Help Replacing an Old Boiler

Discussion in 'Boiler Forum' started by jeffesonm, Sep 14, 2014.

1. jeffesonmMember

Joined:
Jan 4, 2009
Location:
Texas
I would like to replace my 1957 giant beast of an oil boiler (see pics here) There's nothing wrong with it per say, it's just big, old, over sized, and probably inefficient. I'm planning to move away for a few years and rent my home, and I don't want this thing to break while I'm gone and have to pay an HVAC contractor for an emergency replacement. I see 2-5 year old oil boilers on Craigslist all the time from people converting to natural gas, so was thinking I could pick one up cheap and install it myself. I managed okay with the hot water heater install and have a buddy who's a pipe fitter and can lend a hand if I get in a jam.

I had actually spent months planning a wood boiler install with thermal storage tank and a constant circulation ODR-based system. (This was put on hold due to above-mentioned short term rental plans) I did a fairly extensive heat calc using an online calculator and came up with 90K btu/hr at 0 degrees. I then got some input from some other knowledgeable folks combined with past fuel usage and am thinking it's probably closer to 75K btu/hr @ 0.

I also used the Taco circulator selection guide to assess the distribution side. I measured the baseboard length, pipe length and fittings for each of the two zones in my house. I calculated the equivalent length, flow rate and head loss for the two heating zones and one appears to be twice as big as the other (32.5K btu/hr 3.25 gpm @ 3.4' vs 56.5K btu/hr 5.65 gpm @ 20.8'). I was going to split the big loop into two smaller loops so there would be three roughly equal size, all <4 gpm @ <4' and then use a small variable speed circulator like the Grundfos Alpha or Taco Bumble Bee. Zones 1/2 are what I've got now, and 1a/1b are the proposed new zones after splitting Z1.

Based on the amount of baseboard vs the peak heating demand, I figured out I could get away with running lower water temps. The plan was to use several water storage tanks connected in series, batch burn the wood boiler and then have a constant circulation distribution system using a 4 way mixing valve with ODR, drawing hot water from the storage tanks and mixing with cold water returned from the zones.

One day, when I'm confident I'll be living here for a few years, I'll get back to the wood boiler. In the meantime, I'd like to put in a new(er) oil boiler so I'm confident the rental situation will go well.

So here are my questions:

1) What size boiler do I need?

2) Should I still split the one huge zone into two smaller ones?

3) Is there an oil boiler than can provide/accept lower water temps, or is that just a feature of gas mod con units? Should I get a small buffer tank and setup the same batch burn/constant circulation system, or is it not worth it with oil?

Joined:
Nov 29, 2010
Location:
Maine
Typically, any oil boiler you pick up on Craig's list is going to need to maintain around 140 degree return water temperatures or it will condense and Roth the boiler out.

Joined:
Jan 14, 2009
Location:
01609
Unless this is a very large house there's no way it has a design heat load of anything like 89,000 BTU/hr @0F, not to mention that 0F is well below the 99% outside design temp for any NJ location, and even 75,ooo BTU/hr would be on the very high side. Most 1950s ranch homes that had R11 batts in the walls, no foundation insulation, maybe retrofitted to R19 in the attic and some storm windows added will come in under 20 BTU per square foot of conditioned space. With only single-pane windows (no storms) it might be 25 BTU/ft^2, but not 30. So if this is sprawling 3000-4000 ranch house (or a 4500- 5000' 2- story) or a house with multiple gia-normous single pane windows the 75K number might be real. No way is the 89K real.

The very smallest oil boilers out there are typically at least somewhat oversized for most 1950s homes with minor envelope updates.

What Tom said about Craigslist boilers- they will quickly be toast with return water below 140F, and even at 140F return water you would likely need a stainless steel flue liner to keep the exhaust from destroying the chimney. With some near-boiler plumbing (say, a system bypass or primary/secondary) you could run the boiler hotter than the radiation to avoid those issues. This is a real hydronic design problem with lots of moving parts, but if your heating it with an oil burner forget about ODR, stick with something that runs at a fixed temp, and not so oversized that it short cycles on zone calls.

5. jeffesonmMember

Joined:
Jan 4, 2009
Location:
Texas
House is 1800 sq ft ranch. I think 75K is a solid guess but you can read some long involved analysis from another site here if you'd like.

Yesterday I picked up a very slightly used Burnham MPO-IQ84. Previous owner had it installed when he bought his house in 2012, and the next year they put in gas so he switched to the ES2.

This will be vented into an 8x8 masonry chimney with an terra cotta liner that's seen better days, so I do plan to drop a 5" SS liner in there. Any other thoughts as to #2 or #3?

Joined:
Jan 14, 2009
Location:
01609
Clipped from that other forum thread:

"Ceilings - 900 sq ft R40, 900 sq ft R10
Walls - 1272 sq ft R3.4
Windows/Doors - 71 sq ft R1.8, 256 R.75
Floors - 1800 R5
Slabs - 0
Infiltration - 1440 cu ft, 1 air change/hr"

First, the builditsolar heat load calculator is really crude, and doesn't factor in the performance boost you get from the foil they used instead of insulation. If the foil has air gaps on both sides it's better than R3.4, but a fuel-use calculation would be a much better way of assessing the heat load, given the non-conventional construction.

"...the previous owner burned 1000-1200 gallons of oil per year"

Download the NORA FSA calculator from this site, and run it using the basic specifications of your old boiler. When I run it assuming it's output is 150,000 BTU/hr and has a steady state efficiency of 85% (not likely), even assuming 1200 gallons/year and NO domestic hot water heating with that boiler it comes up with a heat load of about 63,000 BTU/hr @ +10F (what the tool assigns for Newark- which is colder than the +14F ACCA Manual-J assigns Newark).

