Will a Bosch Combi 151 unit work for my house?

Discussion in 'Tankless Water Heater Forum' started by Nevil Nami, Sep 9, 2019.

  1. Nevil Nami

    Nevil Nami New Member

    Sep 3, 2019
    Vancouver, BC, Canada
    Hello, I recently moved into our new home that currently has a Super Hot MG-100 Hydronic Boiler (installed in 1986, when the house was built). Clearly, it's done its job and in need of a replacement. We had a couple of boiler companies come give us a quote and one of them suggested putting in a Bosch Combi 151 (ZWD-42) as he noticed that our water heater will also need to be replaced soon and because the combi unit is much more efficient.

    I did some research on the internet and can't get a clear answer if this unit will be able to meet the demand of our house. The house has 3 levels and the boiler (located in the basement) supplies hydronic baseboard heating to the basement and main levels only (the upper level is heated by electric baseboard heaters).

    As for our hot water demands, the basement is a rental suite that has a kitchen sink, washing machine and a single bathroom with a stand-up shower. It is currently occupied by 2 people.
    The upper and main floors are occupied by my wife and I (and hopefully a child in the near future). On the main floor, there is a kitchen sink, laundry sink, washing machine and a half-bathroom (no shower). The upper floor has 2 bathrooms, one with a tub and the other with a stand-up shower.

    Would this unit be okay for us? Any input or advice is greatly appreciated.
  2. Dana

    Dana In the trades

    Jan 14, 2009
    The minimum firing rate output of the ZWB42 is 46,400 BTU/hr, according to p.12 of the manual.

    At condensing water temperatures fin-tube baseboard puts out about 200 BTU/hr per foot, so unless you have 46,400/200= 232 feet of baseboard per zone it's going to be cycling on/off at water temperatures low enough to deliver best efficiency.

    Measure up the baseboard, zone by zone, and see if it's a good fit. For reference on how this works, see this bit 'o bloggery.

    Combi boilers are usually only a good fit for houses with moderate domestic hot water needs but very large design heat loads. That's not usually an insulated newer house. My guess is that unless yours is one of the largest (or absolute crummiest) home in Vancouver you don't have anywhere near enough baseboard in zones (or maybe even the whole house) to run the Greenstar 151 without it short cycling into low efficiency, high maintenance, and ultimately an early grave. With the actual baseboard numbers I may be able to suggest a solution that is more likely to work.

    For reference, my 1923 vintage house is ~2400 square feet of 2x4 framed bungalow over 1600' of insulated basement, and even at -20C outside my heat load is less than the minimum output of that thing! With big high volume radiators or a lot of Euro-style flat panel radiators you could make it work, but those would be rare to find in Vancouver in 1986.

    Since you have a heating history on the place, run a fuel-use based heat load calculation (based on wintertime fuel-use only, which minimizes the hot water use & solar gain errors.) You'll probably be surprised at just how ridiculously oversized the MG-100 is/was. (Expecting another ice age soon? :)) It's pretty common to see boilers 3-4x oversized for the design load of the house, but there is no good reason to repeat the error. Don't expect many boiler contractors to be clued into it either- they're more happy to up-size than down size, since it doesn't require any math (or risk) on their part. Based on the 85,000 BTU/hr output I'll hazard that you have something between 140' - 170' of baseboard in the house (total), and possibly less. I don't know what the total water volume/thermal mass of that thing is, but it wouldn't surprise me if it's been short cycling itself to lower than nameplate efficiency too, but it has to have more thermal mass to work with than the Greenstar 151, which would only make it worse.
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  4. Nevil Nami

    Nevil Nami New Member

    Sep 3, 2019
    Vancouver, BC, Canada
    WOW! Thank you for all that information Dana. I've read your post several times and also read the links you sent me, mind you, there are parts that completely went over my head.

    When you mentioned that the manual said the minimum firing output is 46,400 BTU, what exactly does that mean?....especially in comparison to the 151,000 BTU advertised on the unit?

