Oil to Gas burner questions?

[Jasesun23]

Considering switching to a natural gas from a oil burner (pics below). The boiler heats the water radiator heaters and also the hot water to the sinks and showers. Not sure why the boiler has 2 stickers posted on it but I've included the specs on both.
Was hoping to get an idea if the quote I got seems reasonable since I know nothing about this.

From what I understand the person doing this would remove the old one. Install the new one (model P206- ge12 164,000 btu). I can't seem to find that model online but it seems like models around that size are $2700-3100. I believe he is going to cut up my large oil tank and remove that. I was quoted $8,700.00 with a 1000 dollar rebate from the gas company. Again i'm not sure how much time and effort this involves but is 5000-6000 dollars labor seem right?


Thanks for any advice in advance
Jay

 

[Dana]

There are several issues to address here.

$8700 would indeed on the high side for a simple boiler swap for a ~200,000 BTU/hr boiler , $5K would be on the low side. It probably costs at least half a grand to a grand to dispose of the oil tank, and a similar amount or more to plumb in the new gas lines etc. so it may not be insane.

What WOULD be insane is to install another 200,000 BTU/hr output boiler, since it's unlikely that your house has a 99th percentile heat load even 1/5 that size.

The Burnham P206 is a tiny step in the right direction it's 164,000BTU/hr input, and about 136,000BTU/hr out, but that's still probably 3-4x (or more) oversized.

Oversizing by that large a factor may be great for heating the place up in a hurry, but takes a HUGE bite in efficiency. For about the same money as a P206 you can probably install somewhat higher-efficiency model with smarter controls that would be less than 2x oversized for the heat load, use 20% less fuel, and provide higher comfort to boot. With an 85% efficiency lower BTU boiler you'd be out another grand for a right-sized flue liner, and it may be better/cheaper to go with a sealed combustion side vented version. The smallest of the line 3 plate versions (ES2-3 or ESC3) have 60,000 BTU/hr of output, which is still quite a bit more than the heat load of most houses in NYC/NJ/L.I./Westchester.

Unlike the P206, these little guys are pre-plumbed internally to be able to manage the low return water temps of high mass radiation, but it would be useful do run the math on how much radiator you have just in case it needs a bit more.

But even the 3-platers might be too much boiler if it is a smaller-tighter house with pretty good insulation and current code min windows.

To get a quick & dirty handle on your heat load, how much oil do you use in a year?

Better yet, if you were on a regular fill-up service, what was the "K-factor" stamped on the mid to late winter bills?

Also, how have you been heating hot water?

 

[Sponsor]

 

[Jasesun23]

Found the invoice so I can give a better idea of what is being done

Paper work to gas company
Paper work to buildings dept
run new gas line and meter bar
remove old boiler and oil tank
install 40 gal stainless steel indirect water heater
install boiler burnham 206-ge12
install - pressure reducing valve, back flow preventor, low water cut off, expansion tank, taco air seperator, add serperate zone for indirect water flow control valve, 3 zone relay with wh prority
1 year parts and labor waranty

 

[Dana]

With the indirect HW heater + disposal of the oil tank & boiler $8700 isn't a gouge, but it's still likely to be a ridiculously oversized boiler.

For reference, I had a P206 in my place that was maybe 15 years old when I got fed up with the short-cycling on zone calls & low efficiency and just yarded the thing out and never looked back. Based on both fuel use and I=B=R type spreadsheet calculations my heat load at +5F is under 40,000 BTU/hr. And this isn't some fancy superinsulated house- it's a clapboard sided 2x4 framed ~2400' + 1500' basement 1920s bungalow with the original single-pane double-hungs plus some 1980s vintage clear-glass (not low-E) triple track storms. I've tightened it up a bit since I moved in, but even 20 years ago the heat load was less than 50,000 BTU/hr, before the air sealing and insulation upgrades.

I insulated & air sealed the foundation as part of the upgrades which was something like half the improvement. Heat load currently is in the 35,000 BTU/hr range by fuel-use estimation methods.

