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Cast Iron Oil Boiler to ModCon LP, 2.5x fuel usage increase

glifen
glifen Member Posts: 22
Recently converted from a 25+ year old cast iron oil boiler (last checked was operating at 78% efficiency) with separate oil fired hot water tank to a Navien NHB-80 with indirect hot water tank.

Last December on old boiler we used about 100 gallons of oil, this December we used about 250 gallons of LP (2000sqft log house with quite a bit of glass). The propane usage seems a bit out of control and I have no faith in the company that installed the new system.

The one big change we did make is with the oil boiler we set the heat back to 62 at night and during the day, with the new system we keep the heat at a constant 68. All of the reading I did has pointed towards set backs with a mod con are counter productive (even though we would prefer the colder temps at night...).

Aside from short cycling, what else would be the cause of the big increase in fuel usage? I have the heating people coming in on Friday to take a look and would like to be able to provide them as much direction as possible as.

Also, if anyone knows any experienced technicians in the Albany, NY area, please let me know!
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Comments

  • John Mills_5
    John Mills_5 Member Posts: 935
    What type of radiation?
  • glifen
    glifen Member Posts: 22
    Baseboard, one zone is about ~90ft, the other is about 20ft (upstairs, doesn't appear to be called often)
  • gschallert
    gschallert Member Posts: 170
    glifen said:

    Baseboard, one zone is about ~90ft, the other is about 20ft (upstairs, doesn't appear to be called often)

    the 110 feet of baseboard is all finned? don't count pipe that doesn't have fins in your emitter output.

    what was the heat loss for your house calculated to be? if it's up near your emitter capacity then well, that's not the best situation for a mod-con in my experience.

    also don't look at oil v LP in gallons, that's apples to oranges. You need to convert each fuel to BTU per gallon to compare. not 100% sure but I thought 1 gallon of LP was about 2/3 the BTU of 1 gallon of oil. So right out of the gate you're getting less BTU's per gallon and are going to use more fuel unless you can lower BTU usage with new equipment.
  • gschallert
    gschallert Member Posts: 170
    also you might want to provide more info like

    Outdoor reset enabled? if so what curve?

    central heat supply water temp

    central heat return water temp

    You can get all that info yourself, here's the manual (page 9)
    http://us.navien.com/__DATA/ProductDocument/2014/12/11/[1]_NHB-Users-Information-Manual-English.pdf

  • glifen
    glifen Member Posts: 22
    Thanks, I'll check on that information when I get home.

    The 110ft of baseboard is all finned. Heat loss I believe was calculated to be about 55,000 which is close to the max output of the baseboard.

    For Oil vs Propane...
    Old boiler 78% efficiency - 101,000 btus/unit
    modcon 90% efficiency - 85,000 btu/unit

    Assuming I am hitting 90+ efficiency on the modcon, I should only see an increase of ~15% in fuel usage, not 250% unless I am missing something.

    that doesn't take into count the efficiency gain from going to oil fired DHW to indirect propane either
  • NY_Rob
    NY_Rob Member Posts: 1,370
    If you list the four outdoor reset curve variables the installer (hopefully) set- it will help, because the out-of-the-box settings are probably good for Alaska in winter... maybe, but are overkill for all other areas and waste energy.

    Outdoor high temp?
    Outdoor low temp?
    Supply water high temp?
    Supply water low temp?


    On another note... Consider combining zones, which is very simple to do... and control the upstairs temperatures with the built in registers on the baseboard runs. That will end your short cycling on the 20ft zone, it will also let you lower the supply water temp even more for better efficiency (more time in condensing mode).

