Adding basement zone to hydronic boiler / enclosing boiler

[Miguelito]

I'm finishing my basement (about 1500sf in Norfolk county, MA) . House was built in 2009. Thanks to info found here, I have completely wrapped my foundation walls in 1" DOW TUFF-R polyiso and taped all seams with DOW Weathermate tape. I'm now on to framing. Having pulled all permits for this basement finishing, I plan on having a plumber do the HVAC & bathroom work (non seem to even return calls nowadays - grrr). I just want questions answered so I can frame appropriately.

Important note relevant to this thread: My basement was always warm even in cold winters (upper 50's with mid/low 50's on below zero nights). After the polyiso, the basement is in the low 60's and the coldest I've seen on crazy-cold nights is 59F. The plan is to frame with 2x4's and stuff R13 in between for a total of over R20.

My house has a Burnam Hydronic gas boiler feeding two zones (one for each floor). See attachment below.

Questions:

1) Seeing how warm my basement is already, I was thinking maybe all I need is to add a new zone with a few runs of baseboard to warm it up the few degrees I need (5-10F) to get me to ~68-70F. Can this be done? I mean, hydronic or not, it's just a gas boiler. It should be able to feed hot water to baseboard, no?

2) From the photo below, how much room do I leave for the additional plumbing on the right hand side? I was thinking I don't need more than another foot or so from the capped end near the horizontal yellow valve, right?

3) The town forced me to insulate outside walls behind the equipment to R10 (now covered in two layers of blue polyiso for R13). To the extent that it and all the lumber enclosing the equipment is "flammable," do I have to cover it with something (sheetmetal)? It would seem odd since there has always been lumber above it when it as unfinished.

4) If this get pricey/complicated, I'm tempted to just place electric heaters for the limited use they will see. Thoughts?

 

[Dana]

1> Yes you can use baseboard to heat the basement, but unless you have more baseboard than is needed for the very low load of the basement it's going to short-cycle that ridiculously oversized 4-5 plate boiler. (Is that a P205, with ~110,000 BTU/hr of DOE output, or is it the ~80K-out P204? ) Knowing the boiler's output is necessary to get a handle on the extent of the short-cycling risk. You may or many not have sufficient radiation on the other zones to prevent short cycling too- measure it all up and report back.

2> Not clear on the question.

3> A studwall with half-inch wallboard meets all IRC criteria for thermal barriers against ignition. If the plan was to install R13s in a studwall snug up against the foam, the town was wrong to insist on a second layer of foam- R5 is sufficient for dew point control on the above grade section of wall, and the total thermal performance of R13 +R5 continuous would meet or beat IRC 2015 total thermal performance for US climate zone 5 (all of MA is zone 5). If there isn't room for a studwall behind the boiler, a layer of half-inch OSB through-screwed to the foundation with TapCons meets the thermal barrier requirement.

4> Electric heaters are very expensive to operate at MA type heating loads. A half ton PTHP could probably work if you don't want to cut into the heating system, using half the electricity (or less) of resistance heating like electric baseboards, electric radiant, etc.

Other comments: An indirect fired water heater operating as a priority zone off the boiler would give you superior hot water performance and improve the duty cycle of the oversized boiler.

You can establish a firm upper bound on the actual whole house heat load using this methodology, which would give you an idea of just how oversized the boiler really is. If the boiler's DOE ouput is more than 1.7x more than the fuel-use derived heat load, it's not going to deliver it's nameplate AFUE.

 

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[Miguelito]

Thanks Dana! See my replies below...

1> It's a Series 2 Model 205 with 130,000 input BTU/hr. I think I understand what you mean. Is it that with too few baseboards radiating heat the water temperature won't drop enough and will make the boiler shut off? Is that what short-cycling means?

2> See amended image below. I assume that, to add a zone, another section like the one in the red oval will have to be added to the right where the other (empty) oval is. Is that correct? If so, I need to make sure that when I frame the right side wall (blue line), that I have left enough room.