    I went and measured all the baseboards in the basement and on the main floor. The basement has 38 linear feet and the main floor has 61 linear feet, making a total of 99 feet. I'm not sure what you meant by zones, but if you meant areas controlled by different thermostats, then the basement is on one zone (the tenants have a separate thermostat) and the main floor is on another zone (we also have our own thermostat control).

    As for the square footage. The basement is 1030 sq.ft., the main floor is 1022 sq.ft. and the upper floor is 920 sq.ft. Also thought I'd mention that there is no barrier between the main floor and the upper floor. It's basically a wide-open area where the stairs are, so I assume quite a bit of the heat ends up also heating upstairs, especially since we've never used the electric baseboard heaters on the upper floor. I've attached a mini floorplan of the basement and the main floor to show where the baseboard heaters are and also where the boiler is.

    And finally, I completed the fuel-use based heat load calculation, even though I hardly understood any of it, lol:

    Dec 2018 Fortis Billing Period = Nov 21 - Dec 21, 2018
    Gigjoules Used = 13.7 GJ
    Therms used = 130 Therms
    Outdoor Design Temperature for Vancouver in Winter (Heating 99% Dry Bulb) = 24°F

    Super Hot MG-100 Boiler Input BTU-HR = 100,000 BTU
    Super Hot MG-100 Boiler Output BTU-HR = 84,600 BTU
    Super Hot MG-100 Boiler Thermal Efficiency = 84.6%

    130 therms x 84.6% Efficiency = 110 therms
    110 therms x 100,000 BTU (# of BTU per therm) = 11,000,000 BTU = 11 MMBTU

    Sum of Heating Degree Days (HDD-65°F) for Fortis Bill Period = 616.2 HDD-65°F
    Sum of Heating Degree Days (HDD-60°F) for Fortis Bill Period = 466.2 HDD-60°F

    11MMBTU / 616.2 HDD-65°F = 17,851 BTU per degree-day
    17,851 BTU per degree-day / 24 Hrs = 743.8 BTU per degree-hour at balance point of 65°F

    11MMBTU / 466.2 HDD-60°F = 23,595 BTU per degree-day
    17,851 BTU per degree-day / 24 Hrs = 983.1 BTU per degree-hour at balance point of 60°F

    A balance point of 65°F with a design temp of 24°F is a difference of 41°F
    So the implied load = 41°F x 743.8 BTU/F-hr = 30,496 BTU/hr

    A balance point of 60°F with a design temp of 24°F is a difference of 36°F
    So the implied load = 36°F x 983.1BTU/F-hr = 35,932 BTU/hr

    That's a range of about 15% between the calculation based on 65°F heating degree days and the calculation based on 60°F heating degree days.
    Most 2×4 framed houses will have a balance point close to 65°F, most 2×6 framed houses will balance closer to 60°F.

    For sizing equipment, u se ASHRAE 1.4x sizing factor:
    1.4 x 30,496 BTU/hr = 42,694 BTU/hr (with a 65°F balance point assumption)
    1.4 x 35,932 BTU/hr = 50,305 BTU/hr (with a 60°F balance point assumption)

    Also, if it helps, we usually have the thermostat set to 21.5°C (70.7°F) during the day and 17°C (62.6°F) overnight (from 11pm - 7am)

    Based on the information above, what are your thoughts?
    Thanks again for assisting me with this.

    Attached Files:

  5. Dana

    Dana In the trades

    Jan 14, 2009
    When you mentioned that the manual said the minimum firing output is 46,400 BTU, what exactly does that mean?....especially in comparison to the 151,000 BTU advertised on the unit?

