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Bosch greenstar vs ibc dc series

peppy690
peppy690 Member Posts: 13
I'm looking around at estimates and contractors to update my heating and hw system. It's ancient to say the least. And I want to go with a combi unit for space and efficiency. I was set on the Bosch greenstar, however the most recent contractor recommended the ibc do series instead. I've been trying to find reviews good or bad on both units.
I'm looking for reliability and ease of service.

Can anyone share their experiences with either unit good or bad?
One of the things that makes me look at the ibc more is the feature of learning peak dhw times and keeping the water mode ready for dhw to avoid the cold water sandwich. And the cost is less.

Comments

  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited April 2016
    The smallest Greenstar Combi (Greenstar Combi 100) only modulates down to 34.6 MBH.
    That may be too large for your smallest zone(s).

    Even at design day temps with all three zones running the Combi 100 would be short cycling in my 1750 sq ft home.

    Take a look at the specs for the Greenstar combi vs. space heating only boilers:
    http://s3.supplyhouse.com/product_files/ZWB-28-3-brocuhre.pdf
  • GW
    GW Member Posts: 4,304
    There are always some negative comments on everything. Most people that have an opinion on this say find a good heating guy and just go with what they say.

    Most people are doing indirects in our part of the world.

    Gary.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • kcopp
    kcopp Member Posts: 3,820
    I have installed both, multiple times.
    I do agree that a separate boiler and indirect is the better way to go.... that being said I do prefer the IBC DC over the Greenstar for a few reasons.
    The biggest being that there are fewer parts to breakdown w/ the DC vs. the Greenstar.
    No flat plate HX.
    No 3 way valve.
    The DC control is more straightforward.


  • peppy690
    peppy690 Member Posts: 13
    Thank you all for your input. Sorry it's been so long. I think in going to go with the ibc. I am concerned with the short cycling. Attached is the screenshot of the heat loss results I did with the slant fin app. They should be pretty close.

    My main reasons to do go this route are to update, efficiency, and gain basement floor space. The space is the reason I want to go combi. I also like the idea of condensing/modulating and ibc has a new DC model out that does 20-120k btu. For the design day temp of 10F the 20 will be perfect for 1 zone at a time. And not to forget about the dhw that model will be fine for my hw needs.

    Here are my questions and concerns.

    I'm worried if it's warmer about the shortcycling.

    My first floor zone is all in series and has 2 convectors (attached a picture of the type of convector in my house) about 10ft and 5ft of baseboard. My second floor zone has 4 convectors about 10ft and 3ft of baseboard all in monoflow. I am also having the installer put in a small third zone.. About 8ft baseboard in the basement since the new boiler won't provide radiant heat anymore. I'm wondering what the btu loses are in the piping and if those make the unit I want better on the short cycling?

    Thank you all.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Keep in mind that (according to your photo) the SlantFin app is using 180F supply water temp for it's calculations.
    If you're going with a mod-con boiler the return water needs to come in at or below approx. 130F to be in condensing mode. Even with a DT of 20F that would make your max supply temp 150F vs. 180F.
    May be a good idea to recalculate the radiation potential of your smallest zone using 140F average water temp (assume 150F SWT and 130F Return Water) and then check that against the min fire rate of the boiler to see if it will short cycle.

    Have you run any outdoor reset curve scenarios to get an estimate of your SWT under differing outdoor temps to see when you'll be condensing?
  • NY_Rob
    NY_Rob Member Posts: 1,370
    The 8ft zone will short cycle no matter what you do if it's on it's own, see attached chart to see BTU output for different water temps vs. BTU output for the 8ft zone.
    Maybe combine the 8ft zone with your shortest zone? For added basement heat (if needed)- leave some of the supply and return lines un-insulated?





  • peppy690
    peppy690 Member Posts: 13
    I was thinking about the return temps being to high. Thank you for the curves. I am considering letting the installer tap into the return on one of the zones for the basement. I know it wouldn't make sense to zone just for that small bb.