3> The town (which is a PITA) just wants R10 on that wall. My plan was to not frame that wall at all to take up as little space as possible, so right now the two layers of 1" polyiso are mechanically fastened directly to the block wall. If I need to cover it (with whatever has to cover the polyiso to make it ok), I will just Tapcon through it all into the wall. Is polyiso deemed more flammable than wood such that it cannot be left uncovered? If so, is wood OK as a non-flammable wall covering? If so, I'll just cover the polyiso with OSB as you said and Tapcon through it all to the wall.

4> Ok, I'll avoid electric heaters (that was the plan anyway).

Re: your comment on "an indirect fired water heater operating as a priority zone off the boiler," well, I'm not sure what that means.

Also, I ran through all the calculations and got a range of 49 to 52.5 kBTU/hr, so at an output of 106.6kBTU/hr boiler size is oversized by about a factor factor of 2 or slightly over.

 

[Jadnashua]

Most of the heat load calculators aren't worth the effort to use them...many are way too pessimistic, and overstate the needs of the dwelling.

When you have a boiler, it often ends up being a less expensive option to install what is called an indirect water heater. They call it indirect, because it gets heated indirectly! I.e., it has no on-board heating, as in no burner or electrical heating elements. In this case, you'd treat the indirect as a new zone, and the boiler would heat it. Because your boiler probably has 2-3x more burner, the tank gets heated quicker, and most people also find they can get by with a smaller tank.

You need to be careful with an atmospheric boiler if you then start to partition up the space...when both the boiler and your water heater are running (got a gas dryer, too?), they will be exhausting LOTS of air outside. That needs to be replaced, and partitioning the rooms can become problematic. That's one reason why most newer, highly efficient burners are designed with closed combustion...they have a dedicated pipe to bring in their needed combustion air so you aren't then sucking it into the area through any available crack or leak. You might want to make provisions to bring some fresh air into that utility room from outside, rather than pull it from the basement.

 

[Dana]

Short cycling for a cast iron boiler like the P205 would be burns shorter than 5 minutes, &/or more than 10 burns per hour. To emit the full ~106,000 - 110,000 BTU/hr of output for that boiler takes about 160-200 linear feet of baseboard. Do you even have that much in both zones?

The thermal mass of the water and iron in the system and the high/low limit difference keeps it from being an abrupt staccato of burns, but they add up. Every ignition cycle blows away some heat & fuel up the flue, and puts wear & tear on the ignition components, gas valve, etc. In the bad old days when the radiation all had a lot of water volume and iron for thermal mass oversizing a boiler 3x or more didn't make a huge impact on the system efficiency, but with fin-tube baseboard it does.

At a 2x oversizing factor you're not slipping off the efficiency cliff too far, but when you insulate the basement you'll be knocking another 10-15,000 BTU/hr off the peak load, making it even more oversized, but still do it, since you'll be retaining more of the standby & distribution losses that would otherwise occur. Also, a fuel use calculation does not/can not correct for the fuel used for heating hot water- so that ~50K load number is really an upper bound.

Adding another pump for a small amount of radiation in the basement is simple and works, but it's more than likely that all zones are currently over-pumped. If it's in the budget a single ECM drive pump for the radiation and a manifold with zone valves can cut the power use by more than 75%, not that it's cost effective in the near term.

An indirect hot water heater is a tank with a heat exchanger coil in the middle rather than a center flue. When the tank's aquastat calls for heat it pulls boiler water through the heat exchanger to heat up the tank. An atmospheric-drafted water heater has a huge gap in the insulation at the burner, and convects heat out of the tank into the room (and up the flue) 24/365, which is why they only have an EF50-EF60 type net performance, despite having an ~80% steady state efficiency. An indirect is inherently better insulated, and doesn't convect heat out of the tank and house all day/night, and they are lower maintenance, typically outlasting a standalone like yours by 2x or more.

When it's time to replace the water heater it's time consider an indirect. (MassSave usually subsidizes indirect water heaters.)

