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Old 11-17-11, 10:02 AM   #1
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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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Old 11-19-11, 11:08 AM   #2
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Your peak heat load is not on the worse month, it is on the worst part of the coldest night. Basically when the temperature is the lowest that it you expect it to be. It is the 99% heating load temperature. You can use a search engine and find the temperature for your region. I live where the 99% temp is -20f. Your average is not going to tell you anything if it is based on the average for the month and sun is factored in. At night you don't have sun and if you wake up and the sun isn't up yet you might be cold. The average for January where I live was 11f but the coldest night was -20f and we had about 10 nights where it was in the negative single digits getting close to -10f. If your average for a month is 18750BTU/hr you might get a ballpark figure for finding the weather temperature average for the month and doing some monkey math to the coldest temperature you would expect by the difference in temperature from average to the coldest expected. If your average is 18750/hr be sure you factored in the efficiency of your current equipment versus the new equipment too.

The way I calculated the load with my house is I monitored the run time for the 9 hour period where the temperature was in the -20f region last year on January 21st. I basically paid attention to the forecast for the coldest days and logged the on-off of the call for heat which is very close to the time the gas valve is operating and then did the math for the duration it was on divided by the total amount of time I was logging and multiplied that duty cycle percentage against the output rating of my furnace. That day I calculated 27700 BTU/hr would be required. There was no sun at the time so that was no factor and I had to play with the numbers a little bit as it was -15f for part of the calculation and my house wasn't quite 72 degrees which is what you are supposed to calculate to but once I did the math, I realized that the smallest condensing natural gas furnace I can buy is 40k BTU/hr furnace. Later on a -10f night I raised the house to 80 and calculated then too and the numbers were higher but nothing a 40k BTU/hr furnace couldn't handle. Keep in mind though that my results would be different than yours because I've been insulating and air sealing to get my numbers and my 1985 house wasn't too bad on insulation to begin with, the air sealing made a big difference though.

The professionals size using an ACCA Manual J calculation. There are some heat load calculators on the net but none that I've really liked much so far. Some of the ones that I've had pros recommend to me that are accessible to non-pros. Of course you can download the spreadsheet from here but the details on how to properly fill it out are in the book that is available to $74.95 for mortals and $52.49 for those in the anti-DIY club. I've considered buying the book, then found it was checked in at the central Minneapolis library but I went there and apparently it may have been stolen or maliciously moved to a location it shouldn't be by an HVAC contractor.

Spreadsheets
https://www.acca.org/industry/system-design/speedsheets
Instructions
https://www.acca.org/store/product.php?pid=30

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Old 11-19-11, 11:12 AM   #3
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Could you just have a couple HVAC guys give you "estimates" and have them all do a manual J? That's what I would do. Then if you feel up to it you could install the boiler yourself.
You might want to think about a direct vent combi unit that provides both space heating hot water and water for your showers and so on separately. This way you eliminate the need for a hot water storage tank.
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Old 11-19-11, 11:37 AM   #4
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I guess thats what my post was mostly about. I know my worst heat load is under 50k btu as the house is today. I'm just having trouble finding a unit that is properly sized for the house, and the heat loss is only going to be improved upon while I live here. Does anyone know of any smaller sized boilers? If not, then I buy the 50 or 80k units and I know I have plenty of headroom and also know that I won't find anything more efficient.
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Old 11-19-11, 11:52 AM   #5
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Quote:
Originally Posted by MN Renovator View Post
Your peak heat load is not on the worse month, it is on the worst part of the coldest night...

The way I calculated the load with my house is I monitored the run time for the 9 hour period where the temperature was in the -20f region last year on January 21st. I basically paid attention to the forecast for the coldest days and logged the on-off of the call for heat which is very close to the time the gas valve is operating and then did the math for the duration it was on divided by the total amount of time I was logging and multiplied that duty cycle percentage against the output rating of my furnace. That day I calculated 27700 BTU/hr would be required....
This is really a great approach, very well explained, too.

I used a method that is similar, in that I live in a small house, and I got enough electric heaters at various thrift stores, to heat the house and bought a Kill-a-Watt for each heater. Then I regularly monitored (4-hr intervals) the electric consumption of the heaters during one of our 99% cold snaps. The period that had the largest summation of energy use was what I used for the max heat load.

I also tried several Manual-J based programs, and found that they came close to predicting what I measured, but the differences calculated by the programs was not a confidence builder.

There seems to be a rule of thumb in the heating trade, to do a Manual-J calculation and then increase the result by 50% when you select the furnace or boiler. In fact, one program I tried had the 50% factor built in.

This 50% increase thing works OK when you use fossil fuel for heating (it certainly reduces 'call-backs'), but for sizing a heat pump it is not so good. Heat pumps are best sized slightly smaller than the maximum, and an axillary fuel is used to fill that gap. That method will result in the greatest efficiency and economy. So, for heat pump sizing, greater heat load precision is called for, and from my experience, the greater precision is had by actual measurement.

