Comparing Heating Degree Days

Kjmass1

I'm trying to do some analysis of my energy usage from last year to this year. I've done a lot of improvements but was wondering how to choose a base temperature...it's a single family house. I keep the thermostat at 73. Last winter however the thermostat was set at 55 as it was under construction. What should I use as base temperatures to compare the 2 heating seasons?

Thanks.

STEVEusaPA

I wouldn't worry about last year. Under construction to me means to disregard. Too many factors, too many variables. You kept it @ 55. Did the drywall taper come in and jack it 75 when u weren't there? And now you have it at 73. You're better off comparing next season to this season and go from there. Or compare this season to 2 seasons ago.
Easiest would be to compare how many gallons of heating oil (if it's oil heat) to how many degree days. For example, if you filled up your tank on Sept 30, 2013, and filled it up again on Dec 1, 2013, figure out how many degree days between those days. Then do the same for this year.
There are many websites for your area where you can get Degree Day Info.
What exactly are you trying to figure out? Is it something like the effect of upgrading windows/insulation?

Kjmass1

Yeah- last years data probably isn't of much use but was looking to quantify work I put in...new windows, spray foam roof, blown in walls. Lots of radiator/boiler/insulation updates. I've got some good data and charts for my usage this season. Looks like I'm about 9-13 btu/sf but I thought I would be better than that. I'm assuming my old steam boiler is the last hurdle. It's 80% steady state so I imagine I could get close to 20% efficiency gains with an 82% afue?

Kjmass1

Others on this forum have mentioned that with older boilers that their rating only applies once the boiler has started to make steam...so it is 0% efficient say for the first 5 minutes before it makes steam, then 80% for the next 15 min before it turns off. Kinda like a car getting on to a highway. So in reality it could be 60-70% efficient.

New boilers with AFUE ratings are based on the entire season and should achieve 80%+ efficiency including the run up time. Just what I've read around here.

icesailor

If it is an oil boiler, and you or the owner have been purchasing your oil from a oil dealer that uses a Scully type system that determines when to make deliveries based on past usage and OAT/ Degree days, you can usually get everything you need from them. If oil is purchased from discount cash & drop dealers, all is off.

Where I used to work, one full service oil company delivered almost all the oil. If I did any kind of upgrade, I could go back at some time after and compare before and after.

IME, a boiler change alone never produced the efficiency that a boiler change, tightening up the building, insulation and clock thermostats did. Hands down.

Something to be said for buying from full service dealers. Especially when you have no heat. We always take care of our loyal and faithful customers first.

SWEI

Hatterasguy said:

The ratings on new boilers are typically the combustion efficiency, not AFUE. You get about 82% on a new one. I don't think anyone wants to make a guess on the AFUE of a steam system. Way too many BIG variables that are not well controlled.

Pretty sure he meant old boilers there and not new. New boilers come with those bright yellow AFUE labels on them.

There are things that can be done to improve the efficiency of older boilers. It starts with a combustion analysis and perhaps a good cleaning, but there are other options depending on the system specifics. Maximizing burner run time is a big one, and there are several approaches to that. You need a competent steam pro to take a look at the entire system.

Kjmass1

It's a NG steam boiler. I had someone off this board come out and give it a seasonal maintenance. I've increased main venting significantly with a couple of Gorton #2's. I've replaced all the radiator vents and valves, and the house heats evenly with no pipe hammer. I use a Nest with all the auto away settings turned off and leave it set at 71. There is about a 1 degree swing +/- which I think is pretty acceptable. I recently re-insulated all the mains.

Using the nest and mynestreports.com, I've kept track of daily burn times and amount of cycles. The thermostat call time is accurate as I don't cycle on pressure.

Here is the data from December. On average I was doing 11.4 cycles per day, with a 21 minute run time.