Most zone controllers (basically multiple zone relays all in one box) allow you to assign priority to one zone. When the priority zone is calling for heat it supresseses the calls from other zones, so that 100% of the boiler output is going into the priority zone. With 100K+ of output you can take a 24/7 shower and probably still heat your house, even without an assigned priority but if you ever replace that beast with something more appropriately sized, giving priorty to the water heater makes sense. A typical standalone tank delivers heat to the water at about 30,000 BTU/hr, so the recovery time with 100K of boiler output to work with will be VERY brief, and never an issue for losing ground on room temperatures.

With foam insulation flame spread characteristics and toxicity of the smoke is the rationale behind the timed thermal barrier against ignition. Half-inch wallboard or half-inch OSB qualify, but it's easier to hang the OSB. Make sure you observe the manufacturer's clearances to walls & combustibles in the installation manual. You're probably WAY too close to the foam where the boiler's exhaust vent hits the chimney(!). You can replace it with rock wool, if you like, but fiberglass has too low of a melting point.

A single inch of polyiso doesn't meet current code for thermal performance, neither does two, but it's close. Current code in MA is R15 continuous foam, which would be 2.5" of polyiso. It's unfortunate that it's already up, since there are multiple vendors of reclaimed and factory-seconds foam in MA. You can usually get 3" reclaimed roofing iso for about $15-20 per 4x8 sheet from places like Green Insulation Group in Worcester, or Nationwide Foam in Framingham (the two largest), but there are others.

 

[Miguelito]

Thanks for the info on the indirect water heater. I'll definitely consider it when the time comes. Makes total sense.

As far as the walls, as a side note, I did get the polyiso from Green Insulation Group - again, thanks to info from here. Anyway, thanks for the note on being too close to the exhaust vent. I had left an inch of space all around. I'll make it bigger. Per the sheet the town gave me, I had to have R13 on any outside wall, and R10 on any foundation wall. So with 2 layers of 1" R6.5 polyiso I have R13 and I am good as far as they are concerned. Note that this is only in the less than 10' wide wall inside the utility room. Everywhere else I'll have R20. I'll just add 1/2" OSB and cover the two layers of polyiso.

Other note: this utility room of about 4' x 10' will have 6 to 9 linear feet worth of louvered bifold doors, so air intake will not be a problem.

Now, on to my biggest concern, which is the new heating zone for the basement...

I'll start by mentioning that my current two zones do not heat via baseboard. The hot water is pumped to 2 separate air movers (1 in attic for 2nd floor & 1 on basement for 1st floor) where I imagine hair blows over a radiator to heat heat the zone. This works very well. I'm looking for my 3rd zone to be heated differently from the first two - via baseboard.

As I recall, I was looking to put about 40' of baseboard in the basement, which seemed like plenty (all the linear room I have without introducing huge complexity). Also, note that the basement will likely be kept at 62-64F at all times except when people go there, which on weekdays will be 2-3 hours a day and all waking hours on weekends. When fully insulated, I suspect the basement will be able to hold those standby temperatures with hardly any heat at all. Given that, what do you suggest I do to heat the basement?

Thanks!

 

[Dana]

When you say " Everywhere else I'll have R20", does that mean a 2x6/R20 (which is a serious mold hazard issue), or do you mean 3" of polyiso (which is fine) or an inch of polyiso trapped to the wall with a 2x4/R13 studwall, with no interior side vapor barrier (which is also fine)?

The boiler was probably sized for the size of the coils in the air handlers so that it still returns water to the boiler at a high enough temperature to not condense, but enough coil that it won't short cycle. It's still way oversized for your load, but not a short-cycling issue for those zones. The issue is then the basement baseboard.

