Looking for COMBI Boiler Brand Advice

[DrS1]

So, I have managed one more year out of my ancient Kenmore/Burnham low efficiency boiler and need to do something about a replacement before next season. My tank style water heater is getting up in years as well, so considering a high efficiency combination unit.

I posted last year, and the HTP/Westinghouse UFT design seems pretty nice. Since then, Menards started carrying a Ruud branded unit. I know most of these are rebranded korean or japanese brands, so wanted to get some thoughts on who makes that. The two specifically I am looking at are:

Ruud RUCBH180DVLN
Westinghouse WBRCNG199W

I don't need anything near that high of capacity to heat the house, but both have a 10:1 turndown ratio... the 140k unit actually has a higher minimum btu output.

Any others I should be looking at?

 

[Fitter30]

Nortek Global HVAC (formerly Nordyne), which licenses the Westinghouse name and 100-plus-year brand history. Nortek Global HVAC is a subsidiary of Nortek, Inc.Apr 21, 2020
Ruud is owned by Rheem both good companies. Would want to know how the warranty and parts availability is in your area especially if your going to be the installer.

 

[Sponsor]

 

[Dana]

So, I have managed one more year out of my ancient Kenmore/Burnham low efficiency boiler and need to do something about a replacement before next season. My tank style water heater is getting up in years as well, so considering a high efficiency combination unit.

I posted last year, and the HTP/Westinghouse UFT design seems pretty nice. Since then, Menards started carrying a Ruud branded unit. I know most of these are rebranded korean or japanese brands, so wanted to get some thoughts on who makes that. The two specifically I am looking at are:

Ruud RUCBH180DVLN
Westinghouse WBRCNG199W

I don't need anything near that high of capacity to heat the house, but both have a 10:1 turndown ratio... the 140k unit actually has a higher minimum btu output.

Any others I should be looking at?

The Westinghouse unit is EXACTLY the same under the paint as the HTP. (The Westinghouse tech help line rings in HTP's headquarters.) It's manufactured by Kiturami, a first tier Korean company that competes head to head with Navien (Kyung Dong Navien boiler company) in the home country, which is another vendor to consider. Navien's NCB-E combi boiler series can work too.

Pay attention to the amount of radiation you have to work with, zone by zone, if you hope to run at condensing temperatures without short cycling. The napkin-math version of how to analyze it lives here. If a zone doesn't have sufficient radiation to emit the full minimum-fire output at condensing water temperatures it can be problematic.

 

[Peter20]

Pay attention to the amount of radiation you have to work with, zone by zone, if you hope to run at condensing temperatures without short cycling. The napkin-math version of how to analyze it lives here. If a zone doesn't have sufficient radiation to emit the full minimum-fire output at condensing water temperatures it can be problematic.

Thank you for writing that article, it was very educational!

If I understand correctly, the fact that fin-tube radiators need 180-degree water to emit 600 BTU/hr per foot means that radiators that were originally installed with an old high-temperature boiler will only meet their designed load when a replacement boiler is also providing 180 degree water. That means that any new high-efficiency boiler will probably almost never operate at condensing temperatures with existing radiators, since you'd only be meeting about 1/3 of your heat requirement at 120 degrees. (You'd need an entirely new install with 3x the length of radiators to get full efficiency at full load.)

Do I have that right?

I imagine a well-configured outdoor reset curve might occasionally get you down to condensing temperatures during the shoulder seasons, but that's about it. Or does it end up better than that in practice?

 

[Dana]

Thank you for writing that article, it was very educational!

If I understand correctly, the fact that fin-tube radiators need 180-degree water to emit 600 BTU/hr per foot means that radiators that were originally installed with an old high-temperature boiler will only meet their designed load when a replacement boiler is also providing 180 degree water. That means that any new high-efficiency boiler will probably almost never operate at condensing temperatures with existing radiators, since you'd only be meeting about 1/3 of your heat requirement at 120 degrees. (You'd need an entirely new install with 3x the length of radiators to get full efficiency at full load.)

Do I have that right?

That's the gist of it. Figure on 200 BTU/hr per running foot of baseboard (typical- it varies a bit by baseboard model) at an average water temp of 120F (125F out of the boiler, 115F returning from radiation.)

I imagine a well-configured outdoor reset curve might occasionally get you down to condensing temperatures during the shoulder seasons, but that's about it. Or does it end up better than that in practice?