  • Paul48
    Paul48 Member Posts: 4,492
    Here's another way of looking at it.....what happened to the 60% more btus you produced? I'm in So. CT, and I know last Christmas we were outside in tee shirts.
    kcopp
  • Henry
    Henry Member Posts: 979
    There are 36.39 cu Ft of proane in 1 gallon of LP. There are 2,500 BTU per cu FT. I gallon of LP is equal to 90,975 BTU. One gallon of #2 oil is 140,000 BTU. 100 gallons of oil= 14,000,000 BTU. 250 gallons of LP = 22,743750 BTU. You are using just a touch over 50% more which should not be the case. There is a design and probably a control issue. In my own home, night setback made no difference. I do not do night setback. As once you heat the mass, you only have to make up the loss instaed of using more energy to bring the mass up to temperature.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Paul48 brings up a good point.... on Long Island this December the average daily highs and lows were almost exactly 10 deg colder than Dec 2015 temps which were extremely mild (these figures are listed on my monthly gas bill for comparison).
    10 deg lower highs and 10 deg lower lows makes a huge difference over 30 days time as far as heating your house are concerned.
    With that in mind- with my new mod-con this Dec I actually used two therms (I pay 77cents/therm + a few cents additional for delivery) more gas than I did in Dec 2015 with my 1960's cast iron boiler just because this Dec was so much colder then the previous Dec. Now that Jan billing is done- and this Jan and Jan 2016 average daily high/low temps are much closer (within 1 deg of each-other) and are much better for apples to apples comparison ... I used 30% less gas with the mod-con vs. the cast iron boiler.

    Did you take outdoor temp differences from Dec 2015 and Dec 2016 into account?
  • njtommy
    njtommy Member Posts: 1,105
    I hope they set up the combustion correctly and dialed in your odr curve to maximize your efficiency. I can't see it short cycling with an 8k bottom end but it could be possible for your smaller zone.
  • lchmb
    lchmb Member Posts: 2,996
    just curious when was this system installed and how had usage been in previous months?
  • glifen
    glifen Member Posts: 22
    edited January 2017
    System was installed in August and its been nothing but issues from this company, you can look at my post history to see the first post of the nightmare I went through..

    Also had an issue with the DHW when it was first installed, it would cycle on and off for the hot water call every couple of minutes..

    Here's the full read out of the info display:
    A. 151 - space heating supply temp
    B. 124 - space heating return temp
    C.---
    D.---
    E.---
    F. 28 - outdoor temp
    G. 1 - ODR reset curve - 1 = finned tube baseboard
    H. 0 - boost interval time
    I. 23.9 - water pressure
    J. 30 - DHW priority time
    K. 32 - system supply water temp
    L. 32 - system return water temp


    I also sat and watched the unit for a bit, heres the results:
    5:22 off
    5:30 on
    5:37 off
    5:46 on

    this is for zone 1, zone 2 was never called during that window

    I did notice when the unit cycled off, supply water temp would hit about 160, and then over the next 7-8 minutes it would drop down to 130 and then kick on, watched that happen twice.

    *edited to better format + label chart*
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,810
    With all the checking, check to make sure you're not leaking propane out a valve or fitting-especially outside.
    steve
  • njtommy
    njtommy Member Posts: 1,105
    edited January 2017
    J should be current firing rate I believe. The manual is a little unclear when scrolling through the settings vs changing set point.
  • njtommy
    njtommy Member Posts: 1,105
    Also check burner off set point.
    What type of pumps are you running?
  • Paul48
    Paul48 Member Posts: 4,492
    He said Albany NY Hat.
  • kcopp
    kcopp Member Posts: 3,657
    He said he was near Albany, NY
  • NY_Rob
    NY_Rob Member Posts: 1,370
    A SWT setpoint of 160F at 28F outside temp means they probably used the factory ODR curve for baseboard.

    If you check the install manual the default settings for baseboard are:
    70F Outdoor high temp
    14F Outdoor low temp
    180F Supply water high temp
    120F Supply water low temp

    If you use those numbers in Excel you'll see you're not condensing any time the temp goes below 42F outdoors... and that is just the beginning of the condensing range (assuming a DT of 20F between supply and return temps). To get real condensing efficiency you'd like the return water to to be 120F or lower which means SWT of 140F which on the factory curve doesn't happen till it's 52F outdoors.

    You're not condensing often in chilly Albany during the winter using the default ODR curve.
    njtommy
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Which points out the necessity of taking into account actual outside temperatures... you have to base fuel usage comparisons on total degree days.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • gschallert
    gschallert Member Posts: 170
    glifen said:

    Assuming I am hitting 90+ efficiency on the modcon, I should only see an increase of ~15% in fuel usage, not 250% unless I am missing something.

