Best Tankless Combi Boiler

Simon7500

Hi Everyone.
Thanks for reading this post. I am thinking of converting from Oil to Gas and get a tankless boiler combi. I have gotten three different estimates. Two of the estimates recommends Navien Combi and One recommends Bosch Combi. Navien Installers swears by the quality and blames Bosch for not having Stainless steel heat exchange. Looking at reviews, both boilers have good and bad reviews. However Bosch seems to be little better reviews than Navien. I am leaning toward Bosch Combi. Do you think Bosch Combi boilers are better or are there any other brand I should look in to?
I appreciate any advice.

hot_rod

tell us more about the application. What is the building heat load, how much DHW are you expecting?

Simon7500

@hot_rod Thanks for reading my post. Currently heating system has four zones with about 160' baseboard. we never used all four zones at the same time. most we used two zones at the same time which consist of about 100' baseboard. We are family of four with two kids. two shower at the same time at most. usually one shower at a time. Hope that answer your question

Ironman

A little info about combi's, baseboards, and gas to oil.

The first thing that you need done is an accurate, scientific heat loss calculation that's based upon ACCA Manual J. That will determine exactly how much heat you need. You can download a free app from SlanfFin and do it yourself, but your contractors should be doing it before proposing different equipment - particularly a conversion from oil to gas. I'll explain why as I continue.

First, typical 7" baseboards produce about 500 btus per lineal foot at 170* Average Water Temp when they're clean and in good condition. So, with 160', your radiation can emitt about 80k btus. We don't know how many btus you need until a load calc is done.

Second, it's not uncommon to find that many homes with oil and BBs have under-sized radiation because the installer cheated out and didn't put enough BBs in. To compensate for this, they cranked the boiler water temp up to 195*+. The Bosch will run up to 194*, but most, if nor all other mod/cons top out at180*. The Return Water Temp for a mod/con has to be 140* or below for the boiler to condense and operate efficiently. Outdoor reset can help it accomplish this, but not with under-sized radiation. Over-sized is beneficial.

A third thing to consider is this: most combi boilers have an onboard circulator that can produce about 5 gpm. With a high temp BB system, that translates to 50k btus that the pump can deliver for space heating. It doesn't matter that the boiler can fire at a much higher rate or that the radiation can emitt more. The circulator is the train that carries the btus from the burner to the radiators and it can only carry 50k btus a 20* delta T (180* supply, 160* return).

If a load calc shows that you need 80k btus at design temp (the coldest night of the year), then a combi that has a circulator that carries 50k btus won't properly heat your house when it gets really cold.

Again, the load calc needs to be done first, then the proper type and size of equipment can be made.

With what you've said so far, I'd recommend a mod/con with an indirect tank instead of a combi.

hot_rod

Of course you would want to assume worst case, design or below design day to warm the entire home. Ideally a room by room load calculation would provide a number to size the boiler to.

Assume 500 btu/ ft from 160' of baseboard= a load of 80,000 BTU/hr. is what that much heat emitter could transfer to the building.

Does the boiler you have now run all zones and maintain 70F on a design, or coldest days? What size is the current boiler?

For DHW a general rule of thumb is 50,000 btu boiler input for 1 gpm of flow. You will find the common 100- 120,000 combis provide a bit over 2 gpm of dhw assuming 40F inlet 110F out temperature.

A "legal" shower head is 1.5 gpm. so the smallest comb1s, 110- 120 can and will keep up with a single shower. IF you want or expect multiple HW loads at the same time you would calculate and add those loads and size the combi for that gpm flow rate.

My wife and I get along fine with a 120,000 combi, A family of home with higher flow shower heads may want a larger size.

The challenge becomes not over-sizing for large dhw loads and compromise the heating size.

Granted all combos can modulate to a point. In your case with only 2 zones running, the heating load might be 40K or lower?

Knowing all the numbers (loads) will help make a decision if a combi is right for you. A separate boiler and indirect would handle large dump loads where all faucets or washer run at the same time.

I don't know that there is a "bad" combi being sold, they all have pros and cons regarding design, control options, warranty, local or factory support, skill level of the installer repair guy, etc.

I have two different Lochinvar combis and one Laars combi 110, 120, and 133,000 on my property. The Laars Mascot is 16 years old. They have all been trouble free and easy to live with once you understand limitations of tankless or combi technology.

I like not storing dhw, and the lower potential for legionella growth, small footprint, etc.that combos offer. Not all agree

Simon7500

@hot_rod @Ironman Thank you soooo much for your elaborated expert advise. I have scheduled a heat loss survey tomorrow. I will update the post with the result.

Ironman

Another consideration with a combi is the hardness of your water. Combi's use a brazed plate heat exchanger which has very narrow passages and that will clog quickly from minerals in the water. If you have hard water, it can clog in a matter of weeks.

If proper valves are provided during installation, the heat exchange can be de-sclaed, but who wants the expense and agrivation of doing that regularly?

lchmb

one other thing to keep in mind is your service provider. What are they comfortable with? Do they train on the units your looking at? Are you happy with them? If not, who?...;) just my 2 cents.. I install a lot of combi's, the biggest thing I see is proper sizing...

Simon7500

You guys are Awesome. Thanks for taking your time to replay. Appreciate all your inputs.

hot_rod

Ironman said:

Another consideration with a combi is the hardness of your water. Combi's use a brazed plate heat exchanger which has very narrow passages and that will clog quickly from minerals in the water. If you have hard water, it can clog in a matter of weeks.

If proper valves are provided during installation, the heat exchange can be de-sclaed, but who wants the expense and agrivation of doing that regularly?

I've noticed the combos have really small plate HX, something like a 3X8 " 8 or 10 plate. I wonder that the small size allows high flow velocity that helps prevent lime up.

The Laars Mascot combi in my mother in laws has been providing DHW for 16 years without a HX cleaning? it will still keep up with a shower and dishwasher running at the same time. Our water is around 17 gpg and high iron content.

I know of a number of tankless in my area that have never been cleaned in 5 years or so?

A relative in NJ has a tankless coil in his boiler, been 20 years and it has never been delimed. I did convince him to replace the over sized boiler and add an indirect.

I would certainly add valves to allow a quick flush if needed, but not all need regular cleaning that I have been involved with.

Ironman

Our well water goes through over 300 feet of sand and is so free of minerals that some old ice machines had trouble initiating a harvest. At the back of our pasture, one of the county wells pulls 1500 gpm out of the same aquifer.

Go 10 miles west of us and the water is ridiculously hard. So hard that I refuse to sell someone there a tankless or a combi.

Having the water tested for hardness would be a prudent measure before choosing a combi.