Mod con vs cast iron for my specific home

Gball

Good evening All,
I want to start by saying how much I truly appreciate the dozens of hours of reading that I have enjoyed on this forum.  It is only because of the countless hours of contributions that many of you have made that I am able to have a remotely intelligent conversation with the contractors I have met in regard to my current project.  

My situation is as follows.  I recently purchased a 4500 square foot home in Central Pennsylvania.  The home was built approximately 1920 and has 2 floors currently heated.  Each floor has approximately a 10 ft ceiling.  In addition to the 2 floors being heated there is a basement and an attic.  The unfinished basement has no "heat emitters" other than all of the copper piping that comes off of the boiler and runs its course to where it turns up through the floor and goes to its loop.  It actually keeps the basement quite comfortable.  The attic has no heat emitter at all and is totally unfinished. 
The home is currently heated with a new yorker firing on oil. Hydronic cast iron emitters providing the heat source. That unit is from 1976 and is 375,000 btu.  It has 2 tankless domestic hot water loops in it as well.  I am well aware to base no calculations upon that unit.  I have 7 heat zones in the home.  4 on the first floor and 3 on the second.  The home has 3.5 bathrooms.  In an effort to control monthly costs, I am willing to invest money in the homes energy system.  I am told that my manual j load indicates a need of approximately 160,000 btu to heat my home.  We are a family of 4 and all take relatively quick showers with no large soaking tubs in the home.  

My initial thoughts were the 3 following options
#1 switch to heat pump and achieve a new energy efficient heating and cooling system.  Keep current boiler for extremely cold days when I would turn off electric and run purely on furnace.  This option has been ruled out because I have high velocity ducting and I've been told that those units don't run well on that type of ducting.  I am also told that I don't have enough air ducts off of the trunk line for the size heat pump I would need.  

#2
Remove current boiler and oil tanks.   install a 1,000 gallon underground propane tank.  
Install (2) navien ncb150 mod cons with domestic hot water heater loop and a 40 gallon storage tank.  "Cascade" the 2 boilers to have alternating service times when only 1 boiler is running.  When the firing boiler comes to 80 percent load, the second would kick on and they would work together to provide the necessary btus. Outdoor reset would be part of this install.  There would also be 7 new "ecm" circulators with new flanges and valves. 
This option has been provided by a local contractor that I feel seems knowledgeable and says he is very confident/comfortable installing the mod cons.  

#3 remove current boiler and tanks.  Install 1000 gallon underground propane tank.  Install (2) 100kbtu propane fired cast iron boilers.  New circulator pumps.  line the chimney due to switching from oil to gas.  Alternate service times when only 1 boiler is running.  Install them in parallel.  When the firing boiler is not sufficient, bring on the non firing boiler to provide necessary btus to heat home.  No domestic loop for hot water.  No holding tank.   Hot water to be provided by (3) electric on demand hot water heaters located in very close proximity to the point of use (1) for kitchen, (1) to service 2 of the bathrooms which are next to eachother, (1) for master bath and 1st floor half bath which are located on top of eachother.  I know this is not conventional, but it is a very long house with the boiler at the opposite end from the kitchen and 2 main bathrooms.  The supply to these is 1" copper.  It takes approximately 2 minutes to get "hot" water to the fixtures.  That is with old high consumption fixtures, so new fixtures will take longer.  I feel that with our smaller hot water load the electric will be cheaper when considering many factors such as amount of water that has to flow to provide a hand wash, amount of energy it takes to keep 40 gallons of water hot 24/7/365, short cycling, heat emission of idle pipes into the home when trying to cool in the summer months, etc.  Not to mention the fact that you now have instant hot water.   

In options #2 and 3 I would be buying the propane tank so I would not be in bondage to any particular supplier.  I have calculated the cost of propane vs oil fuel.  Based on current market prices, propane is 60 percent the cost of oil PER BTU in my area.  I am comfortable with the payoff I will see there.  Natural gas not available.

Initially I was going to go with option number 2 and felt very good about the decision.  However as I have plunged further into this, I am having a very hard time wrapping my mind around the 12 year life expectancy of a mod con.  The purpose for 2 smaller units is to have redundancy in case 1 goes down. 

 I have somewhat formulated option #3 based on my own thinking and studying.  Our design day is 13degrees F.  Based on data, Our average temperature from Oct 1 until April 1 is around 40.  With (2) 100k input btu units, I feel that a large majority of the time I will be running on only 1 unit and when it is cold I will run on 2 to provide the necessary btus to properly heat the home.  The reason I feel that 2 small units will be superior to 1 has to do with average outside temp, but also I have taken into account the 7 zones.  With that many zones I feel that a lot of my fire time will be to run zone or 2 that has a low btu demand.  The lower btu units should lessen short cycling of the boiler.  In a sense it creates a modulation turn down of 2 to 1 with a cast iron boiler compared to a single 200k btu unit.  Plus I have redundancy.  The entire decision is predicated upon the assumption that a cast iron unit will last 24 years compared to 12 years of a mod con.  If that assumption is correct, the mod con would have to have save approximately 700 gallons of fuel a year to pay for the fact that it's life span is half of a cast iron unit.

