Boiler size

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cloned1973

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Hello
I'm sure you guys get this question a lot if I'm repeating any forms I apologize
I am going from oil to natural gas in my home , I currently have a peerless set up for net 90,000 BTU (has three options depending on nozzle) I use electricit water heater
When I add up the BTU's on all cast iron radiators I come up with 46,500
My home is under 1100 ft.² , two stories older home no insulation, and I live in New York about two hours north of New York City
Near me is a crown boiler distribution and a slant fin place so I'm going with One of those
Can someone just ask me a few more questions if needed and possibly get me started in this process
The two I was thinking about was the
Crown AWR070 or the AWR105 or
Slant fin sentry S-90
Thanks in advance
 
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Dana

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All of those boilers are likely to be oversized for your load- the AWR070 might not be TOO oversized, but that's not a given. If you have an oil-use history with exact fill up dates & quantities on the Peerless and a ZIP code (for weather history data and outside design temperature purposes) we can get a very good handle on the actual heat load.

When you say " ...two stories older home no insulation...", is that literally no insulation anywhere, not even in the attic? Do you have either replacement windows or storm windows? What type of siding & wall construction? Full basement? Foundation type?

When the radiation was measured up to be 46,500 BTU/hr, at what average water temp was that? What are the high/low limits on the Peerless as currently set up, and does it keep up with the heat load?

A reasonably tight uninsulated 2-story house can still have a design heat load well under 50,000BTU/hr @ 0F, and it could easily be under 35,000 BTU/hr. There are more appropriate options out there. It's also possible that with some amount of retrofit insulation and air sealing the load can be brought down to 25,000-30,000 BTU/hr, assuming it's not already there. (A tight new 1100' 2-story house built to IRC 2012 would come in under 16,000 BTU/hr, and a well designed version might be bumping on 10K.)

The Slant Fin Sentry S-60 or even the S-34 might be a better choice, but we won't know without a better handle on the actual heat load. The S-60 puts out 51,000 BTU/hr, which is slightly more heat than your radiation can deliver at the water temps you presumed in your radiation calculation, and the S-90 puts out WAY more heat than the radiation can deliver, and is a lousy choice (as is the Peerless jetted for 90K.). Oversizing the boiler only causes it to experience more burn cycles- it doesn't deliver any more heat.

If you size the boiler to the heat load and that happens to be undersized for the radiation it adds a small amount of near-boiler plumbing to keep it from running too cold, but it's better for overall efficiency & comfort.

The output of the S-34 is only 29,000 BTU/hr, and would likely need some plumbing tweaks, but rather than cycling on/off all the time it would deliver long and efficient burns, and would be great, if it in fact covers your design-day load with a bit of margin.

The Crown AWR038 is 10% bigger, putting out about 32K, and would be similarly comfy & efficient with your radiation, and it wouldn't surprise me at all if this would be the best choice within those lineups, even before reducing the heat load via weatherization.

But let's put some heat load stakes in the ground based on the fuel use, and see if there are any good ways of reducing that load.
 

cloned1973

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Thank for reply, I'm green I know sorry about that
I will answer all the best I can.
Oil was about three oil tanks (275 tank)
Not exact sorry
Zip code 12550
There is insulation in attic, nothing in walls, plaster walls balloon construction.
All windows are two years old, all exterior door are brand new
Full basement, poured foundation
Two part siding vinyl on top half and they left aluminum on bottom (I going to put vinyl on bottom half one day
My pal before he got sick came up with those numbers 46,500 and he said something about I should add about 20 percent for exposed pipes I gave him all sizes of radiators, and he did the rest. He's sick now and I will not bother him, he has enough to worry about.
The water temp I'm sorry I can't say, high temp is 185
(it has a coil for domestic, not being used but it's there)
As far as the heat, I had no issues that I know about
If you want picture of tag on boiler for any reason let me know,
I'm afraid of going to small on boiler
I thought the AWR070 was to small
No basis for that assumption
 

Dana

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The 99% outside design temp for Newburgh is +10F. (Poughkeepsie's is +6F, a bit up the river and further from the coast than you.) It gets colder than that, but 99% of the time it's 10F or warmer.

