ModCon ODR Sweet Spot

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John Molyneux

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Now that we've had a decent stretch of real New England winter weather I think I've found the sweet spot for the outdoor reset curve on my new boiler, which is a Bosch Greenstar 57. It's installed in a 1940 1400 sq ft reasonably well-insulated cape. We started with a fuel-based heat loss calculation that came in around 30,000 Btu/hr at 4F design day conditions. This was subsequently confirmed by a Manual J heatloss done as part of an energy audit. We are fortunate in that our existing cast iron radiators have an EDR of 560 ft -- obviously way more than enough radiation to meet our heatload under a variety of supply water temps (SWT).

See http://www.comfort-calc.net/userfiles/images/Heating Helper/CI_Radiation_Heat_Emission.png

Based on the above chart I figured I should be able to heat the house on a design day with 130F SWT, so that's where I started. As it turns out I've been able to drop the design day point on the curve down to 114F and have been able to keep the thermostat satisfied during the coldest weather we've had so far (some high single digits, lots of hours in the 'teens and 20's). A couple cold nights I dropped it down to 112F but couldn't maintain the temperature in the house so I bumped it back to 114.

The boiler typically just hums along at low fire cranking out SWT between 95F and 105F with relatively little cycling. (It cycles more than I want it to but that's mainly because we have a small basement zone that needs more radiation to handle such low water temps. It keeps pumping but the burner has to cycle on and off when it's just the basement zone calling for heat.)

The controls on this boiler don't tell me what my return temps are but on the colder days I can tell I have a pretty decent Delta T just by feeling the supply and return pipes. The boiler is condensing like crazy. The condensate pump is the noisiest part of the system and seems to fire every hour or so. I love it.

The very first thing I noticed about the new system is how much more comfortable the house is. I wasn't expecting that. The radiators just stay pleasantly warm and the temps in the living spaces are very consistent. I should note that at the same time we installed the boiler we implemented a number of the energy audit recommendations. The biggest part of that was taking care of our knee walls properly and sealing everything up. So the comfort I'm seeing is part heating system and part envelope upgrades. Nonetheless, I've become a true believer in 'gently heated water.' The other thing I've noticed is that there is relatively little noticeable exhaust plume during normal operations, especially compared with my old unit. During heat calls the polypro exhaust vent barely gets warm. But I can see much more plume during a DHW call when the boiler is at high fire and not condensing.

By the way, I did all this with a combination of very generous rebates and a low-interest energy efficiency loan. Almost no cash out of pocket and an immediate ROI.

Based on how the system (house and boiler) is performing I think I/we probably overestimated my original heat loss and then improved it even further with the recent envelope upgrades. I'll be interested to measure my 2015/16 actual winter fuel use against heating degree days to see what the year-over-year difference looks like. My educated guess says that I will have reduced my heating-related fuel use by half (meaning I'll be able to brag about significantly reducing our household CO2 emissions as well). We switched from propane to natural gas, so my fuel cost would have been cut by half anyway, all else being equal.

As I've posted previously, going through this decision-making process was a real eye-opener. I had done my research here and elsewhere and had a pretty good handle on boiler sizing, installation parameters, etc. Turns out I ended up knowing more than some of the local contractors I had look at the job, two of whom wouldn't even consider installing such a "small" boiler (which replaced a 140,000 Btu clunker). The highly experienced Bosch-certified contractor I ended up with had actually never installed the 57,000 Btu model, even though my house is pretty typical for the area. They all really seem to like the 110 combi's. And despite the fact that they did a nice job with the hardware installation, they pretty much left the control parameters at their factory defaults. It makes me sad that so many fancy modcons will never deliver their promised AFUE because they're getting installed without sizing and tuning them to site-specific conditions.

