Thoughts on practicality of "combi" boiler systems

yosh

I need to replace my ancient gas boiler that is used to heat a Minnesota home and I've been presented with multiple options, one of which is the "combination boiler" that would replace both the old boiler and the water heater. It's by far the most efficient option and also the most expensive, but at this point I'm less concerned about the cost than I am about the practicality.

Does anyone have experience with these high efficiency "combi" systems? Are they reliable? Is the on-demand hot water system responsive? Aside from cost, are there reasons to avoid them?

I've read several of the previous threads on this site about "combi" units and many advise to use an indirect hot water tank instead. That was my first thought, but the last installer said that both Minneapolis and Saint Paul have restrictive codes that require so much insulation in the hot water tank that they are impractical. Am I being misled?

GroundUp

I think the person who quoted that system is blowing smoke. I've installed a couple dozen indirect tanks around the twin cities and have never even heard of that requirement. Assuming you use hot water on a daily basis, a combi isn't likely to be any more efficient than an indirect and far less reliable with a lot more maintenance. There's also the inevitable "wait" for hot water that comes with any sort of tankless. Whether combi or indirect, any high efficiency boiler is only going to be notably high efficiency when the water temps are below about 130 degrees so if you have all high temp emitters like baseboard or CI rads, odds are that a regular old cast iron 80% boiler is going to end up costing you less money over the long run than anything HE- and can still be paired with an indirect.

Jamie Hall

Combis can be wonderful. They can also be horrible. But first, as to efficiency -- any of them, and any "mod/con" boiler should have roughly the same quoted efficiency -- somewhere around 95%, more or less. That none of them actually achieve that is like the mileage on a new car's window sticker -- it's true, but only under specific conditions.

What is not true is that a combi would be "by far the most efficient option".

Now. As I say, when they are properly specified and applied they can be excellent. What is needed, though, is that the heating side be accurately sized to the actual building heat loss (this is true of any hot water boiler). Bigger is most assuredly not better. If your contractors quoting you have not performed a full building heat loss calculation ("Manual J") they don't know what they are doing.

At the same time -- and this is where it gets tricky -- the domestic heating side must be correct for your use, since they are usually intended for "instantaneous" hot water. To evaluate this you must make a good determination of how much hot water you will be using at any one time, and be certain that the rated delivery is at least that much.

What often happens is that the domestic hot water delivery isn't adequate -- or if it is, the boiler is oversized for the building load. You want to avoid that.

On the restrictive codes -- I have no idea. Some cities have gotten pretty wild. However in general, one can get the best results in terms of efficiency, enough hot water, and all that using a properly sized mod/con boiler (see above!!!) and an indirect tank which is heated by the same boiler when it's not heating the house.

hot_rod

DHW expectations are the key. How much do you need or use at one time? Most 150 K combis will provide 2 gpm even with 35f incoming water. Output is based on temperature rise.
Virtually all combos have 10-1 turndown, so a 150 could turndown, even be locked at 15,000 btu/hr heating output. I doubt your heat load  is that low?

it’s all in the numbers and your application. I had 3 combis at my last home. The 130 Laars/ Baxi went 15 years without a single breakdown, and combos cycle a lot. 12-15 years should be the life expectancy if they are maintained properly. I only opened and cleaned the Baxi twice in 15 years. It had good fuel and clean grease air to breathe😃

In most areas of the country you are at design conditions 3-5% of the heating season, so the boiler does not need to run at its highest temperature all season 

A heat load, determine heat emitters output at various temperature is a good starting point.

yosh

GroundUp said:

I think the person who quoted that system is blowing smoke. I've installed a couple dozen indirect tanks around the twin cities and have never even heard of that requirement. Assuming you use hot water on a daily basis, a combi isn't likely to be any more efficient than an indirect and far less reliable with a lot more maintenance. There's also the inevitable "wait" for hot water that comes with any sort of tankless. Whether combi or indirect, any high efficiency boiler is only going to be notably high efficiency when the water temps are below about 130 degrees so if you have all high temp emitters like baseboard or CI rads, odds are that a regular old cast iron 80% boiler is going to end up costing you less money over the long run than anything HE- and can still be paired with an indirect.

This is what I was thinking at first and it's nice to hear it confirmed by someone more knowledgeable.

Do you have an opinion on a Weil-McLain GV-90? It seems like a compromise between a standard 80% boiler and a HE.

Thanks.

jinbtown

GroundUp said:

There's also the inevitable "wait" for hot water that comes with any sort of tankless. Whether combi or indirect, any high efficiency boiler is only going to be notably high efficiency when the water temps are below about 130 degrees so if you have all high temp emitters like baseboard or CI rads, odds are that a regular old cast iron 80% boiler is going to end up costing you less money over the long run than anything HE- and can still be paired with an indirect.

We converted from a Peerless CI to Lochinvar Noble combi (no hot water connected, heat pump water heater). With propane at 3.75 a gallon delivered and probably going up, we are literally saving hundreds of dollars a month and will probably save around $2000 this heating season. We have used 45-50% less propane over the last 30 days compared to 2020.

Lochinvar's combi's preheat the FPHE for DHW and so offer as instant hot water as possible. They also have a DHW response that keeps things moving so there are no cold sandwiches.

to OP: if you go combi, there's some savings to be had vs indirect and certainly vs conventional gas/electric water heaters. But do you have a MNsaves program? We paid $450 for our 80 gallon Bradford White Aerotherm heat pump water heater after the rebate. It replaced two propane fired atmospheric vented water heaters. Approx $1200 a year for the two vs $170 a year for the heat pump water heater. Beats the tar out of any indirect

mrhemi

Another option to consider. Use the mod/con combi with a DHW storage tank. A circ pump for the DHW becomes is another zone on the zone controller, same as an indirect. If you utilize an electric hot water heater as your storage tank, it can be wired but only energized in event of the boiler not being operational, giving you hot water available in that situation. Combi boiler manufacturers show this option in their schematics.