introduced by solar gain coming in your windows during daylight hours, and the overall magnitude of fuel use is small compared to what is used for heating during the winter months. So logging the exact fill-up dates and quantities, then downloading heating degree-day data from a nearby weather station (use base 65F data, which is the long standing standard for a 70F home) using 5th grade arithmetic you can calculate the gallons used per heating degree-day, then divide by 20 hours/day to come up with gallons per heating degree-hour. For purposes of illustration, let's assume that comes out to 0.0065 gallons per degree-hour.

Every gallon of propane has 91,600 BTU of source fuel energy. Assuming it was operating near the condensing zone most of the time, use 90% for the approximate efficiency, so out of each gallon you end up with 0.95 x 91,600 = ~87,000 BTU of heat going into house per gallon of fuel burned. So with 0.0065 gallons per degree-hour, that becomes 87,000 x 0.0065= 566 BTU per degree hour.

The presumptive heating/cooling balance point when using base 65F data is that there is zero heat needed when it's 65F outside. With a heating constant of 566 BTU per degree-hour, for every degree F below 65F would require putting that much heat into the house to keep it warm. If the the outside design temperature is 13F, that means you have 65F-13F= 52F heating degrees (not the 57F delta-T you would use for Manual-J or IBR type heat load calculations), and a design heat load of about 52F x 566BTU/F-hr= 29,432 BTU/hr.

With 155' EDR of radiator and a load of 29,492, which is a ratio of 29,492/155= 190 BTU per square foot EDR. If you refer to the nomograph in that radiator sizing document you'll see that you need an average water temp of about 195F for the radiators to emit that much. That would require an output temperature of about 205-210F, which is probably above the maximum operating temperature of the ALP150, so if it happens that those are the numbers, you'll either need more radiator, a different boiler, or fix up the house to lower the space heating load.

Assuming the walls and attic are already insulated, and all window are at least U0.6 or lower (a wood sashed single pane with a clear glass storm window is ~U0.5), blower door directed air sealing and insulating the foundation (in that order) are two of the biggest and most cost effective ways of reducing the space heating load.