    There are lots of variations on the them for the Greenstar Combi models so I may have been looking too quickly. I believe the one closest to yours is specified on page 25 of this technical spec document. This has a modulating burner- it ramps the flame up and down with load. The maximum INPUT rate is 151,600 BTU/hr for the natural gas version, and 148,300 BTU/hr. But the MINIMUM input rate is 36,000 BTU/hr (NG) or 46,400 BTU/hr (LPG).

    The minimum output at condensing temperatures (the 122/86 °F (50/30 °C) row) is 35,100 BTU/hr (NG) or 46,100 BTU/hr (LPG).

    The smallest independently controlled zone being only 38' means that to get condensing temperature ideally the minimum firing rate of the boiler would be 38' x 200 BTU/hr = 7600 BTU/hr. Anything more than that it will cycle on/off during continuous calls for heat, since the burner can't turn down any lower. The 35,100 BTU/hr minimum output of the Greenstar 151 is more than four times that- it's going to short cycle like crazy. Even with the full 99 feet of baseboard operating at one zone it can't run at condensing temperatures without a lot of cycling, since the 99' of baseboard would only be emitting ~20K of that 35K minimum output, with 15K of surplus heat causing the system temperatures to rise quickly, since there isn't much water volume or other thermal mass to work with. That boiler would be a disaster on your system.

    Without reviewing the load line by line (looks fine on a quick review). A slope of 743.8 BTU per degree F is a credible number for a ~2000' hous over a ~1000 basement. Based on your thermostat settings it's reasonable to use base 65F for the heat load analysis, so you're looking at a design load of ~30.5K, and could reasonably use up to ~43K of burner output to cover cold snaps. Anything over that is fine (even though it is never needed for space heating), as long as at minimum fire it can modulate down to the 7600 BTU/hr range or lower.

    Notice that the Greenstar 151's minimum firing rate is more than your actual design heating load, which is what I expected.

    HTP's UFT-080W boiler (also sold under the Westinghouse label as the WBRUNG-080W) plus an indirect water heater would be an easy choice here, but I'm not sure how well supported it is in Canada. That boiler can throttle down to about 7600 BTU/hr in condensing mode, but can also ramp up to 75,000 BTU/hr when serving the water heater for very short recovery times. From a system design perspective it's easier than most, since it has a pre-plumbed secondary port for the water heater along with some other controls built-in, and it's pretty forgiving on flow-rates through the boiler.

    Navien's NHB-055 also ramps down that low 8000 BTU/hr-in/7600 BTU/hr out at minimum fire, and is widely available in Canada. That size would be fine for space heating, but would have a slower recovery time for the water heater than the UFT-080W. But her slightly bigger sister the NHB-80 has the same minimum firing rate as the -055, and ramps up to the same maximum as the UFT-080W, making it my #2 choice for your system.

    A primary difference between the UFT-080W and NHB-80 is the heat exchanger type, and water volume. The NHBs have very low volume higher pumping-head water tube type heat exchangers, whereas the UFTs have about 25lbs of water in them and much lower pumping head. From a system design point of view the NHBs must get plumbed primary/secondary, which knocks a percent or two off the as-used efficiency due to the slightly higher water temp entering the boiler from the hydraulic separator (Navien sells a pre-engineered manifold to be used as that hydraulic separator), whereas in most systems the UFTs can be pumped direct, where the boiler is taking the full flow of the radiation zones. That may read like Urdu to you, but it's nothing to be concerned about if you trust the competence of the installers.

    Lochinvar's WHB-055 and WHB085 would also work fine here, but they're usually more expensive than the Naviens & HTPs. The WHBs have more bells & whistles to program than you'll ever need on a heating system as simple as yours. Their somewhat simpler NKB080 would be just fine.

    There are other condensing boilers that would still work optimally, but none of them are combi-boilers. The Laars Mascot FTs use the exact same heat exchanger as the UFTs (they are both designed and built by Kiturami a first-tier Korean boiler manufacturer), but for some reason the MFTHW80 only modulates down to 16K-in 15.2K out, which could be a problem when only the basement zone is calling for heat.
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