Standard vs high efficiency combo

Ignoramous · October 12

My 1980s Sears Boiler Furnace went on the fritz and I've decided to bite the bullet going new vs chasing cause of malfunction. It was a good run. After having 2 companies over to quote, I am not sure whether to go with standard or high efficiency combo.

Company 1: High efficiency combo has them on more repair calls than standard, and is significantly more expensive. Thinks they will improve in coming years but he's still recommending standard.

Company 2: Disagreed on repair frequency. Said high efficiency models may have shorter life vs standard which is an actual drawback, but energy cost savings and rebates offset that downside. Quoted Navian NNNC240110H high efficiency/water heater combo. I was surprised only a $ cost difference after rebates hearing warning from Company A. Added upside is replacing a large water heater that is probably <> 3-5 year life.

Any suggestions on how to make the decision or other facts I should consider?

SuperTech · October 12

What type of heat emitter do you have? Radiant floor heating? Baseboard heating? Cast iron radiators?

If you have radiant floor heating then I recommend going with the high efficiency condensing boiler. Anything else I prefer a standard cast iron boiler. Radiant floor heating uses low temperature water, with low temperature heating the condensing boiler always condenses the flue gas and works at its high efficiency. With high temperature baseboard and radiators the high efficiency boiler doesn't condense flue gas as much and isn't worth it in my opinion.

I don't recommend a combi, ever. Or Navien. Combis need to be massively oversized for the space heating load just to make hot water on demand and this leads to excessive cycling and wear and tear on the boiler. An indirect water heater is my preference.

Did anyone bother to do a heat loss calculation to see what size boiler you need? Hopefully they aren't basing the size of the boiler they are quoting off the size of the old one, most boilers are 2-3 times oversized.

Jamie Hall · October 12

Please remove the pricing from your post, @Ignoramous ,

That said, I agree with @SuperTech . While there are rare situations where a combi is right — they are rare. I also second the rest of his comment.

Ignoramous · October 12

https://forum.heatinghelp.com/discussion/comment/1814001#Comment_1814001

Thanks for both replies. It really changed my lean on the options.

I have circulating baseboard/copper pipes only, two zones in a split home <> 2400 sq ft. Unsure on size of the unit as the description on Contractor A quote is only "Entry Level Standard Efficiency–Comfort System" and doesn't even provide brand, which struck me as odd.

hot_rod · October 13

its going to come down to what you are most comfortable with

You will find a fairly even split here regarding standard efficiency vs high efficiency or combis

I have owned a number of combis, got 16 years out of one of them. I have had good results with combis

I strongly disagree that it is a rare situation when they are the right choice. But I talk from actual experience.😉

As for fin tube, 80% of the heating season they can run lower temperature than a design 180 SWT, this combined with modulation, ODR, step firing makes mod cons and combi a good choice.

Some weather data to demonstrate that concept. I believe 0 to -5 is design in this city.

Screenshot 2024-10-12 at 6.59.50 PM.png

Screenshot 2024-10-12 at 7.00.20 PM.png

offdutytech · October 19

There's a few options out there for you. A modcon boiler when properly installed and serviced is going to be efficient to run. The outside air rest and turndown on the burner is going to win hands down for effective control of water temps vs standard boiler.

Viessman makes a nice solution blending cast iron with modcon boiler in their CU3A, it's pricey but a quality boiler. Combis have their place when space is at a premium. Sizing a combi sometimes can be tricky because you need the BTUs for heating domestic water, but the min fire rate on the heating side might be too much for that boiler. This is where a proper load calc needs to be done when replacing any form of heating equipment.

You can also get a modcon and use an indirect tank for domestic hot water. While Navien isn't my first choice in the modcon world, properly installed they do work. I actually just quoted a job the other day for your same scenario. Standard boiler vs Navien Combi at the request of the customer. Not much of a price difference when you factor in rebates / tax credits and what it would cost to replace the domestic hot water tank.

Just make sure if you go the modcon route your quote included flushing the system and a magnetic filter on the heating side.

Ironman · October 19

A combi is generally gonna require a lot more maintenance, particularly if you get one that has a plate heat exchanger like the Navien, and/or if you have hard water. Navien also has a very high failure rate with heat exchangers and 3way valves. I’d avoid them.

Some manufacturers use a miniature indirect tank built into the combi which is a better design.

We prefer using a mod/con heating boiler (not a combi) with an indirect water heater. There’s far less maintenance and a longer life expectancy than a combi.

In either scenario, make sure that the contractor that you choose is competent in diagnosing and servicing mod/con boilers. Most of them are not.

SuperTech · October 19

Ironman is correct. Unfortunately I encounter more problems and poor installations on combis and condensing boilers than anything else. Much more attention needs to be paid to installation and service on them. A condensing boiler when properly installed can be a work of art, unfortunately most of them are not. I would strongly recommend staying away from Navien, they suffer from numerous quality problems even when installed perfectly. Much better options are available. I like the cast iron boilers from Peerless,Weil McLain and Crown. For condensing boilers I like the fire tube heat exchanger boilers like the Triangle Tube Prestige Solo.

Standard vs high efficiency combo

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