Talk me into (or out of) a whole house heat pump to replace dead oil boiler

[NoGoodAtScreenNames]

Heat pumps are basically air conditioners that can run in reverse so they pull heat from outside and put it in the house. Most of them now will be dual use for heating or cooling depending on the season.

Shoulder season is the Fall / Spring when the heating need is lightest. I don’t know why it’s called shoulder season... I hadn’t heard the term before hanging around here.

 

[clancey]

Thanks and taking a guess here about shoulder season now I would think that if the heating season fall and spring is the lightest then it is time to go out and chop wood for the next winter to replace whats been burn"t...Just a guess---one needs to use their shoulders to chop wood and maybe that's how the name came to be but at least now I know when it is fall and spring...clancey

 

[semipro]

I am sorry to hear this. I was considering this unit. We love the quiet operation during shoulder season. With the compressor on low speed and the air handler also running at low speed the system is almost silent.

Although the air handler fan speed does not vary in operation, it does decrease by about 1/3 when you switch configurations from 3 to 2 tons. This is done using DIP switches on the control board of both indoor and outdoor units.

 

[Highbeam]

Although the air handler fan speed does not vary in operation, it does decrease by about 1/3 when you switch configurations from 3 to 2 tons. This is done using DIP switches on the control board of both indoor and outdoor units.

Seems you would always want a certain flow rate to always deliver heat/cold to each register sufficiently.

 

[begreen]

Seems you would always want a certain flow rate to always deliver heat/cold to each register sufficiently.

The higher flow rate is not necessary when the indoor temp is near stasis. In shoulder season weather above 50º our heat pump and air handler are usually running at half speed. This is also true of several high-efficiency furnaces.

 

[Highbeam]

The higher flow rate is not necessary when the indoor temp is near stasis. In shoulder season weather above 50º our heat pump and air handler are usually running at half speed. This is also true of several high-efficiency furnaces.

So those rooms with high demand at the end of the duct run just don’t need conditioning? Do you think this depends heavily on duct design, return location, house shape?

 

[semipro]

There seem to be several HVAC modes where multiple/variable speed air handler fans would be an advantage and various methods for accomplishing that. Here's some related info that I've gathered (though I'm sure this is not all totally accurate.

1. Single (really set) speed AC PSC motor - These usually have multispeed capability depending upon how they're wired. The speed (RPM) changes in operation as the duct conditions change. A restricted air filter will actually cause the motor speed to increase and the flow to decrease because the blower wheel is a centrifugal device. (not intuitive I know).
2. Variable speed AC motors - the blower speed varies depending upon conditions like the temp differential across the coil or setpoint versus outside temp, etc. These typically require a variable frequency drive unit (VFD).
3. Multi-stage systems - both the outdoor compressor/fan and the indoor air handler together run at different preset speeds depending upon the stage. This is what @begreen described above (I think).
4. Variable speed DC motors, or electrically commutated motors (ECM) - These may be more efficient overall than AC PSC motors. These are "smart" in that they can change speeds to maintain motor torque, airflow, or motor speed. Although they can operate at different speeds depending upon the signal/power they get, many are set to operate at a constant speed. The advantage of this is that these motors can compensate for changes in the system like a dirty filter. For example, they can increase speed to maintain flow over the coil to prevent freezing up if a filter is dirty or some registers are closed.
5. True, variable speed in operation air handler motors, whether AC or DC - vary in speed in synchronization with the outside unit, usually an inverter compressor type unit. Rather than operating in single or multiple stages of heating/cooling, they vary infinitely (or with many incremental "stages") to provide operation over a wider range of conditions. I suspect that many mini-splits operate in this manner. It requires special communication between the indoor and outdoor units that is not present with standard thermostat wiring.

In general, decreasing the AH fan speed is useful because it can reduce fan power requirements, reduce evaporative cooling during heating, and reduce fan noise, energy use, etc.

