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Gas boiler sizing - please help/critique

PrincetonPrinceton Member Posts: 9
I’m considering the following boilers for my 1,300 sqft ranch house – located in Doylestown, Penna (ZIP 18901).
• Bosch Greenstar 57
• Buderus GC144/3
There are two underfloor radiant zones – each is ~650 sqft (~16,000 Btu).
Envelope heat loss is ~26,000 Btu @ 50F temperature differential - ~30,000 Btu @ 60F.
Either solution will use an SST150-40 for domestic hot water.
I am heating with propane.
• Are these units sufficient for my needs?
For the Greenstar:
• What is the minimum output rate of these units with propane?
• Is the radiation from one zone sufficient to elicit the minimum output rate at condensing temperatures?
For the Buderus:
• Will I have issues with this unit short cycling as it doesn’t turndown?
Thank you all for your insights.

Comments

  • IronmanIronman Member Posts: 5,005
    Why would you want a non condensing boiler (GC144/3) when only a radiant floor is gonna be connected?

    A mod/con will operate at its best efficiency with a radiant floor and it comes with ODR. The non condensing boiler will require mixing controls beyond the boiler so that it won't condense and will be less efficient. It may also require a metal chimney liner.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • PrincetonPrinceton Member Posts: 9
    The research I've done points to the following potential issues with each unit:
    The minimum firing rate of the mod/con (Greenstar 57) must be low enough to ensure condensing with only one of the zones calling for heat. I'm not sure that is the case here and I'm hoping that someone can confirm that one way or the other. If it does, that probably will be my choice.
    In the case of the Buderus, my concern is the potential for condensation given the low return temperatures with the radiant distribution system. The advantage of the Buderus is simplicity.
    The Buderus has somewhat lower initial cost (and lower efficiency) - the Greenstar has a higher initial cost and higher efficiency. I don't believe that I will burn a lot of gas.
    I'm looking to make the choice that will provide the best service life - both from a cost and reliability perspective.
  • IronmanIronman Member Posts: 5,005
    edited April 2
    Why are you limited to the GreenStar? It's not a bad boiler, but there are others that have a 10 to 1 turndown, stainless steel fire tube heat exchanger, and lower price point.

    Take a look at the HTP UFT 080. The minimum firing rate is 8k btus. Well below what the GreenStar 057 offers and well below your minimum load.

    Are your floors high mass (slab) or low mass (wood). If high, then it really wouldn't matter matter which boiler due to the mass which would produce long firing cycles. If wood, then you'd need a boiler with a minimum firing rate equal to, or below your minimum load.

    Here's a spec sheet for the GreenStar. Please note that the minimum firing rate is much higher for LP than it is for natural.


    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • ZmanZman Member Posts: 4,854
    The Buderus is a garden variety small cast iron boiler. Most major manufactures make a similar model and I don't believe there is much difference from brand to brand. Properly installed with a smart mixing valve for outdoor reset and boiler condensation protection, you should get a long trouble free life out of it. Real world efficiency somewhere in the 80%-84% range.

    The Bosch is another story. It has an aluminum heat exchanger which may have a shortened life expectancy, depending on local water conditions. It will be fairly unforgiving if regular maintenance is not performed. At 12.5k turndown, it should not cycle badly on your 2 zone system. You can expect efficiency in the low 90% range.

    I would recommend a condensing boiler with a stainless heat exchanger and better turndown. Take a look at Lochinvar, HTP, Viessmann....


    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • PrincetonPrinceton Member Posts: 9
    Ironman:
    Greenstar is what my local guy is certified in. I thought the minimum firing for LP might be a typo. I've asked Bosch to confirm. If correct, the Greenstar is out.
    Zman:
    Can you tell me what a smart mixing valve is and what boiler condensation protection is?
    Ironman/Zman:
    Thank you for your insights. I want to get this right.
  • IronmanIronman Member Posts: 5,005
    It’s not a typo. They can’t get it to fire correctly below that on LP. When it’s converted to LP, the electronic key is replaced and that’s as low as it will allow.

    A competent hydronic tech should be able to install and service any mod/con, but I understand wanting to use what you’re familiar with.

