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Boiler capacity vs. heat loss for small houses.

I have a question of boiler capacity vs. heatloss. I would love to install a condensing boiler for the efficiency, but I have a two unit duplex and the heating demand for worse case scenario (Portland, OR, 0 degree F) is not great - about 40,000 BTU/hr for a single unit (40MBH). The units are relatively small, about 850 sq. ft. each.

How close can I size the heating unit, relative to demand?

Most of the winter, the heating demand will be half 40 MBH; and if I ever institute an insulation upgrade, it will go even lower (only blow in insulation in the attic now).

I have been looking at the Viessmann Vitodens 100-W, due to its effiency, and also the fact that a DHW add kit can be coupled with it.

Problem is, this is their smallest unit and it is rated for 37-117MBH.

If I install a single unit, most of the winter, the heat demand will be below the minimum capacity of the unit, such that it would switch on and off.

What is the conventional wisdom of specifying such a unit relative to the heatloss, such that you are getting enough runtime that the boiler will actually condense? Of course the temperature deadband could be quite large, but that defeats the purpose of having a constant temperature and therefore comfort.

I would love to just install one unit, and have a zone for each, but these are rentals, and it is just 'cleaner' to have separate billing for each address.

Thanks
Keith Walker

Comments

  • Ironman
    Ironman Member Posts: 6,868
    edited February 2015
    Select the boiler(s) to match the heat loss as closely as possible to the calculated loss at design temp. The boiler will do the rest: it will modulate as the demand varies.

    If you're looking for a combi, it's input will be higher as it takes more btu's to directly heat the domestic load.

    I think I would be looking at two small combi's in your situation. One may not cover the domestic load.

    What type of emitters do you have? If they're high temp, take a look at the Dunkirk/utica CUB combi: wall hung, 85%, modulating.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • icesailor
    icesailor Member Posts: 7,265
    edited February 2015
    You need to correct your winter low temperature. It is way too low. The average "low" temperature historically in Portland Oregon is about 35 degrees. Not zero (-0).

    Whatever heat loss program you used, they all use zero (-0F) as a calculating factor for all tables. At the end, you correct for the actual outside temperature.

    One thing to be very aware of if you do use any kind of sidewall vented appliance is that there is a lot of snow in Portland Oregon. Therefore, you must have the vent termination a minimum of 12" (or .33 meters) above that listed snow line. A "Average" snow fall in Portland, Oregon might be 11" historically. The vent termination needs to be 23" above the ground to be legal.

    http://www.city-data.com/city/Portland-Oregon.html
  • keithwwalker
    keithwwalker Member Posts: 6
    Thanks for the feedback.
    fwiw, I used 0 F as a design temperature as that is the historic low, and I would think that you want to cover heat load.
    icesailor is correct that on average, the low temperatures are much higher. On average it doesn't get below freezing:

    http://en.wikipedia.org/wiki/Portland,_Oregon#Climate
    http://www.wrh.noaa.gov/pqr/pdxclimate/pg5.pdf

    Please note that Portland rarely gets snow (4 inches per year on average), again look at the climate data. In terms of accumulation, the snow doesn't pile up, it melts.

    http://www.wrh.noaa.gov/pqr/pdxclimate/pg103.pdf
    http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?orport

    Of course my location is close to sea level, there are other higher elevations where there could be snow. Good point on the stack height - I did the same for my gas tankless heater install.

    Anyway, still looking for references for sizing relative to average load.
  • keithwwalker
    keithwwalker Member Posts: 6
    My other concern is boiler efficiency while modulating at low heat inputs. Does any manufacturer publish that data?
  • Gordy
    Gordy Member Posts: 9,514
    How often do you reach that low? You only reach design 2% of the heating season. Oversizing for a 5-10 year below design condition cost money in the over sizing of the boiler.