My challenge, and the reason I've been researching these issues, is that I'd hoped that a smarter system might allow me to shut off registers to bedrooms at our house where we don't want heat while maintaining warmth in bathrooms. There are various systems comprised of electrically controlled duct dampers, remotely controlled electrical registers, and even inflatable airbags within ducts to strategically control flow to individual rooms. However, these systems do not typically coordinate with AH fans, and unintended consequences such as coil freeze-up, increased register airspeeds and evaporative cooling, and increased AH fan energy use may result.

 

[begreen]

So those rooms with high demand at the end of the duct run just don’t need conditioning? Do you think this depends heavily on duct design, return location, house shape?

Yes, proper system and duct design are important Our system is balanced with the longest takeoff runs being under 15 ft from the supply trunk ducting.

 

[EbS-P]

Yes, proper system and duct design are important Our system is balanced with the longest takeoff runs being under 15 ft from the supply trunk ducting.

After removing some of the insulation and ducts I found that every takeoff run was started from the trunk with a manual balancing damper. Restricting the short runs should increase flow a bit in the longer runs. Interesting concept if done correctly. More interesting idea would be to use some type of actuator on these dampers to control each room / run.
Evan

 

[begreen]

After removing some of the insulation and ducts I found that every takeoff run was started from the trunk with a manual balancing damper. Restricting the short runs should increase flow a bit in the longer runs. Interesting concept if done correctly. More interesting idea would be to use some type of actuator on these dampers to control each room / run.
Evan

Exactly, our system has dampers on the shorter runs. It took about 30 minutes to fine-tune the system once it was up and running. Sealing connections to reduce leakage is also important.

 

[NoGoodAtScreenNames]

I talked to the first guy before he comes out to do an estimate. He recommended trying to fix the boiler. It’s leaking from the face plate of the tankless coil (that we don’t use anymore since we have a HPWH). The prior owners installed a seasonal hot water system - oil in winter and electric in the summer. If he can fix the leaking, he advised keeping the boiler officially on during the summer, but adjust the temp down so it only cycles once or twice a day. This would protect it from leaking in the future. While I don’t love burning oil in the summer it will should be a very small amount.

For the heat pump / AC, he recommended two separate air handlers for the first and second floors. One would be in the basement and the other in the walk up attic. I know air ducts in attics are frowned upon generally. The attic is open an spacious - a lot of people in the neighborhood with the same house design have converted them into living spaces. It will be convenient to access if needed. The insulation now is fiberglass batts between the main joists and then a second layer of R-30 layer perpendicular on top with a raised platform walkway going down the middle with exposed insulation near the eaves. The ducts could be under enough insulation that they would essentially be in the building envelope. As it is the attic stays fairly cool in the summer for an attic about 100F on a typical summer day. We have a radiant barrier and solar panels on the south side providing shade. In winter it’s fairly cold up there but warmer than outside and when it’s really cold we probably aren’t running the heat pump with wood with the oil back up. Still, I’m sure that there will be some replies to not put an air handler in an attic. Talk me into (or out of that) I guess as well.

We have someone else coming on site on Friday so we’ll see what they think.

 

[EbS-P]

How many rooms / vents would there be on the second floor? Would the second floor get its own zone then? All the suggestions seems reasonable.

If you don’t keep the boiler running it is just going to keep degrading. Couple thoughts I had. The heat pump without resistive strips while not the most comfortable solution by itself should be able heat the house enough to keep the house livable on its own even when temps really drop outside. I’m guessing when in the single digits your heat pump could pump out 20k btus. I bet it could keep Temps above 50 degrees inside but I’m just guessing. No frozen pipes while you are way. So I could make an argument to do away with the oil burner but if it can be repaired I do think that’s the best plan.

Attic ducts while undesirable aren’t terrible. Insulate well and you’ll be fine.