    I thought the style convectors I have were meant to receive 180f to function. Or will I be ok , just expecting the convectors to not crank like they usually do, with lower supply temps if I set the boiler up w/ lower supply temps?

    On the outdoor reset curve scenarios. I have not considered that at all. Could you provide me direction on a good source to research how to do that?
  • peppy690
    peppy690 Member Posts: 13
    edited July 2016
    Also, How would I get the radiation potential of the smallest zone?

    Thank you for all your help.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited July 2016

    I can't comment on your convectors- the pro's here probably have experience with them.

    You can usually find the default factory reset curve in the boiler's install manual or user manual.
    The idea is to have the mod-con supply just enough BTU's to keep up with heat loss without overshooting/wasting and cycling. Heat loss varies by outdoor temp, so you'll need more BTU's to keep up with heatloss at lower temps vs. higher outdoor temps. You program in via installer mode the four parameters (low outdoor temp, hi outdoor temp, max SWT and min SWT) to make your curve.
    Above is the factory default outdoor reset curve from the install manual for my boiler- they also give you a blank chart to create your own curve. FWIW- you can see my factory default curve goes all the way down to 86F SWT! I can't imagine actually using that low temp for anything.
    For my curve I have set my max SWT at 160F at -4F outdoor temp and 111F SWT at 66F outside. My Design Day (15F) BTU loss is 28k BTU's, the curve I've programmed in will supply 36.5k BTU's at that outdoor temperature. I haven't gone through a winter with this boiler yet... so we'll see how these settings work out... I'm expecting I'll have to tweak it some.

    But before getting involved in reset curve settings, I would recalculate your zones at lower (condensing) return temps to see if your chosen boiler can turn down low enough to prevent short cycling. I had to go through quite a few boiler choices before finding one that could fire down low enough to keep my shortest zone from short cycling.



  • peppy690
    peppy690 Member Posts: 13
    That all makes sense. How would recalculate my smallest zone for the lower supply /return temps. I assumed the heat loss is the heat loss based on the house. The slant fin app doesn't let me change 180f as the supply temp.

    No curve in the manual just those settings you mentioned. To set the curve.
  • peppy690
    peppy690 Member Posts: 13
    How did you get the firing rate btus for the design day temp?
  • NY_Rob
    NY_Rob Member Posts: 1,370
    peppy690 said:

    Also, How would I get the radiation potential of the smallest zone?



    Thank you for all your help.

    If it's standard baseboard, pick a supply temp... subtract 10F to get your average water temp, find that temp on the Slantfin chart I posted- multiply that BTU number by the finned ft length (not total zone circuit length... just the finned tube length) of your zone.
    That will give you the zone BTU radiation at a particular supply temp.
    ie.. one of my zones is 120ft long with 57 ft of finned tube.
    At 140F SWT use 130F for average water temp- according to the chart that gives me 260 btu's per ft of finned tube- multiply by 57 ft- that gives me 14,820 BTU's for that zone.
    Of course as you change the SWT the BTU's/ft of finned tube changes too.
  • peppy690
    peppy690 Member Posts: 13
    Gotcha. That's fine for the baseboard. I need to I figure out how I can get figure the output for my convectors. I'll search around this thread.

    Also for your setup are you concerned about the unit not condensing with your max supply temp at 160?
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited July 2016
    peppy690 said:

    How did you get the firing rate btus for the design day temp?

    I used the actual gas consumption from my utility meter over 61 (IIRC) days from this past winter and looked up the HDD (heating degree days) for my zipcode, and used 75% efficiency for my old 1960's era cast iron boiler. That gave me approx heat loss for the house at design day, so I drew my reset curve to supply slightly (+25%) more than that at design day.
    You want the mod-con circulating 20hrs or so per day during the cold weather.
    Like I had said earlier... it's subject to tweaking :)