The huge dip in heating hours was when we were on vacation. When I came back we lowered the thermostat from 73 to 71 when we got our most recent bill. Notice the savings from the previous high earlier in the month. That's about 20% lower (6.27 hrs vs 5 hrs) for the same HDD). I'd welcome any observations/suggestions.

Kjmass1

It's been pretty mild in Boston this year. The last 3 days however I've been closer to 5-5.5 hrs of burn time with 14-16 cycles. It'll be interesting to see this next weeks data as we are supposed to get a burst of arctic air.

Looks like it takes about 2-2.5 min for the header to be hot to the touch. Obviously a benefit of lots of frequent cycles.

So I guess my question is, why aren't I seeing a better BTU/sf? I've been using 65 as my baseline for HDD...I'm thinking maybe it should match my thermostat setting of 73/71? I thought I read that 65 was the standard, but I can't imagine running the house at 65 degrees.

SWEI

The baseline is the outdoor temp at which you need heat, not the temp inside your house. It depends mostly on the insulation and infiltration characteristics of the building.

SWEI

Hatterasguy said:

SWEI said:

Pretty sure he meant old boilers there and not new. New boilers come with those bright yellow AFUE labels on them.

That bright yellow label typically shows 82% on a steam boiler.

We both know that this is the combustion efficiency. No possibility that the system gets an AFUE of 82%.

The DOE EnergyGuide label has the AFUE on it. I take issue with how AFUE is calculated, but it still makes a better metric for comparing appliances than thermal efficiency does. Older boilers have nameplate ratings showing 80% (their nominal thermal efficiency) but typically have AFUEs more like 60-70%.

Kjmass1

SWEI said:

Hatterasguy said:

SWEI said:

Pretty sure he meant old boilers there and not new. New boilers come with those bright yellow AFUE labels on them.

That bright yellow label typically shows 82% on a steam boiler.

We both know that this is the combustion efficiency. No possibility that the system gets an AFUE of 82%.

The DOE EnergyGuide label has the AFUE on it. I take issue with how AFUE is calculated, but it still makes a better metric for comparing appliances than thermal efficiency does. Older boilers have nameplate ratings showing 80% (their nominal thermal efficiency) but typically have AFUEs more like 60-70%.

That's what I was thinking...but even with the $1800 MassSave boiler rebates, a 15% energy savings would probably have a payback period of at least 10-15 years. Probably best to leave as is and put that money elsewhere until it fails...

Kjmass1

Hatterasguy said:

Kjmass1 said:

So I guess my question is, why aren't I seeing a better BTU/sf? I've been using 65 as my baseline for HDD...I'm thinking maybe it should match my thermostat setting of 73/71?

I think we agree that you cannot compare your data from this year to last year. Too many variables with the house under construction.

You can do a heatloss and run the numbers for a full year. Then you can look at the boiler's fuel consumption at it's current estimated efficiency (80%) and compare the two.

The results are usually surprising. @MarkS had a discrepancy of over 50% between the burn rate and the calculated heatloss. Fortunately, it's in his favor!!

If the burn rate is significantly greater than the heatloss, that's when you need to investigate the system for problems.

Thanks- I'll look in to doing a heatloss assessment.

SWEI

Hatterasguy said:

If the boiler has a combustion efficiency of 82%, let us see if anybody can make the conclusion that the AFUE can be the exact same number?

The AFUE is always going to be less than the combustion efficiency, by definition (unless you're from an alternate universe).

That alternate universe is defined by ASHRAE 103. It needs a re-work but good luck with that given the influence groups like GAMA have on our world.

People believe they are getting an overall efficiency of 82% with a steam boiler when this is simply untrue.

Overall efficiency would have to be something more akin to BTUs per square foot per degree-day.

Older boilers have an honest AFUE.

The older boilers I'm talking about predate AFUE. The yellow labels started in 1980. ASHRAE 103-2007 replaced 103-1993, but I'm somewhat unclear what came before that.

icesailor

The yellow labels from 1980 made good toilet paper in an emergency if you were in the woods and had a roll of them.