So, am I way off track?  What would you do?  Don't hold back at mocking my ignorance, I am not sensitive.  What other options are there?  In advance I would like to thank you for any advice. I am not a wealthy man and this is a very large investment for myself and my family.   I want to do my best to make the right decision.  

jesmed1

I'm just a homeowner, not a pro, but you've obviously learned a lot and have considered many options.

I think you're on the right track with the idea of two cast iron boilers, both for redundancy and a crude form of modulation. I maintain our 4-unit 4800 sq ft 100-yr-old condo building in the Boston area, and we have two cast iron Weil McLain WGO-5 boilers, each heating half the building. Our total heat load is only about half yours, but our boilers are massively oversized at a combined 300,000+ MBH rating. Even so, the system works quite well. Eventually when the boilers fail, we'll replace them with a combined 150 MBH or so, probably the same line of Weil McLains but with fewer sections.

Because you have so many zones, you might consider running two cast iron boilers into a large buffer tank. Then each zone can draw independently from the buffer tank without short-cycling the boilers, and either boiler can run longer, more efficient burn cycles occasionally to heat the buffer tank.

I think you'll find that your old house is massively over-radiated with those big cast iron radiators, with the result that you can run relatively low water supply temperatures. Because our boilers cold start, our supply temps rarely exceed 130 degrees, but because we have so much radiator surface area, that relatively low water temperature heats the building quite well. So you could probably maintain the buffer tank at, say 140 degrees, which keeps the return temps to the boilers from getting too low, and then your zone pumps can run as short or long as needed from the buffer tank.

As I said, I'm just a homeowner, and you'll get better-informed responses from the pros here. But at first look, I like the idea of two cast iron boilers and a large buffer tank. That will maximize the life of your boilers by preventing condensing/short cycling, so you could conceivably get 30-40 years from your boilers that way, while the buffer tank gives you a lot of flexibility in running your zones.

Intplm.

@Gball
If this were my home I would install a boiler or boilers, cast iron.
There are cast iron boilers that are on the market that can modulate or fire to the heat demand. Meaning, ramp up or down to satisfy the heating requirements. Saving money on those fuel bills. And because they are made with cast iron they can last even longer than your expressed thought of 24 years.
I would also install a separate heat source for your domestic hot water.
The biggest bang for your buck doing it this way.
IMHO.

hot_rod

Two mod cons would give you a 20-1 turndown probably eliminating the need for a buffer tank. 
with a good brand properly installed, 20 year life is not un-heard of

Room by room load calc and radiator size assessment would help determine how often you would be in the 90% plus condensing mode

EBEBRATT-Ed

You could also put in 1 CI boiler and 1 mod con. Ha.ve the MOD CON lead and the CI boiler lag. That way you would get savings from condensing. And you will find most of the time you only need 1 boiler to run.

When you find the outdoor temp where the Mod Con can't keep up you can put an outdoor air control wired to the CI boiler to bring it on at 25 degrees or whatever temp you need & use a two stage thermostat

When you need a mod con part and have to wait for it you will still have some heat.

SuperTech

Why propane? At least in my area it's never worthwhile to switch. The L.P isn't less expensive and doesn't provide as many BTUs. I like the idea of two lead/lag cast iron boilers with a buffer tank. If you end up going with a mod con I believe it would be in your best interests to avoid Navien boilers, I've seen too many problems with them that were unrelated to the installation or any other controllable factors. 

Hot_water_fan

You can install a high velocity heat pump. That’s the clear winner here, propane offers very little cost (if any) advantage over oil. Electricity should crush either on cost and you keep the oil for backup. 

Gball

Supertech,
I have done the calculations with propane.  In my area the current cost on propane is 1.39/gallon.  

That brings the cost of propane to $15.19 per million BTU 

Heating oil is currently $3.47 per gallon.  Which brings the cost per million btu to $24.76.  

Propane cost about 61 percent of fuel oil currently.  I have looked over historical fuel bills and that appears to be about average for my area.  

I don't have any of my own historical data, but was told it used approximately 2500 gallons of oil.  I understand the new systems are going to be more efficient.  I anticipate a fuel savings of saving about $1500 to $2000 a year for propane vs heating oil.  That is after factoring all btu conversions.  To me, that is a solid payoff

Hot_water_fan

I have done the calculations with propane.  In my area the current cost on propane is 1.39/gallon.  