If you crank up the high-limit aquastat to the max (probably 210F) the 40' of baseboard will have an average water temp of ~200F, and be emitting (best case) about 700 BTU/hr per foot , which is 28,000 BTU/hr. With 106,000 BTU/hr of boiler output serving just that zone it would be delivering 78,000 BTU/hr of excess. What is going to happen is that it will operate at something like a 25% duty cycle during all calls for heat from that zone. A P205 has about 300lbs of cast iron for thermal mass, with a specific heat of 0.11 BTU/degree-F and even including the distribution plumbing there won't be more than about 5 gallons (~42 lbs) of water with a specific heat of 1 BTU/degree-F. So (300 x 0.11) + (42 x 1)= 75lbs of water-equivalent mass. I'm not sure what the maximum high/low differential on that boiler is, say it's 20F. That means it takes 75F x 20F= 1500 BTU to raise the temperature 20F. At 78,000 BTU/hr (= 1300 BTU/min, =22 BTU/second), that means your longest burns when serving just that zone would be 1500/22= 68 seconds, which is a pretty extreme short-cycle for a cast iron boiler. In reality there is more excess BTU when the average system water is at 180F than when it's at 200F, and the burns will be closer to 60 seconds, even with the boiler temp cranked to the max.

There are ways around this with buffer tanks to provide thermal mass, or by using more (and more thermally massive heat emitters) but it may be simpler & cheaper to just hack the controls so that whenever the basement calls for heat it also runs one of the other zones (but not conversely, or it would overheat the basement.) If you don't mind the cosmetic issues and have the means to move them, you can shop around for some used high volume radiators (on Craigslist or something) rather than baseboard to raise the thermal mass to reduce the total burns per hour (even if they're still sub 5 minutes), which would also limit temperature over/under shoots to the room. To figure out how much radiator it takes, use this guide.

If the air handler & ducts in the attic is above the insulation it adds quite a bit to the actual heat load, even if the ducts and air handler are insulated. To make things works, all of the penetrations in the ceiling for ducts & plumbing add to the stack-effect driven air leakage to the house, and any duct or air handler leakage results in air-handler driven outdoor air infiltration, since it creates a pressure differential between the outdoors (a vented attic is at outdoor pressure) and the indoors. Add it all up it's likely to be more than 10% of your total heat load, and could easily be 25%.

Alternatively, you could do a Manual-J heat load calculation just to confirm the size of the load, and install a 1/2 - 3/4 ton point terminal heat pump (PTHP) to serve the basement zone, provided there is a suitably sized window cut-out in the wall to accommodate the mounting sleeve.

Another alternative is to use the hot water heater as the heat source (isolating the zone water from the potable water with a plate type heat exchanger) to drive the 40' of baseboard. At a storage temp of 140F you'll achieve an average water temp of 120F or so (temperature drop across the heat exchanger, plus across the baseboard), which will still deliver about 8000 BTU/hr, which is about the same output as a 3/4 ton PTHP. With the 350- 400lbs of water in the water heater and the much smaller burner you won't short-cycle the water heater. If/when you replaced the water heater with an indirect you would leave the basement heating system plumbed the same way, using the thermal mass of the potable water to buffer the heat to prevent short cycling the boiler.

 

[Miguelito]

Except for the utility area in question, the whole basement foundation is wrapped on the inside with 1" polyiso and taped, with 2x4 framing in front where R13 batting will go. So technically R19.5. The polyiso is my "vapor barrier."

My attic air mover is above the insulation.

Back to the basement heating. I like your idea of using the water heater to heat the basement - especially if an indirect water heater will be installed later and can still be used to heat the space. Is this something revolutionary that a plumber will freak out about doing, or is it something they should have seen before?

 

[rjbphd]

Thanks Dana! See my replies below...

1> It's a Series 2 Model 205 with 130,000 input BTU/hr. I think I understand what you mean. Is it that with too few baseboards radiating heat the water temperature won't drop enough and will make the boiler shut off? Is that what short-cycling means?

2> See amended image below. I assume that, to add a zone, another section like the one in the red oval will have to be added to the right where the other (empty) oval is. Is that correct? If so, I need to make sure that when I frame the right side wall (blue line), that I have left enough room.