NYC's 99% outside design temperature is +15F, fully 10F warmer than mine- if I air-freighted my place into your neighborhood it's heat load at +15F would be about 30,000 BTU/hr. With the P206 I'd still have enough boiler to stay warm at outdoor temps to about -150F or so, but I'm not expecting an ice age that severe any time soon.

 

[Jasesun23]

thank you for the quick replies Dana. I'll see if I can find the "k" number. Can you point me to the best way to figure out the correct sized burner. Is is based on gallons of water in the system? Number of radiators? Length of pipe? Amount of oil used last year?

Do you think I should be worried that this guy is suggesting such a big burner, would you question his knowledge. I understand AC systems and oversizing them and how they short cycle and don't remove humidity so its very important to size those right. I'm assuming boiler size is something similar efficiency wise.

 

[Dana]

The single most important factor for sizing the boiler is the peak heat load. AFUE testing presumes no more than 1.7x oversizing for the actual loads. If the oversizing factor is much bigger than that it slides off an efficiency cliff due to short-cycling and excessive standby losses. If your boiler room is the warmest place in the house it's a serious as-used efficiency problem. If the boiler room happens to be in an uninsulated basement and is the warmest room in the house it's an energy DISASTER. The above-grade portion of a concrete foundation has literally 10x the heat loss of an insulated 2x4 wall per square foot, and if that room is 5-10F warmer than it is upstairs it's an even bigger multiplier.

If the boiler has been your only source of heat (no wood stoves, pellet stoves or heat pumps, right?) a K-factor on fuel used over a wintertime period is good enough to ball park it. Fuel use measured against heating degree-days over the past year could do it too, but that's skewed a bit by summertime use if you were heating hot water with a tankless coil in the boiler. If the wintertime bills do not have a K-factor stamped on them, if you have the EXACT dates of the fill-ups and a ZIP code we can look up the heating degree-days for that period to come up with the information.

The K-factor is the same information, but expressed differently- it's the degree-days per gallon of oil used rather than the gallons per degree-day. But either can be used (along with the nameplate efficiency of the boiler) to quickly calculate a BTUs per degree-hour number, and apply that constant to the difference between your 99% outside design temp and 65F, the presumptive heating/cooling balance point built into the degree-days base number. In NYC the 99% design temp is +15F, which is a 50F heating-degrees below 65F. Once you have the BTU/degree-hour number you multiply by 50F to get the BTU/hour needed to keep the place warm at +15F. As long as the boiler's output is a bit higher than that calculated load number, it's enough burner.

The other way would be to run a heat load calculation based on the gazillion construction details of your house (framing, insulation, window types, air leakage estimates, etc) to come up with a number, which is more painstaking, and not necessarily more accurate. A fuel use calculation against heating degree-days uses the boiler to directly MEASURE the heat load of the house.

Not number of radiators, but rather the total size, measured in equivalent direct radiation square feet (EDR) is important for deciding how to design and operate the system for maximum efficiency & comfort. The calculated heat load along with the size of the radiation will tell you the maximum water temp needed. Most old-school systems were over-designed for the heat load of the house on day-1 and presumed a 180F water temp. But in most case that was probably before the place got new windows & insulation/air-sealing upgrades, etc which lowered the heat load, and it probably never needed more than 160F water even back in the day.

It's possible that the contractor sized up the radiation and picked the p206 to have burner big enough to actually get those radiators up to at least 160F in order to keep the water returning back to the boiler above 140F. That would be necessary to keep gas exhaust from condensing mildly acidic liquid onto the flue liner in your chimney. If cutting it a bit closer they might have been trying to keep return-water above 125F to keep it from condensing on the boiler plates themselves (which would ruin the boiler in one season). These issues can be prevented by designing and tweaking some of the near-boiler plumbing, but in some of the newer smaller boilers that plumbing is already there, built into the boiler. That internal plumbing is self-compensating down to 110F return water, for the ES2-3 & ESC3. Only if the radiation is SO completely supersized for the heat load that the radiators wouldn't hit 120F would you need to design additional protection for those tiny boilers. (They are literally half the size of the P206, with 3 cast iron heat exchanger plates instead of 6 plates.)