Wow!!! That’s low. 

If you go the two cast iron boiler route, you can size them differently - for example 60kbtu and 120kbtu. This way there’s less cycling.

WMno57

Option #4 The Redundancy Option

• Keep the 1976 New Yorker Oil Boiler
  
• Buy a new Oil Tank (SMALLEST one available)
  
• Buy a propane tank
  
• Buy a propane mod-con
  
• Buy a propane permanently installed genset (SMALLEST one available)
  
• Buy three tank style water heaters, one or two electric, and one or two propane
  
• Buy two yellow 5 gallon Diesel plastic fuel cans
  

jesmed1

Gball said:

I don't have any of my own historical data, but was told it used approximately 2500 gallons of oil.

That is probably accurate. Your design heating load of 160MBH is about twice ours, and your reported oil usage of 2500 gal/yr is about twice ours. So the factor of 2 scaling on both numbers is consistent. (As I mentioned above, this is for a 4800 sq ft old house in the Boston area.)

One thing this tells me is that you have a lot of room for improvement in air sealing and insulation. Our 100-yr-old 4800 sq ft house doesn't have much wall insulation, but we do have 10" blown cellulose in the attic, and all the old double-pane windows have been professionally weatherstripped with spring bronze and silicone bulb weatherstripping on one half of the house, with new storm windows, and on the other half the old windows have been replaced with double-pane vinyl windows.

I always urge old house owners to restore and professionally weatherstrip their original wood windows, instead of replacing them with vinyl. Weatherstripped with spring bronze, original wood windows with a storm window can perform as well as new vinyl windows, but the original wood windows will last another 100 years, while the vinyl window seals will fail in 20 years or less.

https://www.houseandhammer.com/seven-things-i-wish-id-known-about-replacement-windows/

With those air-sealing and insulation upgrades, our design heating load is less than 20 BTU/hr/sq ft. And that's for a 0 degree outdoor design temperature. Your design heating load of 160MBH gives you a loss of 35 BTU/hr/sq ft. That means your heat loss is 80% more than ours, in a house that was built around the same time as ours (circa 1920).

So I suggest you first look at air sealing and insulation upgrades, before the heating system. If you can get from 35 BTU/hr/sq ft down to 25 BTU/hr/sq ft, a goal which I think is reasonable, you're saving over 800 gallons of oil/yr, which at current prices is close to $3k/yr. So if you spent $30k to professionally weatherstrip your windows, air seal and insulate around the sill, insulate the attic, and possibly blow insulation into the walls, you'd be saving significant $$ right off the bat with maybe a 10-year payback period. Then later when this boiler kicks the can, you can upgrade the heating system and gain efficiency there too.

It might be worth your while to pay for a professional home energy audit, with a blower door test, to see where all your BTU's are going. The blower door test will find all your air leaks, which is your biggest source of heat loss. Then you can make an informed cost/benefit assessment.

MaxMercy

Gball said:

In my area the current cost on propane is 1.39/gallon.  

That brings the cost of propane to $15.19 per million BTU 

Heating oil is currently $3.47 per gallon.  Which brings the cost per million btu to $24.76.  

Wow, in Connecticut the oil this morning is just under $3 a gallon while propane is $3.60!!

https://ycharts.com/indicators/connecticut_residential_propane_price

I wonder why is the propane so cheap in your area?? I'd convert to if I could pay $1.39 for propane.

OilfieldHippie

MaxMercy  I wonder why is the propane so cheap in your area?? I'd convert to if I could pay $1.39 for propane.

There are several interstate lpg pipelines that cross central PA. All comes down to transportation costs. Pipeline to terminal to a truck is much cheaper than pipeline to rail to a terminal to a truck. The further north you go from Philadelphia, the less major pipeline infrastructure there is. 

Gball

Jesmed1
Thanks for your reply.  The home had a blower test done.  I am attempting to attach the results to this reply

jesmed1

@Gball Thanks for that blower door test result page. Interesting.

Caveat: I'm a homeowner who is also a mechanical engineer. I'm not a heating pro. I'm not a blower door expert. But I do crunch numbers for a living.

My take on your blower door test is based on thess explanations:
https://www.finehomebuilding.com/project-guides/insulation/using-a-blower-door-test-and-interpreting-the-results

https://energysmartohio.com/uncategorized/what-does-a-blower-door-air-leakage-number-mean/

So at the standard blower door pressure of 50 Pascals (equal to about the pressure of a 20 mph wind), your house leaks 5538 cubic feet/minute. And with a listed floor area of 4038 sq ft and ceiling heights of 10 ft (per your post above), your house has a volume of 40,380 cubic feet. Then 5538 cfm x 60 minute/hr= 332,280 cu. ft./hr. Which means your house experiences 332,280/40,380=8.3 air changes per hour.