3> The town (which is a PITA) just wants R10 on that wall. My plan was to not frame that wall at all to take up as little space as possible, so right now the two layers of 1" polyiso are mechanically fastened directly to the block wall. If I need to cover it (with whatever has to cover the polyiso to make it ok), I will just Tapcon through it all into the wall. Is polyiso deemed more flammable than wood such that it cannot be left uncovered? If so, is wood OK as a non-flammable wall covering? If so, I'll just cover the polyiso with OSB as you said and Tapcon through it all to the wall.

4> Ok, I'll avoid electric heaters (that was the plan anyway).

Re: your comment on "an indirect fired water heater operating as a priority zone off the boiler," well, I'm not sure what that means.

Also, I ran through all the calculations and got a range of 49 to 52.5 kBTU/hr, so at an output of 106.6kBTU/hr boiler size is oversized by about a factor factor of 2 or slightly over.

View attachment 38171

 

[rjbphd]

. Whatever you do,make sure you hire a company with HYDRONIC heating experience, not just some hvac company because it's heating nor plumbing company because the system contain pipes.

 

[Miguelito]

. Whatever you do,make sure you hire a company with HYDRONIC heating experience, not just some hvac company because it's heating nor plumbing company because the system contain pipes.

Good point. Thanks.

 

[Dana]

Except for the utility area in question, the whole basement foundation is wrapped on the inside with 1" polyiso and taped, with 2x4 framing in front where R13 batting will go. So technically R19.5. The polyiso is my "vapor barrier."

My attic air mover is above the insulation.

Back to the basement heating. I like your idea of using the water heater to heat the basement - especially if an indirect water heater will be installed later and can still be used to heat the space. Is this something revolutionary that a plumber will freak out about doing, or is it something they should have seen before?

This is pretty standard stuff, though it's unusual to do it when there is already a boiler installed. If it weren't for the fact that the boiler is probably more than 10x oversized for the zone radiation the cheapest & easiest thing to do would be to install 20-30' of fin-tube and run with it. But if you do that it's going to roughly quadruple the number of ignition cycles it sees in a year, which adds a lot of wear & tear.

Depending on your budget and time frame it might be worth stepping back and taking a look at the bigger picture rather than cobbling up a hot-water heater based solution. Since you're insulating the basement it seems you will probably be keeping that house for a decade or more(?).

Your air handlers are probably so oversized for their zone loads that you might be able to deliver the actual design-day heat feeding them 125-130F water, which would be in the condensing range for a modulating condensing boiler. The output of the coil in the air handler with 125F entering water temp (EWT) is about half the output it has at 160F EWT. The temperatures at the registers will be cooler, and if you go much lower than 125F it might be tepid enough to have a wind-chill effect rather than the "warm summer breeze" you'd be looking for. If you swapped out the beastie Burnham boiler for a right-sized mod-con boiler, there are models that modulate low enough to not short cycle on 40' of baseboard. At an EWT of 125F (or average water temp of 120F) typical baseboard puts out about 200 BTU/hr per running foot, so anything with a minimum-modulation under 10,000 BTU/hr would work just fine. A pretty-good and inexpensive ( under $2k) easy-to-retrofit model that goes that low is HTP's UFT-080W. It's input is 80K at high fire, but dials back to 8K-in at min-fire. There are other suitable boilers, but HTP being headquartered in MA give it a leg up both on the numbers of local installers and factory service & support. The napkin-math on right sizing a mod-con lives here.

Almost all fire-tube mod cons like those can be pumped direct, no complicated near boiler plumbing, and the -080W is pre-plumbed internally with a separate port to hook up an indirect. A single ECM drive "smart" pump operating under delta-T control and 3 zone valves would cut the pumping electrical power use by half from what you're currently doing despite calls for heat taking 2x longer to satisfy than running the air handlers at high temp. But the air handler blower power use would double, so overall it's probably going to be slightly more electricity used.

There are usually substantial MassSave rebates for installing high efficiency boilers and 0%/7 year loans to make it pretty easy too.

