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Will a combi-boiler work for my house?

nnamerznnamerz Posts: 2Member
We recently moved into a new home that currently has a Super Hot MG-100 Hydronic Boiler (installed in 1986, when the house was built). Clearly, the boiler is way past its life expectancy and in need of replacement. We had a couple of boiler companies come give us a quote and one of them suggested putting in a Bosch 151 (ZWD-42) Combi Boiler because it's much more efficient and because he noticed that our water heater is also near its life expectancy. He said we'd kill 2 birds with 1 stone and also take up less space.

I did some research on the internet and can't get a clear answer if this unit, or a combi boiler in general, will be suitable for our house. Some sites said it depends on how many gallons per minute of hot water are used throughout the house since this unit can only output 4.0 GPM. Others said it's based on the number of bathrooms in the house and one site even said mentioned calculating the total length of all the baseboard heaters.

I don't know how many GPM of hot water are used throughout the house or how to even figure that out easily, but I did go and measure all the baseboard heaters.

The house has 3 levels and is located in Vancouver, BC, where it can get pretty chilly during the winters. The boiler is located in the basement and supplies hydronic baseboard heating to the basement and main level only. The upper level is heated by electric baseboard heaters. The basement has a total of 38 linear feet of baseboard heaters and the main level has 61 linear feet.

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.

How would I figure out if we can suffice with a combi boiler? Would this unit work in our household?

Any help or advice is greatly appreciated.

Comments

  • KC_JonesKC_Jones Posts: 4,130Member
    There are a lot of opinions on combi boilers. My opinion is I would never have one. They are always a compromise for something. Unless your house is a mansion, that boiler will be way oversized for the heating load, this is in order to give you enough DHW.

    If you size it more appropriately for the heating load, you most likely won't have enough DHW.

    You would be better off doing a properly sized boiler with an indirect water heater, or just stick with a stand alone water heater. IMHO both of those options are better than a combi boiler.

    For me it's more important to size all equipment to do the job it needs to do, and space shouldn't be factored in. You don't have the space now, so why worry about it?

    Also no matter who you get to install the equipment they should be doing a room by room heatloss calculation to properly size the equipment, this is the only way to do it properly.

    The amount of baseboard you have dictates you can't output any more than ~50k BTU of heat, but the heat loss is still good to know how that compares. If your heatloss is say 40k BTU, then with a modulating boiler you would be able to run lower temps increasing efficiency.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
    https://www.facebook.com/media/set/?set=a.10202744301871904.1073741828.1330391881&type=1&l=c34ad6ee78
  • nibsnibs Posts: 327Member
    We are a couple in the west Kootenai, in a well insulated 1200SqFt house, we have a 50,000 btu combi with Hot Water output of 3+gpm one bathroom 1 kitchen. Works well for us so far, house stays toasty. But with 4 people (+ baby!!!) I agree with @KC_Jones. If you do decide to go the combi route you probably would have to put a small auxiliary electric water heater in each suite. We may do that if wait times for hot water are too long in kitchen.
  • hot_rodhot_rod Posts: 11,684Member
    nibs said:

    We are a couple in the west Kootenai, in a well insulated 1200SqFt house, we have a 50,000 btu combi with Hot Water output of 3+gpm one bathroom 1 kitchen. Works well for us so far, house stays toasty. But with 4 people (+ baby!!!) I agree with @KC_Jones. If you do decide to go the combi route you probably would have to put a small auxiliary electric water heater in each suite. We may do that if wait times for hot water are too long in kitchen.

    Most combis have two operating modes. You can opt to keep the block warm, so DHW production is instant and the only wait time would be attributed to the piping run. It does use more fuel of course, still less than a 50 gallon tank standby energy use.


    Hot water piping in slabs will of course slow delivery time, those lines should always be insulated. Your wait is probably more related to piping loss

    The other combis mode is to fire and wait as you would with any tankless DHW type.

    More and more we see a small, few gallon, tank built into to tankless water heaters to eliminate that cold slug effect.

