Oil to Gas (Radiant Heat System)

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Nathaniel Hieter

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Moderators: My apologies if I chose the wrong forum. I wasn't quite sure where this one fit.

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For the past fourteen years my radiant heat system, fired by oil heat, has served me well. Unfortunately, despite wonderfully low oil prices, I find myself under pressure to switch to propane.

Bear in mind that I am DIYer and NOT a professional.
Here is my current system that I designed and implemented many years ago:

Type: Under framed floor, stapled-up
Zones: 4
Loops per zone: 2/2/4/6 each 250-300ft
Tubing: 7/8in pex
Heat Transfer: continuous 8x12in aluminum plates molded around pex
Insulation: double foil bubble wrap 1in below pex, backed by R19 fiberglass
Pex Spacing: 3 zones have 8in spacing, 1 zone has 16in spacing
Floor: hardwood downstairs, carpet upstairs
Heat Source: Bock oil-fired water heater
Operating Temp: Aquastat set to 140.
Return Temp: Varies but usually less than 110.
Plumbing: 2in manifold, each zone has its own pump
Pressure: closed system, 12-15psi
Location: Hudson Valley NY, well insulated but lots and lots of glass

(Domestic hot water served by Steibel Eltron Accelera 300.)

Possible replacement options (that I can think of, feel free to offer others):
  1. Just swap out the oil-fired water heater for a gas-fired water heater.
  2. Condensing gas-fired boiler
  3. Tankless gas-fired water heater (such as Takagi TH3)

Option #1 seems pretty straight-forward, a project that I can tackle myself. There seem to be direct-vent options with decent efficiency. Since my previous tank corroded at ten years I’m a little gun-shy about tank systems. Price seems in the 3-4k range. Shelling out 4k every ten years would be irksome, but not impossible.

Option #2 is harder. I understand the principles of primary/secondary plumbing, the importance of spacing pump flow-rates etc, but….. realistically I will almost certainly need professional help. I have a good friend who is a competent plumber so this is not a deal-breaker. Price seems in the 5k+ range, but I would expect the system to last longer than 10yrs given reasonable upkeep.

Option #3 has attractive pricing, but would probably cause me the most grief to get it working efficiently. In this configuration I would keep the Bock water heater as a buffer tank. I would insert the primary loop after the outlet of the water heater, pushing into the pressure expansion tank. The aquastat on the water tank would be used to control the pump pushing into the tankless water heater. Power would only reach the aquastat if one of the zones called for heat. This would ensure a maximum temperature feeding the tankless unit. To enable a high set point for the output temperature I would have to ensure the flow rate through the tankless is less than the flow rate through an individual loop via sizing the tankless pump and/or a mixing valve. Temperature settings would require some experimentation. Perhaps 125 for the tank and 165 for tankless output. I would think that a 40 degree differential would be the minimum required for the tankless to work efficiently. The price point is ridiculously (suspiciously?) attractive even considering the need for a buffer tank. (Replacing a simple side-kick is cheaper than one designed to accept a oil/gas burner.) However…… There are so many things that could go wrong!

Questions:
  1. Are there pros/cons in each choice that I have overlooked?
  2. Is anybody on this forum actually having any success with option #3?

Thanks in advance for any feedback!
 

Dana

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Option 3 is a non-starter. Unless you take the time to really design it, you'll either burn the thing out in short years. Primary/secondary plumbing is almost always necessary on tankless based systems, to keep the flow rate low enough to not destroy the flow sensors or have excessive erosion of the internal plumbing. (For the record, I heat my house with diverse radiation types with a Takagi tankess plumbed as a boiler. I did all the system design math ahead of time rather than leaving it to the discretion of the plumbing & heating company that did the installation.) Tankless based systems work better at a high or very-high delta-T, with 1-3gpm flow on the primary loop. Even though they are "rated" for 4, 5, 6 gpm or more, operating them at a high duty-cycle at high flow rates takes a heavy toll on them. To run it off a tankless you'd need to know the design heat load of your house, and set up the flows through the tankless and the flows for the radiation accordingly. Pumping direct to radiation from the tankless is rarely a good idea if you want the thing to last more than a year or three, and it takes quite a bit of system design work to get it to modulate with load. It's really not designed for it.