8.3 air changes per hour (ACH) at 50 Pascals (ACH50) is "moderately leaky." But for an old house, it's probably better than average. Note that the energy consulant in the second link says a good target for your leakge CFM is the square footage for your house. So if you could get from 5538 CFM down to 4038 CFM, that would be good, and that's only a 27% reduction, which shouldn't be too hard to reach.

Now to the calculated heating load in that report. My (limited) understanding is that the heating load of 126,555 BTU/hr listed on the report is based on an estimated air change rate under "natural" heating conditions, ie when you don't have a steady 20 mph wind blowing on the whole house. I may be wrong, but I believe that's an extrapolation/interpolation based on the 5538 cfm test results at 50 Pascals. So I would hesitate to trust it. But if that number is correct, your design heat loss is around 30 BTU/hr/sq ft.

I would put more trust in the Manual J heat loss number you reported previously of 160,000 BTU/hr. Using the listed 4038 sq ft from the blower door report, that's about 40 BTU/hr/sq ft.

So your actual design heat loss is probably somewhere in the 30-40 BTU/hr/sq ft range. Based on your reported oil consumption, I tend to think the high end is more correct, even assuming that a portion of that 2500 gal/yr went to domestic hot water.

With a lot of professional help, you could probably get down close to 20 BTU/hr/sq ft, but that would involve expensive professional weathstripping of old wood windows, professional air sealing, and insulation.

So it might be worth your while to hire an energy consultant like the guy in the second link above. They'll know what the likely cost/benefit tradeoff is for air sealing, insulation, etc, and let you decide where to get the most bang for your buck.

Hot_water_fan

If propane is so cheap there, air sealing and insulation might be value destroying. Tread carefully! 

jesmed1

Hot_water_fan said:

If propane is so cheap there, air sealing and insulation might be value destroying. Tread carefully! 

LOL. There is some truth to that. Where I live, we have oil heat at $4/gallon, so our tradeoff is somewhat different! I just hate to see energy wasted, even if it is ridiculously cheap.

hot_rod

The Midwest usually has the lowest lp prices, that $1,39 seems awful cheap unless you are a commercial account. The EIA website tracks weekly lp prices across the country. Some areas are still just under 2 bucks a gallon. I didn’t see any that low, however?

Gball

It seems some of you are skeptical of my claim of the propane price.  Please see the attached photo depicting the outline of a proposal for the 1,000 gallon underground tank install.  Per forum rules, I have deleted the prices for the work to be performed.  Please see the bottom line which shows the quote of 1.41 per gallon for propane.  Per the date on the quote, that was 11/19/23.  The price of 1.39 I received was 12/7/23 from the supplier I use for my heating oil.  

jesmed1

I don't doubt your propane price. But oddly, reported propane pricing varies widely in a 30-mile radius of Bethel PA, anywhere from your $1.42/gal up to $3.30/gal.
https://fuelwonk.com/cities/bethel/pa

Meanwhile, statewide in Pennsylvania, the US EIA reports an average of about $2.80/gal.
https://www.eia.gov/dnav/pet/PET_PRI_WFR_DCUS_SPA_W.htm

Hot_water_fan

It’s funny because wherever I’m in central PA, it seems every gas station has the exact same price 

Gball

Jesmed,
I greatly appreciate the calculations you did on the airflow.  The quote I received has the cast iron units at about 10 percent more expensive than the mod con units.  

The contractor has the New Yorker cg-f 80k btu cast iron units on the quote.  Anyone have any thoughts or experience on that particular unit?

jesmed1

I assume you mean the contractor quoted you on two 80,000 BTU/hr New Yorkers, for a total of 160,000 BTU/hr. You might be able to squeak by on that, but ASHRAE recommends sizing to 1.4 times your design heating load. So your 160,000 BTU/hr Manual J number x 1.4 = 224,000 BTU/hr.

I like @Hot_water_fan's suggestion to get two different boiler sizes for more flexibility. Say one 80,000 BTU and one 140,000 BTU. Then you get three different output levels depending on which ones you run, and the total of 220,000 BTU gives you margin for the polar vortex blizzard once every 50 years.

hot_rod

Hybrid boiler arrangements is not a bad idea. We see it in commercial applications.
One mod con, one cast boiler. A staging control, or a basic electronic aquastat can switch between them.
So you get redundancy, assumed relatability of the cast, efficiency and turn down of the mod con.

I suspect 85% of the season you will run on the mod con based on you radiator assumptions.