FWIW: I have a large air handler zone operating at ~125F EWT sipping off the stored heat of a "reverse-indirect" buffer tank (as are all other zones, now that I've added some micro zones). The air handler was part of the ridiculously oversized heating system at my house that was short-cycling like crazy on a radiant floor zone. Once the buffered configuration was in place it was possible to correct some of the temperature imbalance issues with the air handler in a few rooms with appropriately sized radiation rather than gutting the house to rework the ducts, etc.

 

[Miguelito]

Interesting info on mod con's. There seems to be an "up to" $1600 rebate for a 95% mod con.

The thing is, my house is only 6 years old, so I am torn about replacing a perfectly good boiler. How nuts is it to just add the 40' feet of fin tubes and if it dies sooner so be it? I can replace it then with a mod con.

(side comment: I think a relay on my basement blower has gone because it will not work on "auto," but will work on "on." The boiler will fire, see the temp has not changed and shut of, which I guess is short cycling. That means I manually have to turn it on/off or have the fan run at all times.)

 

[Jadnashua]

The relay is probably okay. Without knowing your exact configuration, there's probably thermostat (aquastat) that enables the fan...the thermostat's fan ON switch bypasses the alternate path and directly turns the fan on via its control relay (IOW, there's two paths to energize the relay: one, from the aquastat or temperature sensor that detects things are warm enough to turn on the fan, and two, to force it on, but both paths energize the same relay).

 

[Miguelito]

The relay is probably okay. Without knowing your exact configuration, there's probably thermostat (aquastat) that enables the fan...the thermostat's fan ON switch bypasses the alternate path and directly turns the fan on via its control relay (IOW, there's two paths to energize the relay: one, from the aquastat or temperature sensor that detects things are warm enough to turn on the fan, and two, to force it on, but both paths energize the same relay).

I swapped out thermostats (have the same one upstairs) and that didn't help. Yes, there are two relays. I bought them both just in case. I'll replace one and then the other. What the heck, it's a cheap and easy way to check.

 

[Dana]

Interesting info on mod con's. There seems to be an "up to" $1600 rebate for a 95% mod con.

The thing is, my house is only 6 years old, so I am torn about replacing a perfectly good boiler. How nuts is it to just add the 40' feet of fin tubes and if it dies sooner so be it? I can replace it then with a mod con.

(side comment: I think a relay on my basement blower has gone because it will not work on "auto," but will work on "on." The boiler will fire, see the temp has not changed and shut of, which I guess is short cycling. That means I manually have to turn it on/off or have the fan run at all times.)

There is no "payback" in replacing a 6 year old boiler or water heater on a fuel-use savings basis, but it does lower the parasitic heat load- the savings are greater than what the mere AFUE numbers would imply, especially at your likely oversizing factor.

But by being rid of the extra heat exchanger and extra pumps required to set up the basement zone running off the water heater, it saves both materially and labor-wise. That's still probably not going to be fully financially rational if you're not staying there a decade or more though.

If you hacked the controls such that one of the air handler zones was engaged any time the basement zone was calling for heat (and NOT conversely) you would reduce the short cycling problem when serving the basement zone, at the risk of sometimes going a bit over-temp on the air-hander zone. How easy or hard it is to achieve that end depends on the particulars of the system. If the zone air handlers & boiler weren't so oversized for the actual heating loads there would be a statistical chance of the calls overlapping, but as it is the duty cycles are way too low.

 

[Miguelito]

repost

 

[Miguelito]

Well, I had my HVAC guy come (he installed a new AC as well as a new boiler in my last house) and he suggested a wall heat pump unit given how little heat/cooling I would need.

It's a Fujitsu 12RLS3 that puts out 3,100 to 22,100 BTU of heat and 3,100 to 13,600 BTU of cooling. Advantage is not plumbing of baseboards, space taken up and worrying about blocking baseboards, and no more space taken up in utility closet. I'll just need a small electric heater for the bathroom (radiant floor?).

 

[Dana]

A Fujitsu 12RLS3 is overkill for the heat load of an insulated basement, and ultra-extreme overkill for it's cooling loads (which are nearly all latent-load, no sensible load in your location.)