    Where are you located?
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • PrincetonPrinceton Member Posts: 9
    Ironman:
    Doylestown, Penna 18901
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 5,712
    @Princeton

    Double check your design temp difference. 50 or 60 deg seems low for PA but I am guessing. Didn't look it up
  • IronmanIronman Member Posts: 5,005
    I believe that STEVEusaPA covers that area. I'd highly recommend that you contact him. He has several current comments on here that link to his info, or you can try the contractor locator above.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • ZmanZman Member Posts: 4,854
    Radiant heat generally needs to run at low water temps. Outdoor reset control improves efficiency and comfort by lowering the water temp as the outdoor temp rises.
    Non condensing boilers like the Buderus you are considering are not designed for low return water temps. They will be damage with prolonged exposure.
    Smart mixing valves like the taco I-series will doboth outdoor rest control and protect the boiler from low return water temps.
    http://apps.taco-hvac.com/uploads/FileLibrary/102-145.pdf

    Condensing boiler will manage outdoor reset with on board controls. No mixing needed.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • TAGTAG Member Posts: 103
    Princeton: I own a house (weekend) a few miles from you - with all the NG in PA -- frustrating we don't have any distribution.

    What are you using now ?

    My go to boiler for years has been simple Cast Iron Buderus -- supplied with Eccomatic/ Logmatic controls and matching indirect water tanks. Last forever -- minimal maintenance. Full outdoor reset. With NG -- they are still the way I would go.

    The GC line of boilers was developed to replace conventional "American" style boilers. It's not designed like all the other Buderus models before it in the USA to take low temp return. Buderus made low temp famous in Germany -- so it is odd that they came out with this one. It really for retro- fitting into homes with baseboard and other high heat systems. You can't use the ODR control on that boiler. Also -- last I checked it was not available for propane.

    Buderus make the GA line -- it's sealed combustion and uses 3" metal pipe. Comes in three sizes and can use the dogmatic control. I'm building a new place in Solebury and had I the space -- the GA would have been the ticket for me.

    The Logmatic control is an easy set up and it simply fires the boiler on and off to maintain the desired temp of the water based on outdoor reset. domestic hot water priority is built in w/ a sensor that goes into the indirect. In high temp situations where one has baseboard or panel radiators it has an additional module required for the lower temp floors.

    I just don't have the space in my mechanical room for the GA -- looks like I'm forced to use a wall boiler .. most likely Viessmann. More money/ complexity ... yes ... potentially 10% savings in Propane. But, depending on usage .. how much is that? vs the high cost of the unit and ongoing maintenance problems?

    I still own one of my original projects that I now use as an office -- the boiler is 25 years old and I just replaced the indirect about 18 months ago. Replaced with the new SS Buderus tank.

  • PrincetonPrinceton Member Posts: 9
    TAG:
    I currently have a 17 year old oil boiler with a seeping oil tank. Goodbye - and hello to propane. The days of a utility providing service are over. There is city gas one street over - or 300 yards away on my street. Utility (PECO) will not provide.
    I was excited to hear about the GA series - but it is no longer made.
    The GC line will take propane. At this point - I don't see another option.
    Thanks for your insights.
  • ZmanZman Member Posts: 4,854
    Make sure they size the gas line so it will work with natural gas. Eventually they will upgrade the system. You will save money when this happens.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • TAGTAG Member Posts: 103
    Mine was oil as well -- I did not know they discontinued the GA -- I put one in last year. It looks like they still make the X -- standard chimney. Maybe some one has a GA.

    I would call Buderus and ask about low temp and the GC

  • PrincetonPrinceton Member Posts: 9
    Zman:
    Thanks for the tip. Specifying will save money. I will do so - although I don't ever expect to see NG on the street.
    TAG:
    My heating needs are small - X boilers are too large. I have a mixing valve now - I think it will ensure that return temps are high enough. I am also researching the SSB85 - which resolves any potential chimney issues.
  • TAGTAG Member Posts: 103
    Princeton -- my place is larger than your .. but, it's spray foamed. I'm in the 50's BTU for load. That's why I picked up a Viessmann 200 in the smallest size. Forgot the X was not available in the smaller sizes ... Assume Buderus does not sell enough of the conventional to keep making a full gas line. = -- especially with fuel costs in Europe.

    There are so many possibilities --- Viessmann makes the 222 -- 2' square .. sits on floor and includes a 26g water tank for hot water production. The specifications for the Bosch and Buderus wall units are different enough that the venting restrictions eliminated the Bosch as a possibility.

    Everyone has a different idea -- Good luck. Get a big propane tank if possible -- I'm having a 1k buried. There have been ice storms where delivers became impossible.
  • PrincetonPrinceton Member Posts: 9
    Zman:
    What are your thoughts on the Bosch SSB85? Still planning on the indirect water heater.
  • ZmanZman Member Posts: 4,854
    I have to say, right out of the gate, I don't like Buderus/Bosch.
    The first boilers they came out with had aluminum exchangers and in my area, the failure rates were very high. The manufacture did not back them up and I know at least a dozen people that got less then 5 years out of them only to replaced them on their own dime. I would not put one in my house if it was free.

