Combi-boiler replacement

Niels

We have a 15 y/o NTI gas combi-boiler system that is on its last legs. The latest problem is that the valve that switches between DHW and Heat gets stuck on DHW and we have to physically prod it to get it moving back to the heating position. There have been other issues with electronics and a mixing valve as well. We are on a first-name basis with our boiler tech at this point. It's a 2-zone system serving our 2-floor living space and basement rental.

That said, we've gotten two recent quotes for a replacement NTI system, one for a TX200 and another for a TRX199C. The latter quote, from our current company, is 40% higher than the former, which appears to be largely related to the model specified. I say this because they had previously quoted us on the TX200 about 1.5 years ago when we first started thinking about replacement and the quote was comparable to the other company's TX200 quote.

So my questions are:
1. Does the difference in model justify this large price difference?
2. What other manufacturers should I consider?

Thank you for reading my question!

EzzyT

I and many others on forum have never been a fan of wall hung combi boilers since they tend to be to large for the heating side.
I would suggest a stand alone boiler with a indirect hot water tank for the domestic hot water. 

The brand of the boiler isn’t near as important as the right contractor to do the work. 

Where are you located?

EdTheHeaterMan

It appears that he 199C is a new model that is slightly more efficient. Not AFUE efficient. The turndown ratio is 10 to 1 This means that the heating unit can operate as low as 19,700 BTUh. This means that when it is only a little cool outside and your home needs only 20,000 BTUh, the heater is the correct size. as the outdoor temperature drops the heater becomes larger as needed. It has a maximum heating input of 197,000 BTUh.

The 200 is a less expensive piece of equipment. the turndown ratio is from 120,000 max space heating input down to 25,000 BTUh input. So when the outdoor temperature is cool as above, and you only need 20,000 BTU the heater will need to cycle off and on to maintain a comfortable indoor temperature.

This is not a big deal since the difference in operating cost will be less than 2%. The AFUE ratings are the same. That is supposed to mean that when it is colder outside, the fuel usage should be the same.

Some homes do not have space for an additional tank as EzzyT suggested.

Look at Bosch Greenstar. I have had much success with them. Stay away from Weil McLain, I have had nothing but problems with their Combi and their Ultra

hot_rod

Another interesting thing happens when you turndown a large boiler 10-1 you expose a large HX surface to a small flame and get a lot of condensing, so a small additional efficiency bump.

mattmia2

I would vote for the indirect if you have the space.

Big Ed_4

Being a serviceman and worked on most , I could only recommend the German Bosch Green-star. It's a servicemen boiler .... They just work with the best engineerings ...

Niels

Thanks for all the replies guys.
The indirect DHW tank sounds like an interesting idea. Is it mechanically simpler? The closet is 63" x 35", with a 24" opening. Would that be enough?

@EzzyT I'm in Washington DC.

EzzyT

@Niels reach out to @Dan Foley from Foley Mechanical to give you an estimate. 

Niels

Thanks @EzzyT I'll follow up with @Dan Foley

mattmia2

An indirect is always hot so you get hot water immediately but is also can be well insulated so it has little standby loss. It usually means you can use a smaller boiler and can handle larger peak demands. The incoming water temp makes a big difference in what a combi can handle, in a warm climate you can get a good rate out if it, in a cold climate it might be very limited. It isn't less complex per se but the parts are very standardized and very reliable. You can get stainless steel tanks that can last for several decades depending on your water quality. A combi can be a good choice in the right application as well.

motoguy128

Indirect costs more up front because the tank isn’t cheap, but better performance overall, and no 3 way valve and plate heat exchanger to plug up over time.  

I like combi if price is ciritical and they are fine in high mass radiant or former gravity hot water.  But a poor choice  for fin
tube baseboard or multiple temp zones or micro zones.  A 80 or 100k boiler can often replace a 180 or 200k Combi with much lower min fire rate.  

Niels

I ended up hiring a guy to run a Manual J calculation. He came up with 90 MBTU heat loss, which the Navien could handle with 26% safety factor. The NTI seems like overkill given the price difference. $2K saved, so far.