I was thinking a half-ton PTHP (much cheaper than a mini-split, if you have a window opening big enough to install the sleeve) but if it's a mini-split, the half ton Mitsubishi FH06NA or 3/4 ton FH09NA would be more appropriate, since the modulate down to half that of the Fujitsu. A 1-ton Fujitu can deliver over 15,000 BTU/hr at 0F (your 99% outside design temperature is about +10F), and an insulated basement would have a heat load of half that (or less), especially when you discount the load for the amount of boiler standby & distribution loss that is accruing to the basement when it's actually that cold out.

It's not that I'm down on Fujistu mini-splits (quite the contrary!), but you don't have enough load to keep it modulating. When it's cycling on/off rather than modulating it's not much more efficient than a PTHP.

Either run a Manual-J, or an I=B=R load calc on just the exposed above-grade wall area and the window & door areas. If it's not well over 10,000 BTU/hr just on those factors there's no way you'd ever need a 1-ton mini-split. With 2" of polyiso, an 8" concrete wall, and half-inch gypsum/or OSB the U-factor would be about 0.07 BTU/hr per square foot per degree-F. With 70F indoors, +10F outdoors that's a 60F delta, so your wall losses above grade will be no more than 0.07 x 60F= ~4 BTU per square foot of exposed area. If it's 30 'x 50' with 2' of exposure above grade, that's 3000 square feet, x 4 BTU/ft^2 = 12,000 BTU/hr, and that's before discounting for the standby losses from the boiler, the water heater, the lights & warm mammalian occupants. If it's only 18" above grade it's 9,000 BTU/hr. The standby losses of the other equipment is greater than the heat loss through the slab & below-grade walls, it's only a matter of how much more.

Your average wintertime temp in Norfolk County is ~30-35F, or only a 40F difference (max) which brings the average wintertime heat load to something like 6-8,000 BTU/hr max, and by March the Fujitsu will already be cycling on/off, whereas the FH06 or FH09 will still be modulating at high efficiency and super-high comfort.

A 3/4 ton PTHP would always cycle on/off, never modulated, but would cost less than a grand for the equipment, and probably less than $1500 installed, unless it involves a lot of concrete cutting work.

A radiant floor in the bathroom would be pretty cushy for bare feet, but could be more expensive to install & run than a stub of baseboard or wall panel radiator under occupancy sensor + thermostat control, and don't oversize it for the load. Run the load numbers on just the bathroom, and subtract at least 350 BTU/hr for one sitting (but conscious) human plus some lights. If you have 20 square feet of U0.07 above grade wall and 10 square feet of U0.50 window (clear vinyl double panes, no low-E) you're looking at 80 BTU/hr of wall loss, 30 BTU/hr of window, or 110 BTU/hr which means it could be heated by half of one human occupant, in the dark. With 20 watts of LED lighting the lights would be adding 3.412 x 20W= 68 BTU/hr. Does it really need any heating AT ALL? Run the numbers to find out, but odds are even 100 watts (=341 BTU/hr) of radiant floor / wall radiator/ heated towel rack / whatever would be more than enough heat for almost any basement bathroom, but it's common to see people installing 5x that much.

 

[Miguelito]

Thanks Dana. I just saw your reply.

I realize that no matter what unit I get it will be overkill as an AC unit, but it's possible (though unlikely) door may be open a lot on a hot summer night with a lot of people down there and may start getting uncomfortable. I just consider it a side benefit of the mini split unit. It might seldom be used as an AC unit.

As for heat, I had to talk the guy down a model! My basement has 80 linear feet worth of 3' of exposed framed walls with R19 insulation and 60 linear feet worth of 2' of exposed foundation walls with R19 worth of insulation. As I mentioned my basement (currently with only R6.5 worth of 1" of polyiso) is always at a virtually constant 61F (winter or summer).

If you think a 1/2 ton Mitsu unit is the way to go, I will pursue that (seems a LOT cheaper than the 1-ton Fuji - about 1/3 the cost).

Thanks so much for your help!