You imagine incorrectly. Typical existing systems were/are 2x oversized for the load at an AWT of 180F, and can still heat the place just fine with 140F water at the 99% outside design condition, and much lower water temps for the majority of the heating season. At 180F using deep overnight setbacks usually works just fine even in uninsulated buildings with leaky single pane windows due to the excess heat emittance. If you have a heating history on the place it's possible to more accurately estimate 99% load, and dividing the load by the radiation length thereby the water temp requirement needed at design condition (only 1% of the hours in a year would require higher water temps) by referring to the baseboard manufacturers' charts.

Even if it needs to be too hot and out of the condensing zone to cover the load design condition, it can still run in condensing mode the vast majority of the hours in a heating season. The average wintertime load is still going to be something like 50-70% of the 99% load, and the average seasonal load much lower than that, not just during the shoulder seasons.

 

[Peter20]

Thank you for your help. Now I'm on to refining our outdoor reset curve. Thanks to your advice, I dropped it way below the initial setting: it was replacing a non-modulating boiler, and the combi boiler was sized for DHW rather than the space heating load, so I don't need 180 degrees pretty much ever.

So now I have a question about the "ideal" curve. In the comments of your article at Green Building Advisor, you suggested:

...tweaking in the water temp at the cold outdoor temp end of the curve first, dropping it 5F at a time until it's not keeping up overnight, then bumping it up a degree at a time until it keeps up with the setpoint.

If you match the curve exactly to your heat load, it seems like you would never be able to recover from a setback, or other increase in the requested room temperature, since such a curve would never provide more heat than you're losing.

Do I have that right?

So is the solution not to use setbacks, or is the curve in practice never so exact as to interfere with thermostat adjustments?

Is it more efficient to abandon setbacks if that allows a lower water temperature?

 

[Dana]

To be able to use temperature setbacks, boilers have a "BOOST" function that after some (user programmable) amount of burn-time bumps up the water temp by some (user programmable) number of degrees above what the outdoor reset is calling for.

But yes, if the curve is dialed in to perfection you don't need the thermostat- jumper it to always call for heat and the house will just stay within a fairly narrow temperature range.

 

[Peter20]

Interestingly, the Lochinvar combi (Noble) doesn't appear to have a boost function, only the stand-alone Knight boiler does (off by default).

I thought the most efficient scenario was the one with a perfect curve and the modulating boiler running 100% of the time? The existence of a boost mode seems to conflict with that, since (if enabled) it considers burn-time over a certain duration to be abnormal. Is the optimal duty cycle something other than 100%? Or is this just indicative of the unavoidable compromises in life?

On a system without boost, it sounds like I should (a) not use setbacks, and (b) calibrate the outdoor reset slightly higher than "perfect" so that small manual adjustments to the thermostat can (eventually) be satisfied.

As I understand it, the efficiency gain of a setback would be canceled out (or worse?) by the unnecessarily cycling resulting from a water temperature higher than required for the heating load?

 

[Dana]

I thought the most efficient scenario was the one with a perfect curve and the modulating boiler running 100% of the time? The existence of a boost mode seems to conflict with that, since (if enabled) it considers burn-time over a certain duration to be abnormal. Is the optimal duty cycle something other than 100%? Or is this just indicative of the unavoidable compromises in life?

The highest combustion efficiency possible would be a 100% duty cycle with a perfectly tuned curve, but there are many circumstances where setbacks will still yield some additional savings, particularly if the setbacks are long in duration. Most people relying on the BOOST function use setbacks primarily for sleeping comfort, not energy savings.

On a system without boost, it sounds like I should (a) not use setbacks, and (b) calibrate the outdoor reset slightly higher than "perfect" so that small manual adjustments to the thermostat can (eventually) be satisfied.

I think you intend to say setting the water temp curve lower than needed, and bumping it up carefully as-needed for comfort.

As I understand it, the efficiency gain of a setback would be canceled out (or worse?) by the unnecessarily cycling resulting from a water temperature higher than required for the heating load?

It's not so much the cycling that takes the toll on efficiency as it is the lower combustion efficiency delivered by the higher water temps and firing rates needed to recover from setback in a reasonable amount of time. If the setback water temp was calling out 110F from the boiler running a less than 25% of it's max firing rate the combustion efficiency would hit the mid-90s. If not setting back the thermostat with a perfectly tuned curve it might only need 115F water, yielding a combustion efficiency in the low 90s, but if it takes 130F water and firing rate north of 50% to bring the space up to temp in under an hour it'll likely be in the 87%-88% efficiency range. Ponder this representative graphic for a bit:

Whether using BOOST or not, it's clearly in your interest to fine-tune the reset curve a bit if efficiency is a primary goal.