If it's heating your domestic hot water with an indirect that heat load drops back to about 53,000 BTU/hr.

Using a more realistic 80% net efficiency and assuming no hot water it comes up with 57,000 BTU/hr.

At 80% steady state efficiency and heating water with a tankless coil it's more like 44,000 BTU/hr.

Even if your old oil boiler is a best-in-class brand new heat purging 88% burner like a System 2000 it's only coming up with the ~60,000 BTU/hr range.

This tool is based on a fairly well validated model for hydronic boilers created at the Brookhaven National Laboratories- these are not crazy numbers, but would be consistent with a house with s0-so windows, some attic insulation, and next to zero wall insulation. The 44K number is probably a bit lower than current reality, but 60K is surely higher than reality. I'm betting it's 50-55K now, but would be under 45K if you insulate the walls.

The Burnham MPO-IQ84 has an output of about 74K, but that's about as small as you can get with an oil boiler anyway (and the smallest in that line.) It still needs to the return water to be 140F or higher to not self-destruct, and it has heat purge controls built in. As oil boilers it's decent, and should be able to pretty much hit it's AFUE numbers even at 2x oversizing.

But seriously, the U-factor of those walls is probably something like 0.20 BTU/ft^2- degree -F, and blowing the cavities full of 3lb cellulose or 1.8lb fiberglass would drop that to about 0.1. At about 1300' of wall and a 60F delta-T (70F inside, 10F outside) your wall losses would be about:

U0.2 x 60F x 1300'= 15,600 BTU/hr. Insulating that would cut the loss in half, (probably more after infiltration), so it would take at least 10% off the whole-house heat load, probably more like 15%. That would be consistent with the measured results I've seen when insulating houses with 2x4 walls with barely any wall insulation.

Doing it from the exterior is not a big deal (even before replacing the siding) and the cost is probably subsidized in your area. With the foil in there you DEFINITELY want to do it from the exterior, pushing the foil toward the interior side of the assembly or you'll most likely end up with wintertime moisture accumulation in the insulation.Whether it's an econobatt or just a foil, a decent blown insulation installer can dense-pack it fairly easily.

Dense-packing will also reduce your infiltration rates, sometimes by quite a bit. But you may have large air leaks elsewhere to deal with. An air-sealing contractor with a blower door and IR imaging can figure out where the leakage is in the first hour, and probably fix most of them in less than a day. Without a blower door it's a shot in the dark, and even if they fixed 1001 smaller leaks it won't necessarily move the needle on energy use much. It doesn't hurt to read up on air sealing and fix what you can before giving them a call, but an air sealing contractor without a blower door isn't worth much. With a blower door it can be quite valuable- often the most cost-effective envelope upgrade you'll ever do.

When it comes time to replace the siding it's worth putting continuous insulating sheathing over the exterior, but you have to be careful what you use, given that you have a foil layer in the stackup, which is a powerful vapor barrier. The ideal would be 1.5-2" of rigid rock wool, since that would still leave a very good drying path toward the exterior. Next would be 1-1.5" of EPS, which still has enough. Foil faced foam or foam with vinyl/polyethylene facers would be a moisture-trap disaster. You could use up to an inch of XPS (pink, blue, green board) but it's pretty vapor tight compared to EPS . With 2" of either rock wool or EPS it would have about half the heat loss of insulated 2x4 walls. Adding cavity insulation + 2" of EPS or rock wool would be something like 1/4 of your current heat loss through walls.

7. jeffesonmMember

Joined:
Jan 4, 2009
Location:
Texas
I'm betting the 1957 boiler is even less efficient than that. It was serviced every year, but still...

Up until last year hot water came from a tankless coil, so we assume the efficiency is somewhere south of 80%, it does seem likely the 1000-1200 gallons of oil was producing significantly less heat. Good to the heat load is lower than originally anticipated.

I do plan to do the blow-in insulation from the outside at some point... I am planning a 800 sq ft addition in the not too distant future, so figured that would be a good time to address it. Likewise for the sheathing.

The other thing worth mentioning is that I have a wood stove that I use pretty much 24x7 from October through April. It does a decent job heating the house up until the coldest 1-2 months, so I'm really looking to supplement this with the oil boiler.

Joined:
Jan 14, 2009
Location:
01609
Even with a flame retention retrofit burner and fully tuned up it's unlikely that a ridiculously oversized boiler in the pictures installed the year Eisenhower was inaugurated is doing better than 70% net efficiency. (It would probably keeping the uninsulated basement pretty warm though. Your true heat load is probably in the 40-45K range (a ratio of <25 BTU/hr per square foot of conditioned space), which is still credible.

It looks like you have no insulation on that CMU foundation wall too, with quite a bit of it above grade. That too is WELL worth insulating. If you insulate the wall cavities and the basement, tighten it up a bit your heat load should drop to ~15BTU/ft (not counting the basement space), something like 25-30,000BTU/hr . If you also add exterior foam/rock-wool when you re-side you might even duck under 20,000BTU/hr.

There are several ways to insulate the basement, some of which are guaranteed to create mold problems. Check back (search the remodel section of this forum for basement insulating threads) for details.

There is no need to wait for re-siding to insulate the above grade walls, and at \$4/gallon (or even \$3/gallon) it's well worth insulating sooner than later. It will be more comfortable, use considerably less wood/oil and the indoor air quality should improve- particularly in winter. A reasonably tight house in NJ should have no indoor air dryness in winter, staying ~30-35% RH @ 70F even during cold snaps, without the use of humidifiers (which become their own health hazard, creating mold conditions where air leaks out of the house.)