Option #2 would of course work, but you can't ignore the system design math there. Using outdoor reset control would allow you to maximize the efficiency, probably hitting the high-90s if it's set up correctly and the reset curve is dialed-in.

Option #1 would work, but would only go the distance if it's a condensing HW heater. A non-condensing unit would be destroyed in a season at your water temps, unless you did the math on that one too. A glass-lined HW tank (condensing or otherwise) would also have a lifespan of only a decade or so, but a stainless version properly set up should go a couple of decades or longer. HTP makes some well-targeted combi systems and HW heaters that would probably work in your application, but we'd need to know more info starting with the basics:

What's the heat load of your hosue at the 99th percentile temperature bin for your location?

If you haven't calculated it, how much oil do you use per year?

Does the oil delivery company stamp a "K-factor" on the billing? If yes, what is that number, in a mid or late winter fill-up?

If not, what are the exact fill-up dates & volumes for the past heating season or two, so we can correlate fuel-use to heating degree-day data for your location?

Got a ZIP code? (For weather & outside design temp data purposes.)
 

Nathaniel Hieter

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Dana,

Thanks for the detailed response.

ZIP: 12514

Using loadcalc dot net:
36k @ 0F
41k @ -10F

Unfortunately, my oil usage is hard to calculate: Too many changing variables in the last seven years and I don't have records before that. I have used a wood stove in a wide range from primary to backup. I also transitioned from using oil for hot water to a Steibel-Eltron HPHW. To make matters worse, I have two 330 gallon tanks and am on "will-call" for delivery..... so it gets a bit tricky.

Before you invest too much of your valuable time, please note that I have decided to delay the transition for a few years. I plan on using wood stove as primary for the next several years, so my oil consumption will not be large enough to warrant a massive change. By downsizing to one oil tank and moving it outside (yuck! i'll take as much precautionary advice as i can get!) I solve the pressing space issue.

Of course, if the Bock HW unit dies..... then the timetable gets moved up quickly.

As of now, the timetable is 4 years: Retiring my chainsaw and splitting axe will be my 50th birthday present to myself......
 

Nathaniel Hieter

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Forgive my poor description in my first post: I did want to imply a primary/secondary loop in the #3 setup.

Bah..... Just re-read what I wrote there. I would have been better off taking a picture of my drawings. The primary/secondary loop drives the buffer tank to give stable temps to my radiant load. Good grief.... thanks for being gentle!

Not that it matters: Reading a novel's worth of posts on the subject (many of them yours!) has just about completely eroded my confidence in being able to design and execute such a system. It seems like a fun challenge, but I shouldn't treat heating my home like a hobby......
 

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The 99% outside design temp in nearby Kingston is +2F, and in nearby Poughkeepsie it's +6F, so the load at -10F is kind of irrelevant. Even though it gets that cold sometimes, it doesn't stay that cold for enough consecutive hours to really matter. Designing to the 99th percentile temperture bin is enough.

Assuming a heat load of about 35K, that's more than the output of a standard propane water heater. But it's within striking range of the output of a couple of cold climate mini-split heat pumps at 0F, that would be considerably cheaper to operate in your climate than a condensing propane heater. You'd be giving up the cushy warm floors, but you'd be gaining high-efficiency air conditioning too. If you can heat the place comfortably with a wood stove, you can heat it comfortably with a mini-split (or in your case, probably two.)

A Fujitsu 15RLS3H can put out about 15,000 BTU/hr @ -15F, and quite a bit more than that at 0F or +5F. A -12RLS3H can deliver 12,000 BTU/hr @ -15F and about 17,000 BTU/hr @ 0F. A mini-ducted Fujitsu 18RLFCD puts out about 20K @ 0F, if there's a basement or crawlspace for running mini-ducts to doored off rooms. Mitsubishi's FH-series mini-splits are somewhat lower in heating capacity per ton than Fujitsu's cold climate mini-splits, but the FH18NA wall unit type puts out about 20K @ 0F too.