    If you look at that boiler without my prejudice, it looks like a firetube design but it has very low water volume (.78 gal)which makes me think it is their own new design. It also has only a 5-1 turndown.

    If you compare to a Lochinvar WHB85 with 2.2 gallons, 10-1 turndown and top notch support, this seems like an easy decision. https://www.lochinvar.com/_linefiles/WHB-PS-01.pdf

    Your local guy that is certified by Bosch, went to a one day training (sales) class and can program the boiler without taking out the manual.

    Have him install a Lochinvar. I bet he likes it so much he thanks you for suggesting it. :)
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ZmanZman Member Posts: 4,854
    edited April 9
    After a closer look, is a water tube heat exchanger in an unusual vertical configuration. https://www.bosch-climate.us/files/6720884055_SSB_Heat_Exchanger_Cleaning_Kit_Instructions_en_12.2017_US.pdf
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jean-David BeyerJean-David Beyer Member Posts: 2,618
    I have a W-M Ultra 3 boiler that has an aluminum heat exchanger. It was installed in May, 2009, and I have it serviced every year. So it is about 10 years old now.

    Among other things, the heat exchanger is opened and cleaned. Also, the condensate line is a trap to keep CO from leaking into the area around the boiler is examined for evidence of aluminum, and we do not find any. The heat exchanger looks just fine to me and to my current technician. It runs all year because it heats my indirect hot water heater too.

    I do not know what the water quality around here is, but the system does not seem to leak even though the radiant slab was original with the house built in 1950. We use X-100 additive in the boiler and test it every year. It seldom needs to be touched up.
  • starisestarise Member Posts: 4
    I live in Lancaster County. FYI Johnstone supply can set you up with a Navien Modcon.
    My situation is similar. I removed an old oil burner 114.000 btu and installed the Navien which is also a hot water heater. My hydronics are convection fin tube but I'm adding a floor radiant setup to an addition. After my calculations, I found I could go to 80,000 btu. Watch it though.Navian might advertise 100,000 btu. The fine print shows they use the btu from the DHW loop which is higher than the heat btu loop. This gives the perception of a higher output unit. In reality 80,000 is plenty for me sized with slightly more SQ FT. Even when adding a 24x14 radiant loop.

    In my case I need a watts three way mixer valve for the floor radiant because I run my boiler 160/180F. You might not need that. Depending on your use, because you run lower temp radiant /pex heat under the floor.Trun down ratios only matter if you need or use it. I don't see a huge difference in most residential applications between 5:1 and 10:1 when running the demands/heat loads...IOW actual use.
    Here's my setup. I was able to borrow a power press.

  • PrincetonPrinceton Member Posts: 9
    Jean-David:
    The SSB85 has a stainless steel heat exchanger - so I think I'm good there. I'm glad to hear that these installations can have low maintenance.
    Starise:
    A very clean installation. I currently have the three way valve as I was set up for a conventional non-condensing boiler. I am not sure if I will need it going forward. The plan is to use an indirect DHW as the combi units do not seem to deliver enough water flow. I agree with you around 5:1/10:1 - but I understand that the turndown output should be less than your smallest loop. In my case, that is ~18,000 Btu so I am good there.
    I'm not sure what a power press is or why it is needed for this application.

    Thank you both.
  • ZmanZman Member Posts: 4,854
    edited April 11
    Your smallest loop at ~18,000 Btu only applies to the design day temps. On you typical day, using the reduced water temp from the outdoor reset, it will be half that or less. 10-1 turndown is a big deal, you will thank yourself later....

    Jean-David has had good luck with the boiler because he has meticulously maintained it, beyond what any service company will do.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • PrincetonPrinceton Member Posts: 9
    Thanks to all who responded with insights and experiences.
    I am going with the non-condensing solution. As it turns out - I also need to have my chimney relined. I thought that this would swing the decision in favor of the SSB85 - but when I ran the numbers (cost differential against anticipated savings) my payback at 10 years out was only $600 - assuming that there was no higher maintenance costs for the SSB than the low tech non-condensing solution. This did not seem to be a worthwhile bet.
    I will ensure that the indoor gas piping will support both natural gas or propane - positioning me for natural gas if it is ever offered on my street (thanks, Zman). Again, all of your comments are much appreciated.
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