Say you used 3 x 275 gallons = 825 gallons since last August.

Using data from the airport's weather station on degreedays.net, during that period Newburgh experienced about 6522 heating degree days (base 65F.)

So that's 825 gallons/6522 HDD= 0.1265 gallons per HDD.

At 138,000 BTU/gallon that's 138,000 x 0.1265= 17,457 BTU/HDD of source-fuel.

At 85% combustion efficiency that means 0.85 x 17,457 = 14,838 BTU/HDD went into the heating system (the rest went up the flue.)

With 24 hours in a day that's 14,838 /24= 618 BTU per degree-hour.

Your outside design temp is +10F, and the presumptive heating/cooling balance point base is 65F, which means you have 65F-10F= 55F heating degrees.

Which implies a heat load of 618 BTU per degree-hour x 55F heating degrees= 33,990 BTU/hr. Call it 34K.

If you were turning the thermostat down to 55-60F every night or while you at work it might be better to use base 60F. Degreedays.net comes up with 5310 HDD, which would result in a constant of 759 BTU/degree-hour, but only 50 heating degrees, for an implied load of ~38K, but that's still at loooong way from needing the output of the AWR070.

As a rule-of-thumb sanity check, 34,oooBTU/hr for 1100' of conditioned space is 31 BTU/hr per square foot, and 38K/1100' is 35 BTU/ft^2, either of which is a bit on the high side even for an uninsulated house, which implies a lot of air leakage in that balloon framing.

With the Peerless jetted for 90K-in it's not really delivering 85% efficiency, since it's about 2.5-3x oversized for the actual load,. with lots of idling and cycling losses. The real load is going to be bit smaller than that, but it's over 25K-it might be over 29K, but not by much.

With vinyl or aluminum siding the siding is back-ventilated, which makes it safe to install blown cellulose or fiberglass in the balloon framing without running into moisture issues, and it should be possible to install it by drilling from the exterior. Some may be installable from the attic or basement without drilling, but around window & door framing they'd be able to pop off and re-install the vinyl siding without a problem. Aluminum siding might be more difficult to deal with, depending on the exact design, but worst-case you may have to install some from the interior and patch the plaster.

If there is flashing in the window framing it's fine to use cellulose everywhere, but if there isn't it's a bit of a judgement call on just how much roof overhang there is to protect from bulk-water intrusion at the windows. In some ways fiberglass is better, since it dries more quickly from those events. With open stud bays and big air leaks there's quite a bit of drying capacity, but when you install the insulation the exterior wood stays colder (= wetter), and the drying rates slow down substantially. But 95 times out of 100 it's fine to install cellulose, which is generally cheaper than fiberglass.

Air sealing the foundation sill to the foundation with can-foam or closed cell spray polyurethane helps the air infiltration problem a lot, and filling the stud bays with insulation blocks those bays from behaving as basement-to-attic flues driving infiltration. There may be plumbing/flue/electrical chases that need to be air sealed both top & bottom too. Poured foundation walls can be insulated with 2" of closed cell foam (expensive, but quick, not-so green) or with 2-3" of rigid insulation (foil faced polyiso is the greenest, XPS is the worst), or an inch of polyiso or EPS trapped to the foundation with a 2x4 studwall insulated with unfaced or kraft faced (but not foil-faced) batts. Depending on how much above-grade foundation there is, the foundation losses could be as much as a third (and almost always more than 10%) of the total heat load, even if you're not actively heating the basement.

If you air seal and insulate the walls, your total heat load will come down to something between 15- 20,000 BTU/hr, which means you would have huge margin, even with the smallest of those boilers. And the warmer exterior-wall temps would raise the average interior radiation temperatures, making it more comfortable at ANY air temperature.

