We 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 151 (ZWD-42) Combi Boiler as he noticed that our water heater is also near its life expectancy 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, 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.

  • Is there a reason you're looking specifically at a combi boiler, and not a standard modcon feeding an indirect tank in addition to the space heating zones? – ThreePhaseEel Sep 11 '19 at 1:30
  • From my end, there's no reason at all. To be quite honest, I had never even heard of a combi boiler. We just called a heating company to come give us a quote on a replacement boiler and the worker looked over a few things and noticed that our water heater would need replacing soon, so he suggested the combi-boiler as a 2-in-1. He said we'd also save money because of how efficient it was and that our Gas Company would give us a $1500 rebate for going greener and that we'd also save space since it takes less space. What is a modcon? – nnamerz Sep 11 '19 at 4:44
  • "modcon" is short for "modulating-condensing" -- modern boilers are of this type due to their ability to a) "turn the fire down" under low boiler loads to conserve fuel and b) operate with condensing or near-condensing exhaust temperatures using a secondary heat exchanger to extract the maximum possible amount of heat from the combustion exhaust stream. – ThreePhaseEel Sep 11 '19 at 11:46
  • While researching some more, I came across a few sites that said most of the standard boilers sold these days are oversized for the average home and that installing one will cause the boiler to short cycle into low efficiency, high maintenance, and early failure. Is this accurate? And if so, how would I know the proper amount of BTUs for my house? – nnamerz Sep 11 '19 at 21:23
  • "standard" as compared to what? Also, have you had a Manual J load calculation run for your house? (That's how you figure out how many BTUs it takes to heat or cool it) – ThreePhaseEel Sep 11 '19 at 23:09

You estimate the hot water requirements ie start with some of the “rules of thumb” such as each person uses between 40 to 60 litres of HW at 60 deg C per day and include all the other HW demands... once you have that then you can equate what you need.

  • Thank you for giving me a starting point. I ended up looking online and found some average values for how much water certain things require or consume: Shower = 2.0 - 2.5 GPM, Standard Kitchen Faucet = 2 GPM, Standard Bathroom Faucet = 1.0 - 1.5 GPM, Standard Washing Machine = 23-25 gallons/load, Energy Star Washing Machine = 13-15 gallons/load. So, if I'm not mistaken, it seems that the Bosch 151 could almost handle doing any 2 tasks at once. My wife and I can easily plan not to do any 2 things at once, but that's going to be much harder to accomplish with our tenants. – nnamerz Sep 11 '19 at 21:18

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