Adding a Hybrid W/H to my oil fired furnance question

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Bigal41

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Here's the deal... my son recently switched from elec to gas on his W/H. he had just purchased a GE Hybrid W/H (elec) last year ..it's a GE Geospring 50 gal I think it is red on the outside . I currently have a 10 yr old Slant fin boiler Liberty model L-30PT, oil fired with coil insert to heat my domestic H/W and the baseboard heat is from the boiler ( separate from the domestic water) ). Here is my question, I am having a plumber install the W/H... I believe he will bypass the domestic coil (Yes ??). What will happen over time to that coil that is in the boiler ?? will blow from over heating due to the fact water has nowhere to go or will it melt..LOL. Also because the Hybrid will take over for all my domestic water, can I turn down the temp controls on boiler during the summer so that the boiler doesn't come on as often.. then come winter return the temp to where it was . Hope all this makes sense.
 

Dana

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Just curious, but is it set up for steam, or is it pumped hot water?

If the there is no plumbing connected to the coil it'll just sit there. There is no reason to keep water in it, and it's fine to drain it, leaving it un-capped (or lightly capped, not gas-tight.)

There is no point to keeping the coil plumbed in series with a hybrid HW heater. Heating with the heat pump water heater will be cheaper than oil, even during the heating season. But put it in the same room as the boiler, since it will "harvest" the jacket losses of the boiler in winter, and dehumidify the boiler room in summer, reducing any oxidation issues from letting it cool off completely.

If you're not using the boiler in summer, turn it OFF, not just down. Since you don't need it for hot water, during the winter you can crank the minimum temp down (unless it's steam). Whereas you probably needed to maintain 160F or higher to have reasonable hot water performance, you can safely set the boiler to 140F at idle. (Below that you run into copius flue condensation issues, and if you go WAY below 140F you'll end up with corrosive exhuast condensation on the boiler's heat exchanger plates, shortening it's life.)

With most of the pre-packaged burners it came with L30 can be jetted for either 134MBH or 151MBH output, according to the manual, either of which is ridiculously oversized for the space heating load of any normal sized house in CT that has glass in the windows and doors that close. The consequence of that is that when it's tuned up you probably won't get better than 75% efficiency out of it, even with a heat-purging retrofit economizer (only appropriate for hydrnonic, not steam), and with the standard aquastat controls it might not hit 65%. Do you have an oil use history on it, or mid to late winter oil bills with K-factors stamped on the slip?
 

Tom Sawyer

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1 - the coil won't overheat, blow up or do anything of concern at all

2 - the low limit control on the boiler should be bypassed so that the boiler cold starts

3 - now would be a good time to add an outdoor re-set control to the boiler.
 

Dana

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ODR control on a ridiculously oversized boiler (which it probably is) doesn't do anything for efficiency, and sometimes cuts into efficiency as it'll short-cycle the boiler at lower water temps, though it improves comfort somewhat. A heat-purging economizer control is a better approach to dealing with oversized boilers.

The average home in CT has a heat load of 35,000 BTU/hr or less, and this is a boiler with over 130K of ouput, so the oversizing factor may be quite large. Unless the radiation is high-mass high-volume radiators (and not fin-tube baseboard) and similarly oversized for the load, you'd have to do something other than just ODR control on the boiler to get the comfort benefit without the efficiency hit.

It's doubtful that this boiler is cold-start tolerant, and if the radiation is high-mass and oversized you'd still want to use low-limit control during the heating season. With low-mass not-oversized heat emitters it would probably be fine to cold-start it during the heating season, but with high mass oversized radiation it would never come fully up to temp and you'd be potentially looking at copious exhaust condensation in both the flue and on the boiler's heat exchanger plates, shortening whatever life is left in the thing.
 

Tom Sawyer

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Dana, where are you getting your "averages from". While 35,000 btu is somewhat typical for homes constructed in the past twenty years or so, it is way off the mark for the majority of homes in New England unless you're talking about a two bedroom ranch or bungalow. The actual loss on my home is 135,000. No matter how I fudge the numbers the K factor alone verifies that. Yes, it's older and not the tightest home in the world and yes there are 15 rooms but it's not unusual. It's an old farm house with barn attached. Something that there are thousands of in New England. I have heard you put those numbers out many times but I'll be damned if I can find any factual reference. I've gone through RSES, ASHRAE and CARE publications as well with none of them stating such numbers.
 