I'm pretty sure NYSERDA still has pretty good subsidies for weatherization & insulation upgrades- I haven't surfed their site regularly enough to know the details, but it can be surprisingly cheap to air seal and insulate the framed walls of a house that size. If they won't spring for foundation insulation, using used-once reclaimed or factory-seconds roofing foam you can DIY it for cheaper than 1" virgin-stock foam + 2x4 & batts from box stores. IRC 2012 code-min for your location (US climate zone ) would be R15 continuous foam, or R5 foam + R13 batt studwall (or R19s in 2x6 studs but that's a mold hazard), but even R8 or R10 would be a huge improvement over the ~R1 naked poured concrete foundation. If you end up doing the basement as a DIY, check back for details on how to do that without creating a mold hazard.

Even if you do the weatherization in stages, it seems highly likely that you'd get within range of the 29-32K output boilers pretty quickly. If there is knob & tube wiring in the place it could become an issue for the insulators, so get that checked out. But it would be silly to install a 70K-90K boiler just to cover the "before insulation upgrades" picture, when a 29K output boiler would cover the load down to -20F or colder at the "after" picture. If you oversize the boiler for the space heating load it will only run less efficiently than the smaller more appropriately sized boiler.
 

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BTW: With the boiler's high-temp set at 185 I'll assume that the estimated radiation output was for 175F average water temp (185F out, 165F back.)

Looking at the nomograph on page 2 of this document, at 175F AWT you get an output of about 160BTU per square foot of equivalent direct radiation (EDR). The stimated 46,500 BTU of radiator capacity then implies he came up with about 46,500/160= 290 EDR feet of radiator- let's call it 300EDR'.

Assuming the true heat load is about 30K, (as determined by fuel use) that's a ratio of 100 BTU/EDR'. Looking at that same graph, it means you could heat the place at 10F outdoors with an AWT of about 145F even before insulating. When you get the heat load down to about 18K, that's a ratio of 60 BTU/EDR, which means even at 10F outdoors you could heat the place with 125F AWT.

This is very much in the condensing temperature range for a condensing gas boiler, which is something else you might consider. There are some pretty good tiny modulating condensing boilers out there, as well as some pretty good heat/hot water combi systems that can work just fine with small loads such as your "after" picture, as well as the moderate load of your "before" picture.

Of course it's more money than a small cast-iron boiler, and at current gas prices there's not much of a cost reason to go with condensing gas for small heat loads like that, but with sufficient rebate incentives it can be "worth it". When all else is equal, putting the real money into upgrading the thermal efficiency of the house with air-sealing and insulation buys more comfort than spending it on higher efficiency equipment. Yours is a classic case where cutting the heat load nearly in half with insulation is better money spent than the 15-25 fuel savings you'd get out of a modulating condensing boiler.

I know it's leap of faith to cut the size of your boiler output by 2/3, but that's really what's called for when the current boiler is nearly 3x oversized, and there is a clear path toward dramatically lowering the heat load in a cost-effective manner. There are a lot of 2x4 framed 1000-1500' houses that have been retrofit insulated to heat loads less than 25K without gutting the place and starting over. This guy's 1400' house in Portland Maine is just one of many that have gone down that path.
 

cloned1973

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I'm speechless with the amount of info and work you provided. I will be able to work on sealing up the basement the best I can, but as for everything else that will be some projects in the future
That crown 70 is starting to not look so good
 

cloned1973

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And the final thing that I need to ask is about the the liner
Going from oil to gas is that a must? And if it is can I use aluminum?
Chimney is brick and its inside the home not on exterior , there is a clay liner but conditions are unknown
Thanks
 

BadgerBoilerMN

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You can use a flex aluminum liner for the low-efficiency gas boilers. Anything over 50k input would be a mistake.

A new condensing boiler, Crown and Slant-Fin not withstanding, can be vented in PVC or PP allowing you to close off the old chimney if you add and indirect-fired water heater or use electric.

Just giving Dana a breather :).
 