I kept trying to think of what you could do with 100 amp service if you didn’t have a wood stove and wanted to ditch the oil heat. I kept thinking a complete boiler replacement is in the same ballpark as an electrical service upgrade. New boiler will need annual maintenance and have a 20-30 year life while the upgraded service would probably be fine for the rest of your life.

just some thoughts.
Evan

 

[NoGoodAtScreenNames]

How many rooms / vents would there be on the second floor? Would the second floor get its own zone then? All the suggestions seems reasonable.

If you don’t keep the boiler running it is just going to keep degrading. Couple thoughts I had. The heat pump without resistive strips while not the most comfortable solution by itself should be able heat the house enough to keep the house livable on its own even when temps really drop outside. I’m guessing when in the single digits your heat pump could pump out 20k btus. I bet it could keep Temps above 50 degrees inside but I’m just guessing. No frozen pipes while you are way. So I could make an argument to do away with the oil burner but if it can be repaired I do think that’s the best plan.

Attic ducts while undesirable aren’t terrible. Insulate well and you’ll be fine.

I kept trying to think of what you could do with 100 amp service if you didn’t have a wood stove and wanted to ditch the oil heat. I kept thinking a complete boiler replacement is in the same ballpark as an electrical service upgrade. New boiler will need annual maintenance and have a 20-30 year life while the upgraded service would probably be fine for the rest of your life.

just some thoughts.
Evan

Not sure about vent placement but the second floor would be a single zone. Upstairs is probably around 1,200 sq ft - 4 bedrooms, 3 baths plus a small office. So a lot of wall divisions - especially since my daughters door only opens twice a day...

I thought about the cost of a new boiler vs the cost of upgraded service as well. I would want something that was able to maintain at least 65F without a fire going. That could be because the boiler kicks in or resistance heat is used. In reality I’ll be burning wood so the house will be comfortable regardless.

For resistive heat with 100 amps is it possible to upsize that later or over do it now but set it so it doesn’t ramp up too much? So could I get something that could run up to 40 amps but keep it from ever using more than 20 until the service is upgraded? If not could I add resistive heat to a system down the road without a complete replacement? I still keep thinking though that my outdoor condenser is on a 50 amp breaker and the air handler is on a separate 15 or 20 amp breaker. How much resistive heat could I get with switching those over to a heat pump?

That all being said a service upgrade is probably somewhere in my future but the budget would like to find another way around that if possible. Doing solar, a kitchen remodel and this all within a year is eating into the kids college fund...

 

[Highbeam]

The resistive strips are super cheap, simple, small and upgradable. You could buy a low wattage set and a high wattage set now to be swapped in later just to sure the bolt holes all match up. Be sure the wiring being run allows for the amperage draw of the higher wattage strip heater. We call them strip heaters but it’s more like a few coils.

 

[EbS-P]

If not could I add resistive heat to a system down the road without a complete replacement? I still keep thinking though that my outdoor condenser is on a 50 amp breaker and the air handler is on a separate 15 or 20 amp breaker. How much resistive heat could I get with switching those over to a heat pump?

I’m guessing that on most air handlers thave a spot to install the strips and can accommodate strips with different power levels. So it seems plausible that you could install /upgrade at a later date. Would the installer want to do it that way? I’m not sure how low the the strips go but my 10Kw strip is on a 60A so you could get 3 kw on a 20 easy if they make them. I would have them install and pull wires from the recommended strips now but leave them out of the main panel for now. I think having everything their now but not being used would be a simpler install down the road. Slide in strips, pull wire into panel and connect to appropriate sized breaker. 20 minute job.

I’m guessing that a 4 ton unit will be on a 40 maybe 50 amp. Still need the air handlers on a separate breaker if air handler is separate from the compressor (again guessing).

I’m right with you in the upgrades. New roof big renovation upstairs, Diy renovation on 1000 sq ft basement that just won’t end. My record is 5 trips in a single to “get more done store” but sure doesn’t feel like it.

 

[woodgeek]

OP: "We use about 300 gallons per year with burning 2-3 cords. I would want to size the new system as if we were not wood burners though."