Being fully modulating units with reasonable turn-down ratios, they all "play nice" with woodstoves. You can set & forget the temperature on the mini-split, and it'll cut in as the fire dies down.

Mini-split as total HVAC solutions are somewhat rare in this climate except for super-insulated types of houses, but that's not to say they can't work. A co-worker of mine just installed 3 mini-splits to heat & cool his 1920s antique, replacing the oil-fired steam system, in a Boston suburb with a 99% outside design temp of +9F (a bit warmer than your location, but not by a lot.)

I don't know if NYSERDA has any subsidy support for mini-split heat pumps in homes heated by oil, but there might be. (There is modest support for that in MA.)
 

Nathaniel Hieter

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Those are the very same Fujitsu mini-split units my plumber friend is putting in his house this week and lives just slightly northeast of me.... I'll admit I thought he was living on the bleeding edge but this is the third time I've seen those units referenced in the past month.

A few other factoids to throw in:
(1) I have a 25-30yr old Carrier A/C unit (came with the house when I bought 20yrs ago)
(2) My solar panels (tracking, ground-mount) run a slight excess (when measured over 12 months)
(3) Two-thirds of the heated space is on the first floor with full-basement access under its entirety.

A mini-split suddenly seems rather..... intriguing.

Central Hudson (our small local electric company) has rates only exceeded by companies in Alaska and Hawaii, but if I have sufficient solar capacity.... or can extend it....

An interesting note about the "cushy warm floors". My wood-stove is in a prime location. There is so much thermal mass and the water conducts so well..... the floors are remarkably warm and the heat is evenly distributed. Not quite the same as when the radiant is on, but still quite pleasant. It would indeed be difficult to give that up (mostly due to the hellish man-hours running all that darn pex).

Wow! Thanks, Dana! You have given me a great deal to think about......
 

Dana

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If you have Hawaiian style electric rates you can beat that hands down with solar at this year's rates in my area ($3.50-ish per watt on the roof top, installed, prior to any subsidies or tax rebates.)

Better class mini-splits use less than half the power for cooling than your quarter-century old 1-speed Carrier.

For a whole lot o' money you could install a 4-5 ton geothermal heat pump and probably still cover your heat load with the existing floor radiation, but at those water temps it would be less efficient than a mini-split.

For less money you could run the floor with a cheap 1-speed 5 ton chiller with anti-freeze in the hydronic loop to raise the floor temp most of the season, but it would crap out on capacity well before your 99% outside design temp. You'd still need mini-splits or something else to cover the full design load.
 

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Could you help me understand why the air-delivery system of a mini-split is so much more efficient (and better capacity it seems) than the heat pumps that deliver through hydronic systems?

The same friend that I worked with on the solar also does geothermal....... but the shale on this mountain is not conducive to trenching so I am leery of the drilling costs. (Not that I really priced anything out. I just assumed it was out of my reach.)
 

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The Daikin Altherma air-to-water heat pump is about as efficient as air-delivery mini-splits given sufficiently low water temp requirements, but it's expensive and doesn't have sufficient capacity at your (apparent) water temp needs. If it was a radiant slab that only needed 90F water on design day it would be worth pursuing, but you're talking ~140F water, which won't cut it.

The efficiency of the Altherma is higher than a cheap chiller because like air-coil mini splits, it modulates with load, and at part load the efficiencies of the "oversized" heat exchangers is much better, and the power of the pumps & blowers much lower per BTU delivered. A 1-speed heat air source pump uses the same power whenever it's running, so it's efficiency & capacity dependency is fixed, depending only on the air temp and output water/air temp, whereas modulating systems' efficiency also varies with load.

A mini-split running at max speed at 10F isn't going to pull a coefficient of performance (COP )better than 2, often more like 1.5, but if it's loping along at 1/3 power at that temp it can be bumping on a COP of 3, even at 10F. At 40F running at minimum modulation they can beat a COP of 4 or sometimes 5 (depending on defrost cycle losses.) Optimally sized to the load a best-in-class cold climate mini-split will deliver a seasonal average COP between 3.0-3.5 in your climate, even though when it's below 0F it won't be doing better than ~2, even when modulating.