Dana

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( Thanks Morgan- feels like carpal tunnel syndrome setting in ... ;) )

The existing clay lined flue is almost certainly WAY- oversized for a ~25-35K boiler, and any boiler with a combustion efficiency over ~82% needs a liner that can tolerate at least some flue condensation. If you right-size the boiler it's duty cycle will be fairly high, which will keep the liner warm enough to not condense much in the first place. Short intermittent burns into a cold chimney (or liner) of an oversized boiler contributes to flue condensation problems, but that won't happen with a correctly sized boiler & liner.

Assuming you opt for something like the AWR038, it's 32K output into 300EDR' is 107 BTU/ft. If it's running a 100% duty cycle it will stabilize at an average water temp of about 145F (155F out, 135F back or something), which would be fine. But since it's high-mass radiation there is a huge slug of cooler water in the system at the start of the burn, and most of the time the radiation will never come up the full temp before the thermostat is satisfied, so the boiler has to be protected from destructive condensation inside the boiler due to that cooler temp. There are several ways to do this, but for a simple single-zone system a boiler or system bypass branch approach will work.

A system-bypass or boiler-bypass is a branch of pipe between the boiler output and return ports, mixing boiler output water into the return water to raise it's temp. In your case it's best done with with a thermostatic mixing valve where the bypass connects to the radiation return to guarantee that the entering water temp at the boiler is at least 130F. The existing system may or may not have a bypass branch (they often do), but the mix/flow is probably hand-tweaked with a ball valve or something, which isn't ideal when you have high mass radiation and a small burner. The smaller the boiler and greater the radiation oversizing for the load, the more important this is. A thermostatic mixing valve isn't super expensive but it's still quite a bit more than a ball-valve. But in your system it'll be worth the up-charge.

If you request thermostatic valve on the bypass branch to be specified as part of any proposal, a competent boiler installer would understand what that's all about. If they give you a blank stare or tell you that you need a much bigger boiler, you might want to use a different contractor.

An example schematic of a bypass branch with a thermostatic mixer, clipped at random from the web:
 

BadgerBoilerMN

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All true, but before you dive in a condensing boiler (95% AFUE) will address all these concerns without the extra labor and probable head scratching.

If you can't afford a condensing boiler, a copper boiler e.g. RayPak or Laars JVS at (85% AFUE) will tolerate lower return water temperatures without condensing and the Laars offers two-stage burning in a residential boiler. If your contractor has the cast iron religion (most do) you could use a Burnham Series 3, which features a built-in by-pass and can be had with outdoor reset for an extra fee. Again, this package will come close to a real high efficiency condensing boiler and allow you to shut down the fuel-wasting old chimney.
 

Dana

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The smallest of those li'l direct vented Burnhams is about 60K-out , but the fact that they tolerate 110F return water without exterior plumbing makes it a dead-easy retrofit here. (The Series-3 are chimney vented, and would still need the liner.)

It would be 2x oversized for the current heat load, more than 3x oversized after the house is tightened up and insulated a bit, but with the smart controls that it comes with and high thermal mass of the radiation the fact that it's also a bit oversized for the radiation isn't such a big deal. With the AWR070 you'd be at the same ridiculous oversizing level, but would have to add heat purge control & cool temp boiler protection to buy back some efficiency. The AWR038 it's about right-sized for the current heat load, and would still pretty much hit it's efficiency numbers after insulating, But it's not direct-vented like the Burnham, which keeps the parasitic loss of the chimney convecting 24/365.

BTW: The installation manual implies the the AWR0xx can tolerate 120F return water. See the discussion around Figure 8.4, p16 (p18 in PDF pagination.)

Putting off the weatherization projects until later isn't necessarily the right approach. Tightening up and insulating would cut the heat loss almost in half, and provide more comfort. The cost of air-sealing & insulating those empty walls (even without NYSERDA rebate subsidy) is less than the cost of a cast-iron gas boiler installation, and would cut the heat load by 30% or more. At that point the basement insulation is probably the next-most-cost effective step, and is still "worth it", whether heating with gas or oil. With rebates the cost of air sealing and insulating walls becomes an instant no-brainer type of investment, even for short-timers looking to sell in 3-5 years.
 
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