Dana

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Southern New England isn't very much like Maine. CT is warmer, has more industry and has a LOT of 1200-2000 square foot post-WW-II housing development skewing the averages to the low side. Most homes in CT built in the past 100 years have either been updated with insulation or were built with insulation. An average ~2500' home in CT built in the past 20 years has a heat load under 30K, many under 25K, as verified by K-factors.

A survey of existing single family homes in nearby MA done about a decade ago by one of the major utilities came in at design heat load average of about 13kw or ~45,000 BTU/hr, which was heavily skewed to the high side by the large number of late 19th century homes in the Boston 'burbs.

When I moved into my house (a ~2400' 1920s bungalow) it's heat load was about 45-50K @ +5F (the local 99% temperature bin), but with tweaks (like insulating the walls & foundation) it's now a bit under 35K, and will be under 30K by the time I'm through messing around with it.
 

Tom Sawyer

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I think we need to be careful with blanket statements though. Sure, a whole lot of oversized equipment is out there but not all. Im in southern maine, right on the NH border and ive been running manual J's for years. Very few come bsck anywhere near that low and ill bet if i dug a few out, the average is closer to 80,000.
 

Dana

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I think we need to be careful with blanket statements though. Sure, a whole lot of oversized equipment is out there but not all. Im in southern maine, right on the NH border and ive been running manual J's for years. Very few come bsck anywhere near that low and ill bet if i dug a few out, the average is closer to 80,000.

And yet an older 1400' Portland ME area house analyzed on this forum came in at about 30K, with a plan for cost-effectively reducing to the low 20s.

I'd be very surprised if the average home in ME is anywhere near 80K, (though I could believe 50-60K), and I'm quite sure the average house in CT is on the order of half that number (or lower.)

But nobody lives in the "average house", they live in their house, which is why blanket recommendations for outdoor reset control on a boiler with 150K of output is premature and needs to be analyzed further, which is why I brought up the comparatively low heating load of the typical or average CT house. It's very common (too common) to see 3-5x oversizing factors on boiler, and 2-3x oversizing on the radiation, and even on design-day the water temp needed to heat the house can be well below the safe operating temp for on oil boiler. Outdoor reset isn't safer or useful for many (or even most) systems in CT with a boiler that size, where the boiler was likely sized to be able to deliver reasonable domestic hot water performance with an embedded coil.

The EIA doesn't have a handy 2-pager for CT (or ME), but they have one for MA. The average home in MA is about 2076 square feet, and uses about 110 MMBTU/year for total energy use, 59% of which (or 65 MMBTU) is for space heating. Over about 6500 HDD that 10,000 BTU/HDD, or 417 BTU/degree-hour. With 99% design temps in the 0-10F range that's about 65 heating degrees, or an average heat load of 65F x 417 BTU/degree-hour, for an average load of about 27,105 BTU/hr. That's a ratio of about 13 BTU/hour per square foot of conditioned space. And that's SOURCE FUEL. BTUs. Assuming an average of 80% efficiency (probably way high), you're looking at 10-11BTU/hr per square foot.

Mind you, nearly half of the housing in MA is multi-family, bringing the average load down a bit, but there's no way that the average single-family home (even the average pre-1980 home) is 2x that number. While rules of thumb are a lousy way to design, the rule of thumb for heat load that comes closest for older homes in MA is 15-20 BTU/hr per ft^2, and most of those older homes can be retrofit air-sealed & insulated to something like 12-14 BTU, which is still somewhat higher than the statewide average of 10-ish per foot when multi-families are included.

The average house size in CT is probably smaller then 2000', but say it's 2500'. At 15 BTU/hr per foot that's still only 37-38,000 BTU/hr, and at 20 BTU/hr per foot it's only 50K. Even with the lower design temps for western ME, I sincerely doubt the average house in western Maine has a heat load over 60K.
 

Dana

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"Slant fin boiler Liberty model L-30PT"

The L-30 series (with the exception of the L-30H) can be jetted for either ~150K in (134K out) or ~150K out, but in either case it's output is WAY over the likely heat load of the vast majority of single family homes in CT.
 

Tom Sawyer

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It absolutely can. Gotta watch stack temps to make sure the net isn't below 400 ish but other than that it'll be fine. Drop the firing rate, constant circulation, modulate return temps, differential bypass to keep the boiler from condensing. Piece of cake.
 
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