This bit of bench testing is now bit dated, 1-2 generations back from the current state of the art, but it's instructive for seeing how much difference modulating can make. The clearest picture is figure 5, p10 (p18 in pdf pagination), where the COPs of the 12RLS2 are tested at 3 different speeds at a number of outdoor temperature conditions. At 35F outdoor temp a the lowest compressor & blower speed the efficiency is a COP of about 5, nearly twice the max-speed efficiency at that temp, but at +8F it's in a tight band between a COP of 2.2-3 at all compressor & blower speeds. With a mini-split sized to have sufficient capacity at 0F, it'll be running near it's minimum speed and highest efficiency most of the season, even if it has crummy efficiency at 0F (which is less than 1% of the time, in your area.)

The latest-greatest cold climate mini-splits run 15-20% higher efficiency than the units tested, but the temperature and modulation level dependencies are similar.
 

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Very interesting reading, particularly the section on modulation. It was also a pleasant surprise to see heating/cooling measured capacity to be ABOVE the manufacturers reported data.....

I have spray insulation going into the attic next year (and around/underneath the bedroom over the garage). I will experiment with dropping the water temp in the radiant system. Unfortunately, in my retrofit setup I have to deal with 3-3.5" of wood downstairs and carpet-over-1"wood upstairs. Even with continuous aluminum plating, the rate of heat transfer seems (anecdotal) slow. However, I also no longer have three infants crawling around..... so my wife may relax the heating requirements a bit.

The bulbous air handlers would be a tough sell, but one of the units you linked had an in-wall/ceiling setup with a low profile. I am certain I have several options available for tucking them in.

I am a bit concerned about my cathedral ceilings. 60% of the downstairs footprint have walls that start at 10ft and go up from there. I have well positioned ceiling fans and the floor plan is extremely open (just doors to laundry/bathroom), but I still worry about hot/cold zones..... perhaps needlessly.

That leads to my next question (thanks for indulging me!). Traditionally I've seen heating vents on the floor and cooling vents in the ceiling. With a dual heating/cooling system where is the preferred location?
 

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I have spray insulation going into the attic next year (and around/underneath the bedroom over the garage).
You may have the alternative of putting the spray foam on the underside of the roof rather than just above the ceiling. That turns the attic into "conditioned space" but unheated space. That action requires doing away with the eaves and other attic vents. It might be worth some reading. What you gain is an attic that stays clean by not having vents.
 

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Spray foam insulation is good at air-sealing, but it's not the only way to air seal, and quite a bit more expensive than open-blown cellulose. Blower door directed air sealing can make it pretty tight with very modest amounts of foam/caulk/ or other materials, after which you can load it up with the cheap stuff for higher performance/lower cost.

Insulating at the roof deck is more expensive and more complicated. In this US climate zone 5 location for an unvented attic you'd want a minimum of 40% of the total R to be on the exterior of the roof deck, and keep the path from the roof deck toward the interior comparatively vapor-open to provide a drying path for the roof deck. (Code requires R20 above the roof deck in an R49 code-min stackup.) If you used closed cell foam on the interior it's more expensive (16-18 cents per R-ft^2) than open cell foam (11-13 cents per R-ft^2), and WAY more expensive than low density celluse on the attic floor (3-5 cents R-ft^2.) If you installed R49 of closed cell foam it's about 8" of foam (installed in passes of no more than 2" per lift) and creates a moisture trap, since the 8" foam would run about 0.1-0.15 perms, which is almost a true vapor barrier. (6-mil polyethyene vapor barriers run about 0.05 perms.) Roof decks can't dry toward the exterior through #30 felt under asphalt shingles (also about 0.1 perms), so it has to be set up to dry toward the interior.

With a vented roof deck interior moisture drives can be mitgated with lower permeance materials, and the moisture that does make it through the materials gets diluted by the outdoor air in the vent space before it can reach the roof deck. It can be complicated to build a higher-R vented cathedralized ceiling, which is why it's often preferable to go with insulation above the roof deck, and no venting. Rigid EPS runs about 9-10 cents per R-ft^2, gains performance with falling temps and has a stable R-value over time, which makes it the preferred above-deck solution. R20 takes 5" of foam and a nailer deck, which would allow you to use R30 of open cell foam or batts/blown fiber under the roof deck.