    Another thing to keep in mind even design temps seldom last a 24 hour duration. The homes thermal mass carries you through a longer period. Most load calc programs are padded 15-20%.
    Jean-David Beyer
  • icesailor
    icesailor Member Posts: 7,265
    Zero is way too low. No matter how you understand it.
  • Jason_13
    Jason_13 Member Posts: 299
    I like the Burnham Alpine. The ALP80 goes down to 16k input so the output would be a bit lower. You can turn the high side down so instead of 80K input you can de-rate the fan to 40K leaving the DHW at 80K or less if doing an IWH. They also have a 2 minute low fire hold on start-up.
    Secondly the vent can go vertically outside the building stated in the I&O manual if you need to gain some height.
  • WillieJ
    WillieJ Member Posts: 15
    icesailor said:

    You need to correct your winter low temperature. It is way too low. The average "low" temperature historically in Portland Oregon is about 35 degrees. Not zero (-0).



    http://www.city-data.com/city/Portland-Oregon.html

    Your outdoor design temp for Portland is around 22* F. At least across the river in Vancouver it is 22* F. See the link that follows. I'm a few hundred miles north of you in Bellingham, WA. We get Fraser River outflow winds (we call it a Northeaster here) which are similar to the Columbia River outflow winds through the Gorge. Our design temp here is 19*F.

    http://app.leg.wa.gov/WAC/default.aspx?cite=51-11C-80100

    You might consider the Lochinvar Whn 055. It might meet your size needs well with an indirect, but I defer to the pros here for that kind of advice.
  • Gordy
    Gordy Member Posts: 9,514
    Just to give you an idea in my area -4 is design. Not uncommon to get -10 each winter for a night or two. Historically it's been -27 below. I have seen it get that cold, and in the upper -20s that's in a 50 year period those -20s came, and went pretty quickly. Basically you would be a little lower than set point if the boiler was perfectly matched. And that's only if you knock off the 10-20% built into the heat loss program.
    Ironmanicesailor
  • Ironman
    Ironman Member Posts: 6,868
    I'm in full agreement with what Ice and Gordy have said. You should use the design temp stated in the heat loss, not the historical low.

    Where I live the manual gives me two options: one that covers 97% of expected low temps (17*) and one that covers 99% (14*). I use 9* because those are in town numbers and a lot of the jobs I do are in the outlying mountain areas which get about 5* colder.

    But, design temp only happens on the coldest NIGHT of the year and it always gets warmer the following day. At any other temp, the equipment is over-sized.

    I've seen it get -14* here, but I would never use that number to design. You use the design temp number for your locale.

    The federal government has done more than one study that proved the average home has a heating system that's 2&1/2 times the size that's needed. That wasteful, inefficient and actually reduces the comfort of the home.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Paul48
    Paul48 Member Posts: 4,469
    Lochinvar Cadet CDN 040?
  • icesailor
    icesailor Member Posts: 7,265
    Ironman said:

    I'm in full agreement with what Ice and Gordy have said. You should use the design temp stated in the heat loss, not the historical low.

    Where I live the manual gives me two options: one that covers 97% of expected low temps (17*) and one that covers 99% (14*). I use 9* because those are in town numbers and a lot of the jobs I do are in the outlying mountain areas which get about 5* colder.

    But, design temp only happens on the coldest NIGHT of the year and it always gets warmer the following day. At any other temp, the equipment is over-sized.

    I've seen it get -14* here, but I would never use that number to design. You use the design temp number for your locale.

    The federal government has done more than one study that proved the average home has a heating system that's 2&1/2 times the size that's needed. That wasteful, inefficient and actually reduces the comfort of the home.

    That's true. My new car has a 2.0 liter 4 cylinder, turbocharged engine. It carts my @$$ around just fine. But if I need to get on the turnpike, and some driver doesn't want me to be in front of him, a gentle nudge on the Turbo resolves any issues over power. When that Steptronic 8 speed transmission gets the revs up so you have maximum turbo boost, the performance is truly stunning.

    Or, why I wish that MOD/CON's would modulate UP. Like they do down.

    You need nuts.

  • Steve Minnich
    Steve Minnich Member Posts: 2,756
    I agree with Ice, Gordy, and Ironman in terms of not adding any more fudge factor to the already present fudge factor. Do the math using the correct parameters and then trust the math.
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
    RobG
  • Gordy
    Gordy Member Posts: 9,514
    As Paul suggested IF your math is good with the NEW design day, The CDN 40 is big enough
  • keithwwalker
    keithwwalker Member Posts: 6
    I went back to my heat loss calculation and using the higher design day temperature of 23F, the heat loss is now only 30MBH (30,000 BTU/hr).

    I agree that the Lochinvar CDN40 units is sized well for my application, but other comments on this forum and others have not been too complimentary about the heat exchanger longevity.

    Are there any other mod/con units out there that have a lower MBH ratings, but good quality?