 

[Dana]

BTW: It's par for the course for boiler-swappers to just take a WAG at the radiation sizing and pick something not too different from the pre-existing boiler that works. A better contractor would run a real heat load calculation and measure the radiation, but those are the exceptions that prove the rule. Most of the clients neither know nor care enough about it, and in some respects taking the time to do the full analysis isn't in the contractor's best interest unless they're being paid for that service- time is money, and most clients neither know enough about nor care enough to push them to do it more rationally.

Not all plumbers are really hydronic heating designers, and it's possible your contractor learned some rules of thumb that work well enough such that the customer never gets cold, and the boiler lasts at least until the warranty is up.

But that's not to say doing it that way is ever in the customer's best interest. It clearly fails from an up-front cost, operating cost, and comfort point of view.

For pretty much all cast iron boilers efficiency measured against duty-cycle looks something like this:

You can see that at a 50% duty cycle the 82% boiler is stil running about 80% efficiency, but at a 25% duty cycle it's only making 75% efficiency, and below that it gets a lot worse (and fast!). If you sized the thing exactly to your heat load at +15F, given your average winter outdoor temps in the mid-30s you would have a seasonal average duty cycle above 50%, which is pretty good.

But if you oversize it 4x it means even on the coldest 1% of the heating season hours you're only at the 25% duty-cycle mark, and at your average mid-winter temp you'll be down in the 10-13% duty cycle range, where you'll be lucky to average 65% efficiency over the season, no matter what the AFUE or steady-state efficiency of the boiler happens to be.

The smarter controls of the tiny units will heat-purge the boiler's heat into the radiators at or near the end of a call for heat to lower the boiler' standby temp. This moves the knee of the curve a bit to the left, and keeps the flat middle-portion closer to the steady-state combustion efficiency. So even if you're 2x oversized for the load with one of the little guys you'll still come close to hitting the efficiency numbers. SFAIK those controls do not come standard on any version of the P206, but even if it did it can only help so much- it might raise the average efficiency from 65% up to 75%, but probably not as high as 80%.

The fact that the original boiler is so oversized for the load a fuel-use against heating degree days type heat load calculation will be an upper bound. There is built-in margin, since the boiler was not operating as high as it's steady-state efficiency.

 

[Jasesun23]

Thanks for the detailed reply Dana. I'm going to have to read it a couple times to fully understand but I think I get the jist.

Here is some more info if it in anyway changes what I should be doing or looking at



There are 2 zones. 1 zone is the full house. The second zone is just an additional room that was added to the house. It probably 200 sq ft and has baseboard heating.

Below is a list of radiators and some pics - all radiators are around 23 inches high
1st floor
TV room - 3 radiators (48" / 48" / 32")
hallway - 1 - 48"
kitchen - 1 -67" (this one is different from the rest, its is a full cast iron one)
1/2 bathroom - small floor type radiator

2nd floor
7- 36" radiators (2 in each bedroom, one in bathroom)

The front of the radiators are just grills behind them are small fin contraptions. I took a pic of one

The boiler is right next the chimney so I'm assuming it vents into the chimney.

The old looking radiator is in the basement and i'm not sure its hooked up and there is also a radiator in the garage i forgot to take a pic of

 

[Dana]

None of those heat emitters in the pictures are radiators- they are all fin-tube convectors. You can estimate their 180F water from the tables & correction factors in this reprinted guide book section. If that's too messy, figure out the most-comparable units in this catalog to come up with output numbers at 180F. Run the numbers, and add it all up- it'll probably be a large fraction of the output of your oil-boiler, but the number is useful for determining whether a right-sized boiler would need some near-boiler plumbing to protect it from condensing exhaust.

The 200' zone with the baseboard will likely cause most boilers to short-cycle, but the smaller the boiler, the less of an issue that is.

Any progress on digging up fuel use figures?