View Full Version : New "high efficiency" system operating costs are HIGHER -- help!
12-12-2011, 10:40 PM
Replaced an aging oversized forced-air gas furnace and gas water heater with a high-efficiency Triangle Tube Prestige Solo 60 boiler, TT Smart indirect water heater and runtal radiators. We were annoyed to see that our gas and electric use have gone up and not down!
The bill provides a comparison of the same billing period the previous year, as well as average temperatures for the two periods. The average temp was the same (42F in October-November). The bill says that we used an average of 1.9 therms per day last year (for a total of 62), compared to 2.1 therms this year (for a total of 70.4.
What other causes might there be for this? We didn't make any other major changes to the house and there are the same number of people living here so I am suspecting something in the installation of the new system isn't right. Any thoughts or questions are appreciated. Thanks!
Man, I really thought we'd see our energy use go down....
Runs with bison
12-13-2011, 12:38 AM
I would wait to see another, colder month first. The difference given is small enough that it isn't really clear that efficiency has really fallen, although you have reason to suspect it has. If on the other hand you see similar results for one or more of the three coldest months of the year (corrected for relative delta T compared to the balance point), then start digging deeper. Not that you can't start digging into it now, I just wouldn't call the contractor until you have some specific things to point to. Having this past and the next month both come in on the high side would suggest a real problem.
I can't speak for residential boilers, but one thing I've noticed about new AC units is that they often have electrical resistance heaters on them to keep the compressor warm when not in use and thereby prevent starts flooded with liquid refrigerant. On two stage units in particular that is an incredible waste of electricity for at least 6 months out of the year. (The crankcase heaters run about 40W each, 24/7 based on my measurments and some descriptions I've found online.) During cooling months the heater elements don't run as much because the compressor(s) run periodically and the heaters only run after X number of idle hours. The solution to the problem is simple: during non AC months I flip the AC breaker to the off position. When it is time to cool again I flip the breaker on 8-12 hours before I anticipate needing AC again. I leave it that way until we exit AC season. Ideally they should be instrumented to do this sort of thing on their own...but the controls aren't that smart at present and instead seem to default to heat pump protection settings.
The difference is on the order of 10% which is significant, since the mod-con should have a ~15% higher average steady-state efficiency than an ~80% furnace or an atmospheric-drafted HW heater.
What temp are you running the radiation? It takes return water under 120F to get substantial
Using "average temp" numbers during the shoulder seasons can sometimes be deceiving, if there was a warm period of near-zero heat load in one of the comparative months, and not the other. A better method would be to use base 65F heating-degree-days for the exact billing periods, and work from fuel use per HDD. You can download historical HDD data for a weather station near you in various formats from degreedays.net (http://www.degreedays.net/). See if that works out any better.
At fuel use numbers that low a few extra showers (or longer/shorter showers) in a week can insert an incremental error as well, but not 10%.
12-13-2011, 09:39 PM
With a new indirect, have you been luxuriating in longer, hotter showers?
12-13-2011, 10:41 PM
Nope. We're still the same tree-hugging eco-freaks :-)
The installer came by today to check in on the "thud noise" I posted about here:
It turns out that although the controller had been replaced, it was not a corrected controller -- they sent one that had the same problems!
Apparently, a new one is on the way. The correct units have a green dot or sticker on them, which our new one did not. I am hopeful that we'll get better efficiency/performance out of the unit with a proper controller. I'll report back on that.
(As usual) Dana has provided another lead -- that the return water temperature needs to be 120 degrees or lower otherwise the unit will not achieve its highest efficiency. As far as I can tell, the return water temperature is not nearly low enough. Leaving is 154-deg and returning is 150, according to the display on my TT Prestige Solo 60.
I need to do some more reading/research to understand all this, but my current understanding is that if higher BTU radiators were spec'd that they'd pull more heat out of the water, make the rooms toastier faster (which is something we'd really like -- it seems like it takes too long to heat up the house), AND the return temperature would be lower, making the unit operate more efficiently. Is that correct? It almost seems ironic that we'd get higher efficiency out of pouring more heat into our livable space!
12-13-2011, 11:27 PM
only losing 4 degrees on a loop seems very off. I don't diagnose radiant systems like that very much, so can't give a lot of input, but I know that my radiant floors are supposed to drop 20-25 degrees, and they're a more efficient system than individual radiators. it sounds like something is wrong that your radiators are not using the heat going through them very much. you would lose 4 degrees pretty easily just by looping the water into the house and bringing it back again w/o any radiators.
fast heating and efficiency are mutually exclusive on a radiator. You're not using a setback thermostat are you? that works well on a forced air system, but on radiant systems you don't want to turn the temp up and down... just set it and forget it. recovery times on radiators are slow and take a lot of energy... you'll lose all the benefit of the lower temp just in heating the space back up. if you were to be leaving for a week or something, then it would make sense to turn it down for that, but don't cycle temperatures on a daily basis.