Feedback on boiler and water heater replacement

[Faderus]

Hi everyone, thanks in advance for any help.

I have a 1960 split level house in Poughkeepsie, NY (12601). The insulation is original in the exterior walls but I had the roof deck and garage ceiling and common wall Spray foamed (open cell, 8" on roof deck with joists encapsulated 1-2", 6" on garage ceiling and 4" on common wall.) Approx 1100 sq ft' upstairs and 600 sq finished basement area with about half the wall height below grade.

Currently I have a 160k BTU 80% NG boiler and 12-15 year old NG water heater. Everything is working well but the water heater is getting old, and I'd like to reclaim the space where the water heater and boiler live (immediately at the foot of the stairs.) There are 2 zones, Upstairs and down. The bedrooms (above the garage) tend to be somewhat colder than the living area upstairs so I was considering splitting the upstairs into 2 zones.

I used the SlantFin calculator and came up with the following numbers for the upper floor:

Living area (LR, Entry, Kitchen and Dining area):

20,344 BTU 42’ BB

Master BR: 5400 BTU 17’ BB

BR 2: 4500 BTU 9’ BB

BR 3: 2600 BTU 7’ BB

Bath: 1395 BTU 3’ BB

I haven’t run the basement calculations yet but expect them to be around 8k.

I looked into this project 18 months ago and got a variety of boiler recommendations but now EVERYONE is recommending Navien systems of various sizes for HW on demand and baseboard heat.

I was initially looking at a Buderus GB142 with an indirect tank under the stairs to maximize space. I was told by one installer that Buderus is having issues with the heat exchanger warranty and that was why they stopped recommending them.

Overall my goal is to maximize space, and save some money on utilities but I do not want to introduce a lot of headaches for the sake of saving a couple hundred dollars a year. I am also suspicious of the longevity of the Navien systems, but hesitant to overrule the equipment recommendations of the installers.

I have a Solar array being installed later this week so will have access to relatively inexpensive electricity if that makes a heatpump water heater make more sense.

Any input or suggestions?

 

[Tom Sawyer]

I think that unless you are willing to rip the baseboard out and install European style radiators and re-pipe the entire house that going with anything that condenses to achieve maximum efficiency will be a waste of money. Besides the unit not achieving maximum efficiency, it will most likely short cycle too which drastically shortens it's life. Even under ideal conditions, most of these condensing wall hung boilers are only going to last ten years or so. Look into a system 2000, gas fired or a Biase cast iron three pass boiler with a power gas burner.

 

[Sponsor]

 

[Faderus]

Are you saying there will not be enough heat given off by the existing, slant fin baseboards at the temps required to make the Navien run in condensing mode? If so, that was one of my concerns. If I were to leave the top floor as a single zone would it help that issue at all?

When you refer to European baseboards is http://www.radiantheatproducts.com/Runtal_European_Baseboard.php what you had in mind? Why would the house have to be repiped?

 

[Tom Sawyer]

Yes, your slant fin baseboard is designed to give right around 600 btu per lineal foot (slightly less) @ 180 degree inlet temperature. At inputs of 150 or less it puts out considerably less heat. Making the entire house one zone would help with condensing temperatures but would probably lead to uneven heating. Runtal is exactly what I was talking about although there are several other companies making the same basic product. Re-piping is necessary because you want to be able to control each individual radiator with thermostatic radiator valves.

 

[Daniel Collick]

Dunno, but this may help:
http://www.triangletube.com

 

[Faderus]

Dunno, but this may help:
http://www.triangletube.com

how would using a different brand of condensing boiler help with the issue mentioned above? Did I miss something in the link you provided?

 

[Faderus]

Yes, your slant fin baseboard is designed to give right around 600 btu per lineal foot (slightly less) @ 180 degree inlet temperature. At inputs of 150 or less it puts out considerably less heat. Making the entire house one zone would help with condensing temperatures but would probably lead to uneven heating. Runtal is exactly what I was talking about although there are several other companies making the same basic product. Re-piping is necessary because you want to be able to control each individual radiator with thermostatic radiator valves.

Ok, thanks. You are basically mirroring the doubts I had in their recommendations. Based on the info provided what would be your recommendation for a wall hung (space is a consideration) that would also provide hot water sufficient for a family, specifically 2 showers simultaneously and/or a dishwasher? If its a better option, is there an indirect that would fit under the stairs in a split level and still provide sufficient volume?

 

[Daniel Collick]

how would using a different brand of condensing boiler help with the issue mentioned above? Did I miss something in the link you provided?

Sorry, just noticed you're in NY. I'm sure your winter temps are much cooler than in the PacNW. During much cooler winters, I think you're right, a condensing unit would likely be overworked and unlikely to achieve the performance as well as a shorter lifespan.

 

[Faderus]

Sorry, just noticed you're in NY. I'm sure your winter temps are much cooler than in the PacNW. During much cooler winters, I think you're right, a condensing unit would likely be overworked and unlikely to achieve the performance as well as a shorter lifespan.

Ok, thanks. Do you have a recommendation for an appropriate combination of pieces, or a recommendation as to what to avoid? Either way helps to narrow down my options a bit.

 

[Tom Sawyer]

forget the wall hung, condensing boilers. Look into system 2000 or any two or three pass boiler with an indirect for hot water.

 

[Dana]

If you want to cut the thing up into a bunch of different zones with tiny amounts of low-mass heat emitters like fin-tube baseboard, you're better off going with a buffer tank centric system design, or a self-buffered unit like the HTP Versa Flame or Hydro combi systems. The System 2000 units will do pretty well due to it's excellent heat-purging control strategy, but even those can be short-cycled to death on zone-calls with the miniscule zone output of your existing radiation when cut up into zones.

Your total heat load is on the order of 35,000 BTU/hr (the Slant-Fin tools always overshoot reality by double-digit percentages, being basically an I=B=R methods tool). It might be 40K when you include the basement (more if the basement isn't insulated), but probably not.

To get condensing efficiency out of the Navien the average water temps need to be under 125F. Typical fin-tube baseboard only emits about 240-250 BTU/hr at that temp. Right now you have only about 80' of baseboard, so to get condensing efficiency without cycling the min-fire output of an un-buffered mod-con has to be less than (80' x 250BTU/ft-hr=) 20,000 BTU/hr. The smallest Navien combis have output lower than 20K, but just barely. When you start cutting it up into zones the min-fire number has to shrink to balance with the smallest zone, or it'll short-cycle like crazy, cutting into efficiency and boiler longevity.

With the zoning as-is your smallest zone is already asking for a min-fire of less than 10,000 BTU/hr, and you need/want to cut one of the zones in half. Since the room over the garage has different heat characteristics than the rest of the house (a common temperature balancing problem for "bonus rooms") it really wants to be it's own zone for comfort's sake, which pretty much rules out mod-cons without a buffering thermal mass. If you're going that route with a combi, you either find a hydronic designer you trust to design a system around a condensing hot water heater like a Polaris or Vertex, but it's easier (= less design risk) to go with an HTP Versa, which are purpose built as combi systems, complete with outdoor reset, etc.

With a design heat load of 35,000BTU/hr and 80' of baseboard that's ~435BTU/hr per foot of baseboard, at the design peak, which means under those weather conditions (the coldest 88 hours of the year) you'd need an average water temp of about ~145F (155F out, 135F return), which would be above the condensing zone. But at your average winter temp you'd be able to get condensing efficiency out of a condensing boiler.