On an off topic subject, but related to my interest in energy efficiency -- we've also recently seen a pretty dramatic drop in our electric usage. This might be partly due to the new boiler but is probably more related to the fact that our 20-year-old washer and dryer reached the end of their useful lives within days of each other just before Thanksgiving. We picked up an LG front-loader on sale and I can pretty much trace the electricity reductions to that date. These new washers are not only efficient at washing but they spin so fast that it substantially reduces dryer time. Nothing else has changed but our bill dropped by around $20/month.
 
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Dana

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If the hydronic system is now running only with ECM drive circulators that's definitely part of the electricity savings. But you're right- the new LG washer/dryer with the high spin cycle and the dryness sensing are probably the lions' share of the savings.

When calculating the heat load from fuel-use against weather data on old and oversized boilers it's usually an upper-bound type of number. Most Manual-J's overestimate too, usually with higher-than-reality air infiltration numbers.

With 114F output water even on the colder nights it's no surprise that it's condensing copiously, and sending fairly dried-out exhaust out the vent, with no plume to speak of. You're probably hitting the high-90s for average combustion efficiency. In a high wind it might still lose a bit of ground to higher infiltration losses, but you might not notice with the evenness of the heat flux off the radiators. For yuks you might want to run a fuel-use against HDD calc using the new boiler, using it's nameplate AFUE as the estimated efficiency. (I'd be very curious to see where it comes in!) It's actual as-used efficiency can even be higher than it's tested AFUE since your operating temp is so low, but it won't make a huge difference.

As you have discovered are too few mod-con installers who even understand the principles and can do the craft work well, and fewer still who are willing to sign up for multiple call-backs to tweak in the ODR curves. But its not rocket science to just play with it until it's pretty much dials, just the way you did.

Enjoy the modulating even-temperature comfort, and the fuel savings!
 

John Molyneux

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And it's not just contractors. Bosch sets 168F as the default design output when using radiators, and 176F for baseboard/convectors, which results in a curve that probably won't condense during much of the heating season and may never, ever operate at AFUE.

(The laboratory conditions they use to calculate AFUE seem to be designed for comparability between boiler models and are not based on real life operating conditions, at least according to this helpful video from Caleffi):

Folks really need to understand that ModCons only start condensing at around 130-140F return water temps, meaning that's when they just begin to become more efficient than a non-condensing boiler. If your goal is efficiency you need to aim much lower (see attached graph). Seems like 100F RWT is a better target. And also keep in mind you won't ever get maximum efficiency at high fire. (I assume this is because of air flow through the heat exchanger?)

Having said that, it might be helpful if our local experts would comment on the stand-alone benefits of modulation, irrespective of condensing. I expect there are various benefits to having a modulating boiler even if it doesn't condense much. Any opinions on that?
 

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Dana

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Every burn cycle (condensing or not) has a fuel-cost in every ignition cycle and an energy cost penalty in every flue purge, as well as idling losses. Short cycling a boiler makes those ignition cycling losses a double-digit percentage of the fuel use. Ridiculously oversizing a boiler can turn the idling losses into high single-digit percentage of the fuel use.

Modulation of a right-sized non-condensing boiler guarantees that it's as-used performance hews very close to it's raw combustion efficiency, since the number of burn cycles and flue purges per day/season are low, and it spends more of it's time firing rather than idling.

Most natural gas mod-cons don't hit the condensing zone until the return water is in the mid 120s, which corresponds to 130-140F out, not in.
 
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BadgerBoilerMN

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This is like a going to hydronic heaven; where every boiler is right-sized for the radiation and heat loads, with built-in weather responsive controls, direct vent, sealed combustion, silent operation, lower emissions, lower operating cost and perfect comfort in all weather.

Just shoot me now...

Thanks for the link Dana.
 

BadgerBoilerMN

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Not to pick, but condensing boilers are always more efficient than conventional atmospheric boilers. You start by shutting down the ancient chimney, the controlled exhaust modulating output and by electrical draw, and there is the outdoor reset--standard equipment--so thermal efficiency 96%, combustion efficiency 86% and going up as the return temperatures go down.

There is the ideal, but nearly any application is enhanced by a ModCon.
 
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