The KEY factor in all this is your winter BTU demand, both for sizing the HP compressor tonnage AND figuring out how big strips you need (and thus if your panel can carry it). And this info from the OP is key.

Let's say a normal winter is 2.5 cords and 300 gallons total per year. If we figure each cord displaced 150 gallons of oil, then this is 150*2.5 = 375 + 300 = 675 gallons of oil/yr if OP never burned a twig. If this figure includes the boiler is running all summer and it has a standby oil consumption of at least 0.75 gal/day, or 100 gal during the non heating season, we would discount this. But it sounds like the OP shuts it down, so I'll run with the 675 gal/yr figure for heating. Summer shutdown also gives a higher seasonal eff rating (no parasitics).

675 gal * 138,000 BTUs/gal * 0.75 (true eff) = 70 MMBTU/year. DEMAND

If we assume that OP has an annual HDD of 6000, then we can figure: 70 MM/ 6000 = 11,600 BTU/HDD. This is ~500 BTU/hr°F for his house envelope.

In MA, I would design strip heat to carry a -10°F temp with no heat pump, to 70°F, which is +80°F dT. Given that loads are slightly quadratic (due to stack driven losses going like dT^2, while conduction losses are linear) I'd round up to 90 or 100°F dT. This implies that heating with strips during a polar vortex event would be 45-50,000 BTU/hr. Since strips are 100% efficient, this is 13-14.5 kW of strip heat. 14.4 kW of strip heat pull 60A at 240V.

So, if he rips out the boiler, OP needs 15 kW strips, wired with 75A wiring and breaker (80% service rating).

CHECK: I have an older, 2340 sq ft house with a HP, it would call ~500 gallons per year, and I have 15 kW strips. I have estimated that my demand at 0°F is about 12 kW, based on timing the thermostat duty cycle. So 15 kW is sensible.

OP question: the strips themselves are just small coils like in a toaster that live in the air handler, and cost like $200, and are easy to swap out. The cost is in the heavy wiring run to the air handler. You are looking at 75A wiring and breaker. IF you decided to install undersize strips (while the boiler remains as a second stage), you would still install 75A wiring, but say put in a smaller coil and breaker to match. At a later date (when your service is increased, say), the coil and breaker can get swapped out to 15 kW for a few hundred dollars of parts.

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The other decision is tonnage. If you get two units/zones, total tonnage. I'm on a 4 ton system, and based upon OPs colder climate and 15-20% higher estimated demand, he needs 5 tons minimum, which is the max size for a single compressor unit. My system carries me to about 24°F outdoor temp. I run the HP down to 5°F, and the second stage makes up the difference.

The issue to come up is the installers. They are all gonna spitball the heating load, as I have above. Some might say OP needs 6 tons (like two 3 ton units) and 20 kW strips (like 10 kW in each air handler). Others (fearing a call back) might spec in 8 or 10 tons (two 4 or 5 ton units) and 30 kW of strips (two 15 kW strips, 150A).

I"m saying the 6-7 tons (two 3 ton units, or a 3 upstairs and a 4 downstairs) and 15 kW total strips is probably fine. Higher tons will get you a lower balance point, and save operating cost (less strip calling).

OP...what is your AC tonnage (its on the compressor label) Its probably 3 tons. This tons heat versus tons AC is why you need a variable speed compressor and more ducting. Its (possible) that you wire the two units so the upstairs unit is covering most of the AC (hot humid air enters at the top, and cool dry air falls) and get away with single or dual speed units. Maybe.

Don't worry about ducts in the attic...get them **masticked** so they don't leak, wrapped in code-level insulation, and bury them. I had to tear mine upon to mastic them after install (seriously), but then found I was losing <2% heat on my longest run. If your installer doesn't mastic ducts in the attic...look for a new installer.

You still need to call an electrician to confirm your service wiring is 100A, not just your box. Or you can read the gauge off the wiring and post it here. If you are limited to 100A service, then you are looking at fixing the boiler, wiring it as a second stage, and putting in heavy wires to the air handlers, but no or dinky strips (to take the chill off). And as said, burning 100 gallons during the summer to keep the boiler from leaking? Or you can bite the bullet, get a new service run and a 200 A panel, and be ready for the future.