For the underside of the room under the garage and it's walls you'll almost certainly be better off going with open cell foam, if foam it must be. When thermally bridged by a 25% framing fraction the more expensive and environrmentally damaging closed cell foam only adds about R1 in whole-wall performance to a 2x4 wall compared to open cell, performance that can be gained more cheaply in other ways. High density blown cellulose might still be a better option though. We can go into the details of why if you're really interested, and the material stackup of the wall may make a difference.

The Fujitsu RLFCD mini-duct series are pretty flexible, and have far better air-hander drive than most of the competition. Heating & cooling the first floor with a mini-duct unit mounted in the basement should be pretty easy. Upstairs it can be a bit more tricky, but the fact that you can mount the Fujitsus vertically as well as horizontally helps.

Ceiling fans are great for feeling cooler at higher room temp, but are pretty useless for anything else. During the heating season the wind-chill effect makes it less comfortable, and although it breaks up stratification, it doesn't necessarily reduce heating energy use. With a wood stove the stratification is a lot higher than it ever would be with a mini-split (ducted or otherwise), since the output temps of mini-splits is in the ~110-125F range, no rising columns of 300F air like you get above the wood stove.

Similarly, there's no point to putting duct registers both high & low in this climate. Put them in the floor, which puts the heat where you want it during the heating season. In the cooling season it'll keep the air-temp near the ground cooler too- the higher radiant temp of the warmer ceiling will have a modest effect on comfort, but you can always turn on the ceiling fan, or drop the setpoint on the mini-split a degree or two to compensate. This is a very heating dominated location- most of the cooling load is the latent load (humidity), so having the thing running long cycles (or continuously) at low speed will be what keeps it dry & comfortable.
 

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Thanks for the info. I had a conversion of my vented attic to conditioned space, and I was thinking about it. This involved removal of the rock wool and spraying closed cell foam to the underside of the roof. $1 per board ft for the foam. Also closing off vents.

The closed cell foam offered less loss of sparse attic headroom compared with open cell.

No roof top alterations other than closing vents.

Sounds like my hesitation has been a good idea.
 

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In climate zone 5 northern IL if you do it all on the underside of the roof deck you can do the same 40% of total R as closed cell foam against the roof deck, with 60% fiber below. If all you have is 2x6 rafters (5.5"), a shot of 2" ccSPF (R12-R13) with R15 rock wool below works just fine, for about an R27-R28 total center-cavity R, with over 40% of it being air-impermeable vapor-retardent foam. That is a noticeable performance uptick from R15-R20-ish rock wool or fiberglass with a vented air gap above. At 2" the SPF runs about 0.5-0.6 perms, which is still sufficiently vapor open to dry at reasonable rates toward the interior, and it would not preclude you from putting rigid foam over the roof deck later. In climate zone 4 southern IL it only needs to be about 30% closed-cell foam. In these assemblies the vapor retardency of standard latex paint on the ceiling is sufficient to limit condensation and moisture accumulation on the surface of the spray foam, since the average wintertime temp at that level in the stackup is at or above typical indoor dew point temps. It might condense a film of moisture overnight on a cold night, but it's dry by noon the next day.

With 2" of closed cell foam the roof deck is fairly well protected from interior moisture drives, and air sealed, and you can then cheat on the R-ratios a bit if you then use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus between the fiber insulation and the ceiling gypsum. so if you had 2x12 rafters (11.25") you could then put 9" or so of R4.2/inch high-density fiberglass (R38-ish) and meet the IRC R49 code-min, but would need the smart vapor retarder detailed as an air barrier to limit condensation or frost accumulation on the surface of the closed cell foam. In the spring when the roof temps warm up the relative humidity inside the fiber layer goes sky-high, which makes the smart vapor retarders vapor-open, letting that moisture out before it's warm enough to support mold growth.
 

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I was interested in foaming the attic rafters (and gable ends) because the air-handler of my A/C is in the attic. However.... if I transition to mini-splits this is less of a concern.