Thanks again for you clear approach.

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Old 11-19-11, 12:35 PM   #6
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50% over the manual J? I don't understand that at all, that is far too much over. Usually you size to the manual J which already has a small amount of fudge factor built in, albeit a small one. If the manual J is done right it should be accurate enough to use the same size or if it is right on a size, say a 50k output furnace has a man. J of 48k, one size up would be fine if an owner insisted on a setback. I think setbacks are great but I'd rather have a 30k unit and then just lose the setback when I'm within 10 degrees of my design temp which is less than a week out of the winter.

With a heat pump you usually have to size for your air conditioning load at a minimum so you are covered there and if your heat load is reasonable you'd be set. In my case natural gas is so cheap that the balance point for a heat pump would leave me with a 24k air conditioner because that is what my house needs for 78 degrees inside at 95 degrees outside(experience with my 23k at 95 outside) and once the temperature gets low enough to where the heat pump can't keep up and is defrosting often I would be on natural gas anyway. Problem is that it that it is 30 degrees out right now and my furnace has would have run so little if I wanted things at 70 degrees that if I wanted the increased efficiency of a heat pump the savings are limited for me and the cost would never be made up unless natural gas prices changed significantly for my area. If I was running off of oil or propane I'd be all over heat pump. Of course I'm talking about the central air heat pumps including the Carrier Greenspeed aren't worth it not to mention its extreme cost, an mini-split inverter heat pump like the 12k Fujitsu would be a different story as the A/C and heating savings might make a bit more sense and the unit cost is small and I'm not running the expense of swapping the current A/C and heat equipment to go that route.

...ok, I just realized I'm off topic to the boiler discussion. How are you sure that your under 50k as a worst heat load. If you are 18750BTU/hr on the worst month and that is a number reduced by sun through the windows and warmer days, have you done any measuring or calculating to tell you that you are under 50k? My January was 76therms with the house at 50 degrees outside of load testing because that is the temperature I'm comfy at with pants and a hoodie, add a blanket from time to time. Average temperature was 11.4f so 50 degrees - 11.4 = 38.6 degrees of rise. If it was 72, that would be 60.6 degrees rise and about 57% more fuel on paper to 119 therms $117 versus $63 for 76 therms. ...scary comparison, I haven't done the math to calculate the savings yet and didn't figure I was saving that much.

My point 119 therms = 11900000 BTU/730 hrs(month)=16301BTU/hr. In reality it was 7600000/730=10410BTU/hr. Similar climates but probably different sun exposures so the numbers aren't as comparable as I'd like to think they'd be. Plus the numbers might not be as linear as multiplying the temperature differences otherwise 18750/16301=114% * 27700 BTU/hr = 31578. ...I wouldn't bank on that number, too much fudge. I suppose for a heat pump calc if I used 50 degrees further into the future and my average was 10410, then a Fujitsu 12k mini-split shouldn't have much of an issue from the time the sun shines on it from the southwest in the winter mornings until it starts to get cold at night for most nights. Most heating happening when the sun is down and its cold though, which is when you have no solar water heating or solar window heating benefits but if I had a setback at night and let the heat pump recover most of it during the day, that might help. ...again sorry I'm blabbing off topic about heat pumps. Maybe it could apply though, what are your electric and natural gas rates? Mine are 75 cents up to 110 therms and electric is $.11 in the non-A/C months.
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Old 11-19-11, 05:05 PM   #7
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I am not sure that I'm really under 50k as a worst case. I am fairly confident I'll be there by next winter though. I probably won't be purchasing the boiler this winter as only one room in the house currently uses hydronic heat. I'm just trying to cover all the bases when considering sizing a heating system (thanks for the help guys!), and to get an idea of what is out there product wise.

I also totally negated to mention that I will still have my gas forced air furnace. I just don't plan on using it much at all.

I will definitely try your technique of monitoring furnace duty cycle during a cold snap though. That is a great idea.
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Last edited by Daox; 11-19-11 at 05:08 PM..
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Old 11-19-11, 05:47 PM   #8
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Are you planning on heating the rest of the house with water after some of the renovations you've mentioned? Personally I like radiant heat, baseboards or floors, because it's much more peaceful (biggest downfall with minisplits as I see it). But it's also nice to have the duct work in place for an HRV or central air.
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Old 11-19-11, 06:15 PM   #9
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50% over the manual J? I don't understand that at all, that is far too much over.
I didn't intend to recommend such a practice, I just noticed from lurking on some HVAC blogs that plenty of tradesmen were recommending the practice to avoid callbacks.

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Old 11-20-11, 07:50 AM   #10
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Are you planning on heating the rest of the house with water after some of the renovations you've mentioned? Personally I like radiant heat, baseboards or floors, because it's much more peaceful (biggest downfall with minisplits as I see it). But it's also nice to have the duct work in place for an HRV or central air.
Yes, it will eventually all be hydronic floors. Its just going to take some time to get there.

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