    I doubt you're condensing enough to be getting 90% efficiency on avg. You're still getting the advantage of the burner modulation but if your radiation capacity doesn't exceed your heat loss by enough to run with low enough water temps that condensing efficiency is lost.

    I second Rob_NY in getting rid of that short zone and having the whole system run based on outdoor reset.

    If you want to learn more about condensing boilers, how they work and the best applications you should check out this blog post: http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/sizing-modulating-condensing-boiler

    To find Navien service specialists try their contractor locator service by using your zip and radius. I don't know if the company you used will be listed but maybe it will give you some alternatives.
    http://us.navien.com/nssfinder/
  • Brewbeer
    Brewbeer Member Posts: 612
    edited January 2017
    ODR curve is definitely not set up correctly. At 28F with 90 feet of baseboard, the boiler should be running constantly at low modulation maintaining a constant (within a few degrees) supply water temp. The 8 minutes on/8 minutes off cycling is reducing efficiency, and causing excessive wear on the boiler. My mod con will run all day long (no cycling) between 20-35% fire with an outdoor temp of 28F.

    The boiler is only achieving 95% efficiency if it is at low fire and return water temps are low (~100F). While your boiler is a little different, take a look at this efficiency graph to get an idea how boiler efficiency varies as a function of return water temp and firing rate:
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • glifen
    glifen Member Posts: 22
    Thanks everyone for the great information.

    I originally asked to have the 2nd floor zone added to the first, but the installers talked me out of it.

    I've checked all propane connections multiple times and have never smelled any gas so I do not think there is a leak.

    Hatterasguy, I noticed your calculations don't take into account the different in efficiency, wouldn't that skew the numbers by an additional 10-20% or so (assuming proper operation and efficiency gain of going from oil fired DHW to indirect)?

    The old boiler was serviced a few months before it was replaced and the tech rated it at 78% efficency.


    I had a question on these two settings as well:
    K. 32 - system supply water temp
    L. 32 - system return water temp

    I can't find any good information in the manual on those. Is it normal for those to be set to the same and to be set so low?


    For the ODR curve, is it something I should try configuring myself? My knowledge on this stuff is minimal and my research isn't producing much of a return.

    If I understand most of these comments.. I should be aiming to get the boiler to be running constantly at a low temp with the return temp under 130 degrees (as close to 80 degrees as possible?). And even if I do get the configuration down, I probably won't see a decent savings in fuel usage?




  • njtommy
    njtommy Member Posts: 1,105
    K and L are optional supply and return sensors that get attached to the piping.
  • glifen
    glifen Member Posts: 22
    I know this is probably a loaded question but..

    how do go about starting to calculate SWT requirements? Even just ballparking it. I'm researching it now but there's a lot to dig through.

    Assuming the baseboard needs 160F+ SWT resulting in no efficiency gains, is it still normal in this circumstance for the unit to cycle on/off every 8 minutes or so? If not, what is most likely the cause of the unit cycling so often?

    Thank you for all the help, much appreciated!
  • Hilly
    Hilly Member Posts: 412
    Trial and error would say keep turning down the swt until you find you are no longer comfortable. At you coldest day of the year design termperature the boiler should never cycling. 8on/8off says you can likely reduce the temperature. You need to find a balance of longer cycle times and the rooms being able to achieve and maintain setpoint temperatures.
    njtommy
  • gschallert
    gschallert Member Posts: 170
    glifen said:

    If not, what is most likely the cause of the unit cycling so often?

    What thermostat(s) are you using and what is their CPH set to?
    CPH=cycles per hour
    njtommyBrewbeer
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,491
    Cycling could be part of the problem. Should not this boiler be running and modulating?

    The advantage of mod con is modulation especially if they are not able to condense due to high return temps.
  • Brewbeer
    Brewbeer Member Posts: 612
    edited February 2017
    glifen said:


    For the ODR curve, is it something I should try configuring myself? My knowledge on this stuff is minimal and my research isn't producing much of a return.

