Will a combi-boiler work for my house?

nnamerz

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.

KC_Jones

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.

nibs

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_rod

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.

nnamerz

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_Jones

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.

hot_rod

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

Yonk

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.

DanInNaperville

If you have an old boiler (cast iron) probably with a cast iron air scoop and possibly with some other cast iron components, there's rust lurking in the heating system. Combined with some inevitable mineral scale build up over the years, there will be some crud in the pipes.
Boilers are generally one of 3 types;
- cast iron - big old cast iron "bottles"
- water tube - tubes filled with water that are heated by an "open" flame
- fire tube - vents (tubes) through a container of water that flames are directed through
All of the water tube high efficiency boilers (like the Noritz, but they all are of the same design) use a heat-exchanger in the form of a coil built up of flattened tubes of about the cross section of a soda straw - and any crud in the system will clog them up. Crud that doesn't clog them all the way up will make for narrow spots in the tube that can become hot spots in operation that overheat and fail.
Unless your system is extremely thoroughly flushed, and all valves, air scoops, etc. that aren't clean copper are replaced, that new high efficiency boiler could need replacement in 3 to 5 years.
Then you get to buy a new one and pay to have it installed. The warranty doesn't cover running it when clogged with gunk.
There is a hybrid boiler called the GV90+ (forgot who makes it) that can work in older systems and provide higher efficiency than a standard cast iron boiler. It mounts a large secondary heat exchanger in the boiler flue to improve efficiency. There are some fire tube high efficiency boilers with somewhat wider water passages that are a little less likely to clog, but they tend to be more expensive than "standard" water tube high efficiency boilers.
US Boiler (that makes many brands and types of boilers) is replacing the water tube heat exchangers with fire tube heat exchangers in some models, (e.g. K2WT vs. K2FT) possibly due to this issue.
An "old style" cast iron boiler may be your best choice. Their weight makes them difficult to move to where they need to be installed, and they take up more space than a high efficiency boiler, but they easily last 30 years or more and usually tolerate spotty maintenance fairly well. A modern high efficiency boiler, if installed into clean, iron free plumbing and carefully maintained every year, should last at least half that long, maybe longer.
A high efficiency boiler will save you 10% on your heating bill, but you'll need to pay for a fairly involved maintenance session every year which will eat up some of those savings. An old cast iron boiler will often need a much shorter service call for maintenance and is a lot less likely to leave you without heat on a cold night.
A high efficiency boiler is better for the planet (less fuel burned = less CO2) but it's not a huge difference. Make sure your windows and door seals are in good shape and it will make up some of the difference.