    I get along fine with my combis, two of use. A small 6 gallon buffer smooths out the heating side cycling, added with ramp delay, and electronic derate they can work just fine when oversized for the heating load. Certainly better cycling control compared to a "single speed" non modulating, non adjustable type boiler. A 110- 120 combi should supply around 3 gpm or so, and turn down to around 8K for heating.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • nnamerznnamerz Posts: 2Member
    edited September 13
    KC_Jones said:

    ...Unless your house is a mansion, that boiler will be way oversized for the heating load, this is in order to give you enough DHW. If you size it more appropriately for the heating load, you most likely won't have enough DHW.

    Forgive my ignorance, but why is oversized bad? I thought it would be good in the sense that it could handle whatever we threw at it.
    KC_Jones said:

    Also no matter who you get to install the equipment they should be doing a room by room heatloss calculation to properly size the equipment, this is the only way to do it properly.

    None of the companies that came to give us a quote mentioned heatloss calculations. In fact, none of them even asked to see the rest of the house. They only saw the basement and boiler room. How would I go about trying to figure out the heatloss calculations?
    KC_Jones said:

    The amount of baseboard you have dictates you can't output any more than ~50k BTU of heat, but the heat loss is still good to know how that compares. If your heatloss is say 40k BTU, then with a modulating boiler you would be able to run lower temps increasing efficiency.

    Is there a standard amount of BTUs each linear foot of baseboard gives out?
    hot_rod said:

    More and more we see a small, few gallon, tank built into to tankless water heaters to eliminate that cold slug effect.

    Would you happen to know of any specific brands that sell combi-boilers with the tank built-in?

  • KC_JonesKC_Jones Posts: 4,130Member
    The baseboard dictates max heat output. So in your situation the max output of the system is around 50k BTU. So even if you hook up 1 million BTU, you get 50k BTU. That is based on max output. It can be less with lower temps, this is where outdoor reset comes into play.

    Heat loss is the only way anything else is a guess. So if they aren’t going to do it they are guessing. For me, I pay professionals to do a job not guess. It doesn’t take much skill to guess.

    If the boiler is too large it will short cycle if it’s dramatically over sized it can shorten the life of the equipment. Think of a light bulb, turn it on and off 50 times a day it will last a while. Turn it on once and leave it on it will last longer. In my experience nothing mechanical likes on and off cycles, it likes to be on and left on.

    Keep in mind even though the equipment can modulate it will only go so low. For me a better comparison is the lowest boiler output, compared to the lowest heating requirements for the house (based on heat loss). With the heat loss you can figure the requirements at any outdoor temperature. Say you need the full 50k on the coldest day, you might only need 5k on the warmest, so how does 5k compare to the lowest output on the proposed equipment? Those are just random numbers for illustration.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
    https://www.facebook.com/media/set/?set=a.10202744301871904.1073741828.1330391881&type=1&l=c34ad6ee78
  • hot_rodhot_rod Posts: 11,684Member
    I was mentioning a small tank added to the tankless water heaters, not any combi I know of.

    Here is one example of a real nice high mass, mod con boiler, no onboard dhw however. Seems Viessmann would offer an optional flat plate hx for it to generate hw with that 199 capacity. Sort of an external tankless option :)

    HTP Heat Transfer Products has a tank style combi also.

    I think the lowest turn down on any mod con that I have seen is 7 or 8,000, much better than what we have had with fixed speed, non modulating type.

    nrhttps://www.viessmann-us.com/en/residential/gas-boilers/condensing-boilers/vitocrossal-300-cu3a.html

    A heat load calculation would pin down the actual load as accurately as the data input :)

    If you like crunching numbers you could get the weather data history for your area and determine some hours of occurrence to see how of the you are at various load condition, a bit technical.

    It helps determine how often you could be in condensing mode on a system with ODR enabled.

    Caleffi Idronics 25 is the source if you want to read more

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • YonkYonk Posts: 1Member
    I have almost the same setup as you: Three floors, (I'm guessing a smaller) 2600sqft house, basement suite, 3 showers, and two dishwashers. We also have the superhot 100 installed in 1990 and live in Vancouver BC.

    We had recommendations fora Noritz NRCB199 combi and an IBC SL 10-85 G3 with an indirect tank. I don't know how to compare products properly but get the feeling from posts here that the IBC with the indirect tank would be the leading option.

    I'm still looking so advice or info exchange on what you found would be great.
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