    Yes, you should definitely configure this yourself. One way I've heard described here on The Wall is to set the thermostat at 80 degrees so the boiler stays on. Adjust the ODR curve until the heat coming out of the baseboards is just maintaining your desired inside temperature (e.g., 68-70 F). Based on my experience setting the curve for my boiler (and not knowing anything about your boiler) set the low end of your curve at 85F supply temp for an outdoor temp of 75F, and set the high end of the curve at 150F at an outdoor temp of 0F (or what ever your design temp is), and see what happens to the temp inside your house on a day when it's below 32F and there is little to no solar gain (e.g, cloudy day, or at night). If the temp inside the house climbs above the desired inside set point temp, lower the high end to 140 and see what happens. Conversely, if the inside temp falls from the desired set point, raise the high end to 160 and monitor the results. Continue with this procedure until the inside temp maintains. Eventually, you should be able to get it dialed in so that the boiler stops cycling and runs continuously at a low rate of fire.

    This description is probably a little simplistic, but it will at least get your boiler into a mode whereby it stops cycling the way it currently is. You can adjust the low end, too, but be careful since fin tube baseboard heat output drops quickly at below 90-95 degree supply temp. You can also adjust the design temp down, etc., and monitor results.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
    Hilly
  • Paul48
    Paul48 Member Posts: 4,492
    If it were mine.....Next spring I would be balancing the system and adding emitters. I'd get it to condense, all the time. Maybe some nice comfortable floors are the ticket.
  • glifen
    glifen Member Posts: 22
    Thanks Brewbeer, that makes a lot of sense. Being doing a lot of research on setting the ODR curve and think I will tackle it once I get the tech who installed it another shot at it.

    One question though... how do all of these settings impact the indirect DHW? Do I have anything to worry about there?

    Paul48, I'd love to add more emitters.. but I'm maxed out on my 1st floor. I can probably add another 20 feet upstairs, not sure if that would really help all that much though.

    I'd love radiant floors but I have 5 sets of french doors that need replacing first and a bunch of windows.
  • Paul48
    Paul48 Member Posts: 4,492
    edited February 2017
    Slant/Fin Base/Line 70?
    http://www.ratheassoc.com/PDFs/Slant_Fin/Brochure 70.pdf
    Just some possibilities for down the road.
  • glifen
    glifen Member Posts: 22
    I have about 40 feet of high output baseboard installed, this was put in with the new system. 20 feet downstairs and all 20 feet upstairs are high output.. or should be, I never really checked to confirm it was and not sure how. The new baseboard they installed isn't slantfin and I couldn't really find any markings on them.

    But you're right, swapping out the rest may be a good cost effective solution...
  • Brewbeer
    Brewbeer Member Posts: 612
    Adjusting the ODR shouldn't impact hot water production, which is typically set up as priority on mod con systems.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • rbeck
    rbeck Member Posts: 57
    If the boiler is oversized try adjusting the inducer rpm's to match the heat loss of the structure. This will only help when the set point temp and return temp are far apart due to first demands after thermostat shut down or additional thermostats call bringing back cool water and dropping the boiler temperature. Like I said limited but it all helps. If the boiler is bigger than the IWH requires reduce that rpm's there also to restrict short cycling when making hot water.
  • glifen
    glifen Member Posts: 22
    Tech came out on Friday and wasn't very helpful. He had no idea how to set a custom ODR curve, he verified that it was set to the default and that was it. He also spent too long arguing with me that if the system wasn't putting out 180 degree water then it wasn't functioning properly.

    So I started down the trial and error path of figuring out a good ODR curve myself. I was able to get it to run constantly (or almost constantly, haven't had time to sit there and watch it for extended periods of time) with a SWT of about 130 during our recent 30 degree days. Temp in the house did dip down to 65/66 during the night but was back up to 68 by the time I got home from work.

    However, I did notice the difference between the supply and return temp is always 10 degrees even after changing the settings a few times. Should I look into slowing down the circulator pump? The Grundfos pump is currently set to medium.
  • Gordy
    Gordy Member Posts: 9,530
    All good advice above. However none of the fine tuning will equate to 50% more fuel usage.

    This is a log home with a lot of glazing. Is it a full log?

    Full log homes can have high infiltration if chinking is not maintained.

    So I think you need to look back at the weather in your two comparison months of different years.