    There are really only two cold months for Portland, the rest of the time it is relatively mild - the Viessmann 100 unit modulates down to 37MBH. It still seems too large to condense, in the more mild months.

    fwiw, regarding Ironman's statement "The federal government has done more than one study that proved the average home has a heating system that's 2&1/2 times the size that's needed. That wasteful, inefficient and actually reduces the comfort of the home."

    That describes my duplex. The unit I live in has a 75MBH unit, and the other unit has 85MBH(!). They are both forced air units, that are way too loud, and cycle too much. Both oversized at least 2.5x

    Anyway, I am looking to have concentric intake/exhaust, as it would be easiest, as there are already is a stack chase for the old forced air units. The intake/exhaust would shoot straight up, no bends needed. The units are within 10 feet of the bath and kitchen faucets, so hot water runs are incredibly short as well.

  • SWEI
    SWEI Member Posts: 7,356
    There is no direct competition (yet) for the CDN040 that I am aware of. Nothing wrong with the HX as long as it's installed and maintained per the manual. The Lochinvar WHN055 modulates down to 11k, but costs ~50% more than the CDN040. It also has more control options (multiple temps, etc.)

    Dunkirk has a 50k that modulates down to 10k - not sure of the price on that one.
    Gordy
  • Rich_49
    Rich_49 Member Posts: 2,692
    HTP EFT55 (Elite Fire Tube) will get down to 11K . Not quite 9K but very nice . Has the firetube HX as opposed to Giannoni . Dave Davis from HTP says they will have an 80K firetube that will modulate down to 8k quite soon . There is one option if the HX design is a deal breaker or just a consideration .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Gordy
    Gordy Member Posts: 9,514
    Looking forward to that 10:1. Now get it into a 55k
  • Rich_49
    Rich_49 Member Posts: 2,692
    That would be nice huh ? I guess flame stabilization is more difficult than we know . Gordy , did you watch that video of Dave from ACH that I posted ? They have come up with some real innovative stuff for us to use soon .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • SWEI
    SWEI Member Posts: 7,356
    From what I hear, flame stabilization and air control both present challenges at high turndowns.
    icesailor
  • icesailor
    icesailor Member Posts: 7,265
    As anyone using their outside gas grill in a windstorm can attest to. The wind just blows the flame out on low fire.
  • Gordy
    Gordy Member Posts: 9,514
    Rich yeah I did it's exciting. Wanna know what I got out of it most? They are listening. That says ALOT!
  • Rich_49
    Rich_49 Member Posts: 2,692
    They seem to be the only ones from where I sit . Everyone else is just advancing the SOS .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • keithwwalker
    keithwwalker Member Posts: 6
    edited February 2015
    Regarding smaller units, I checked on Viessmann's UK site, and they have a Vitodens 50-W unit. It seems to have the upper limit of the 100-W, but can modulate down to 22MBH, still not optimal, and not sold here in the USA.

    http://www.viessmann.co.uk/content/dam/internet_uk/Brochures/Vitodens%20050-W%20Web%2002-2014_GB.pdf

    It's been a while since I checked the Rinnai site, but they seem to be importing a european boiler (forget the parent manufacturer) that is rated at 50MBH, and can throttle down to 13MBH; and comes with the circulator:
    rinnai.us/documentation/downloads/E50c_sp.pdfhttp://
  • SWEI
    SWEI Member Posts: 7,356
    ATAG is the OEM for the Rinnai boilers. Nicely built, and very easy to tear down. If they had a Q50 with the variable speed pump that modulated down to 10k...
  • John Mills_5
    John Mills_5 Member Posts: 935
    I can give you a suggestion on a local contractor you might want to chat with. Sky Heating.
  • keithwwalker
    keithwwalker Member Posts: 6
    Thanks John for the reference. I am still in the planning phase, wanting to nail down heat loads and emitter design. I will continue to analyze the part load operation, as the balance of winter in Portland is not very cold.

    fwiw, I am not doing this for any expected payoff, but more for the floor space that will be opened up, by converting to a wall unit (reduced clearance to non/combustables), noise/allergen reduction, and comfort. The conversion will eliminate the existing burner room/ducting, and the associated closet with an electric water heater.

    This will open up 50 sq.ft. in each unit; not a great amount, but it will be noticeable as each unit is only 850 sq.ft. and the liveable space increase will be appreciable.