I have very very little attic space since much of the first floor is "floor-and-a-half" cathedral ceilings and a measly 5in pitch for the roofs. While this makes for a (relatively) inexpensive attic insulation job, I was a bit anxious about the proffer vents that extend into the attic from the cathedral ceilings. I had planned to run them up snug to the ridge vent, but quite frankly I wasn't sure if the expanding foam would crush them.

The ceiling joists are only 2x6 which means swimming in the cellulose or using a fair amount of the rigid EPS you mention. With the A/C handler removed from the attic I would have very little reason to visit the ghastly place, making me more amenable to piling up the cellulose.

However, if the absolute price difference is reasonable (despite the large relative price difference) I may still end up going with foam just to give me easy (and clean) access to the 2nd floor ceiling.
 

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To be clear, EPS only really makes sense above the roof deck. It's nearly impossible to make it sufficiently and reliably air tight cut'n'cobbled between the rafters, tight to the roof deck, even when sealed in place with can-foam. It's been tried often, works sometimes, but fails way too frequently.

With 2x6 rafters the R12 cc foam on the underside of the roof deck with R13-R15 fiber to completely fill the cavity works just fine, and won't be a show-stopper if you wanted to add some foam above the roof deck later. With the condensing surface being the sprayed closed cell foam, the above-deck foam can be anything you like, it doesn't need to be R20.

In my area there are several vendors of reclaimed roofing foam from commercial re-roofing and demolition, which makes the cost of above-deck foam pretty cheap, since it's 1/4-1/3 the cost of virgin stock goods. You may not have to go too far to find some either. Any amount of foam above the roof deck will improve the thermal performance significantly, since it thermally breaks the rafters. Unlike between-the-rafters sprayed foam, you get the full-R out of a continuous layer that isn't thermally bridged by framing. With 2x6 hemlock or pine rafters you're looking at only ~R6.5 for the framing fraction, which doesn't change whether the insulation between them is R30 closed cell foam or R23 rock wool, which moves an oversized fraction of the heat through the assembly relative to the fraction of the roof are that it covers.
 

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Thanks for the tip on the reclaimed foam! It would seem a very cost effective solution for adding significant R-value to the bedroom floor/walls that are exposed to the garage...... a project I could tackle this winter instead of waiting for next year.
 

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I just became aware of another more sophisticated small modulating chiller that might be of interest for keeping the floor warm, even if you still needed mini-splits to keep the place fully heated on design day:

F-performance-curve.JPG


heat-table.jpg


Mind you, those are wet bulb temps- the outdoor air temps are warmer than that, but it looks like it might handle the whole load at your average January temp, depending on how much heat you get out of the floors with 122F-131F (50-55 C) water.

I'm not sure how it handles defrost modes, etc. and you'd have to use glycol to avoid freeze-up, which also affects heat transfer performance. Not a slam-dunk, but it's probably sorta-doable in an efficient way if you want to play Jr. Hydronic Designer and do at least all of the napkin-math on the system.

If you backed it up with an electric boiler you could do the whole thing with the radiant floor, but the system efficiency would go completely to hell if the chiller wasn't delivering at least 90% of the heat at your outdoor design temp.
 

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I think I can safely say that I will have some percentage of my future heating needs delivered through the heat pumps that replace my ancient A/C unit.

Using a chiller like this one for the extra heating capacity (and cushy floor comfort) would be really interesting.

If the mini-splits work out really well, then I may only need marginal supplement with the oil-driven radiant heat. I'd like to ditch oil..... but that urge is directly proportional to how much oil I am burning..... if it is not much per season....

Of course, when the oil-fired water heater goes, then I would be forced to make a decision. By then I would hopefully know (1) how much excess KWH I am generating, and (2) how much (if any) I need to supplement the mini-splits.

At that point, a chiller such as this one or perhaps an htp pioneer? It seems that the heat-pump technology continues to improve (or perhaps just our manufacturing/pricing of a well-known technology) year over year. The performance charts you linked are very encouraging. As you say, it is probably sitting in the "sorta-doable" bucket right now. But if continue to see efficiency/capacity/range improvements......
 
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