    Factors include

    solar influence.
    Winds.
    Of course temps.

    Btu/hdd/sf comparisons can only paint part of the btu usage.

    Winds, and solar influence can make huge differences on leaky structures.

    Solar influence with a lot of south glazing also plays a big role.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    glifen said:

    ...I did notice the difference between the supply and return temp is always 10 degrees even after changing the settings a few times. Should I look into slowing down the circulator pump? The Grundfos pump is currently set to medium.

    10deg delta isn't bad, especially if you're getting long burns. You can try a lower pump setting- but keep an sharp ear on the heat exchanger- it you hear hissing, popping, percolating, etc... go back to the next higher setting right away before you fry the HX.

  • Paul48
    Paul48 Member Posts: 4,492
    @glifen
    Taco has FloPro University online. You should sign up for it, it's free, and they have many simple tutorials. I think you'll be amazed at what you can learn. It will help you with the initial set up of your curve. You'll learn the Universal Hydronic Formula. It's the simple formula that's the basis of everything related to the transfer of heat. If you're a knowledge geek, like me, you'll find it addicting.
    Canucker
  • flat_twin
    flat_twin Member Posts: 284
    Hi Gilfen, Just a homeowner here (retired telco technician) with a recent oil boiler to natural gas modcon conversion. My experience is similar to yours in that the installer left me with default max/min water temp ODR settings but he explained enough about how to adjust those settings that I felt confident to educate myself further and give it a go.
    I read the entire installation manual for my boiler and particularly the parts about outdoor reset. I dug up all I could find on forums like this one about modcon boilers and ODR.

    At the start my boiler was running 10 min on / 10 min off. The installation manual said for cast iron radiators the water temperature settings should be 180 max /at heating design temp degrees outside and 130 min/ 70 degrees outside.

    I started by lowering both max and min by 20 degrees and got longer run times. Again I reduced the water temperatures 10 degrees at a time allowing the system to adjust to the new settings until the boiler ran non stop and maintained 71 degrees inside. I also figured out the new Honeywell thermostat had a tight temperature variation setting. Changing that to a broader setting helped to reduce cycling. In my case the thermostat has settings of "Steam, 1,2,3,4,5" Steam is the widest temp swing allowed and 5 is the narrowest. I set it to 1.
    As the outdoor temps fluctuated I made small adjustments to the water temps and ODR. I also used the "raise the thermostat to 80 so it runs all the time and see how the boiler reacts to nothing but input from the ODR" method.
    I'm done adjusting things now and have a boiler that runs nearly non stop. I found a nice compromise where warmer calm sunny days will make the boiler cycle on and off maybe once an hour. And there are rare days when it's extremely windy and the indoor temperature sags to 69 degrees though the thermostat is set at 71. I'm fine with that.
    I have an advantage in that my cast iron radiators are capable of 105,000 btu output while my heat loss is 77k btu so I can run the boiler at lower temps, low enough to make the boiler condense all the time. If your baseboard output matches your heat loss, you won't be able to turn your boiler down nearly as much but I suspect you can get longer run cycles and use less propane by doing so.

    Currently my boiler is set like this...
    Max water temp 130 at -5 degrees outside
    Min water temp 85 at 63 degrees outside

    Output from the burner is usually 20-30%
    Boiler is a WM Eco 110, one zone with 2.5 in main pipes feeding 10 cast iron radiators.


  • Gordy
    Gordy Member Posts: 9,530
    edited February 2017
    The proper way to dial in the outdoor reset curve. It takes time, but in the end the thermostat turns into a high limit device. Its sole purpose being to compensate for solar, and internal gains from other heat emitting devices. The outdoor reset then drives the system.

    With a cast iron radiator emitter set up like @flat_twin there is much more mass than a baseboard emitter this helps buffer the system. Also if a EDR survey was conducted, and proves the radiators are oversized for the loads. That is a huge benefit in lowering the AWT the system needs.

    So raise your thermostat setting to as high as it will go. Then adjust your reset curve in small increments allowing the structures mass to equalize to maintain desired indoor temps. This will take time however it will give you the longest burn times at the lowest modulation that the system will allow. Of course provided that flow rates are dialed in.
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