When you get quotes... share the tonnage and total kW strip they say you need...

Cheers.

 

[woodgeek]

So yeah, again, I did all this in 2009-2012.

I ran for two seasons with on a new HP (ducts in attic) the old oil boiler as a second stage, while improving my envelope. They installed 15 kW strips (and a new 200 A panel) which only got called on defrost (not wired to the tstat at all). My guy put in 100 A wiring and breaker (!) worried that I would come back and need bigger strips.

My oil demand plummeted, and I could estimate break even temps by when the second stage got called (which improved as I improved the envelope).

I was still using the oil boiler for HW 12 mos a year.

In 2012 I tore out the old boiler and tank, put in a HPWH and hooked up the strips as the second stage by moving a small wire behind the tstat. And I was all good.

 

[NoGoodAtScreenNames]

Thanks @woodgeek - that’s a lot of great info to think through. You are correct. We’ve been shutting down the boiler in the off season. Usually comes on in December and off in March - though I probably do it too early and end up turning it back on during some cold spells. Early spring can be tough. It’s 55 and rainy, the house feels cold and I’m not turning the burner back on but it’s too warm for a fire. This is where a heat pump would really help for comfort (or tbh just sucking it up and keeping the boiler on more).

For the service, upgrading was an option last year when we went with solar. The panel is compatible with a new 200 breaker in and I think the wiring from the meter to the box is also upgraded with 200 amps in mind. The issue is the cable size from the transformer to the house and specifically the conduit that it is in. They said it is undersized for a 200 amp cable by at least a half inch by code.

I’m meeting with someone later today. I’m going to pull some of your and @EbS-P numbers to make myself sound a lot smarter on this stuff than I really am.

 

[NoGoodAtScreenNames]

I don’t see tonnage on the AC condenser. My main takeaway from the label is that it’s really old and uses the old refrigerant. So other than switching out capacitors this is one repair away from the junk yard.

 

[woodgeek]

Separate from all this, do some reading on 'attic airsealing'... older houses (pre 2000) typically have several square feet (!) of openings between the conditioned space and the attic, and that alone contributes 20-30% to total winter heating demand/bill. I got that fixed when I did my oil to HP swap and it made a HUGE difference on my winter comfort (draftiness). summer comfort (humidity) and bills.

Tells: You say you have R-49 attic insulation, and no air handler up there.
--In the winter, an R-49 insulated attic should have NO snow melting on the roof. That is, the snow on an R-49 roof should melt at basically the same rate as the snow on the ground in the same sun. If yours melts faster than that (or gets a joist pattern showing through) you do not have an 'R-49 attic', you have conditioned air just blowing through all that fiberglass.
--In the summer, if you turn off your AC at noon on a warm sunny day, the upstairs floor in a house with an R-49 attic should not get uncomfortable very fast. Like maybe rise a few degrees by the late afternoon and not get noticeably humid (even when the attic is roaring at 120-130°F). If it is heating up faster, you have superheated and humid attic air getting pulled down into the framing through gaps in the attic.

Attic airsealing is either a large DIY or a small trained pro project. If you have a big open flat attic, it will be relatively easy/cheap to do and have a very fast $$ payback. I'd get it done before installing ducts, tbh.

 

[woodgeek]

I don’t see tonnage on the AC condenser. My main takeaway from the label is that it’s really old and uses the old refrigerant. So other than switching out capacitors this is one repair away from the junk yard.

Googles the model # and came up dry. ACs in New England don't get much use and can last a LONG time.

If you DON"T upgrade the service:
Get a new boiler, you could also 'drop in' a new single speed (cheap) HP (and **variable speed** airhandler) on the **existing** AC ductwork. You would max size the unit to the existing ducting, and skip the strip heater. Get a smart stat that calls the HP when outdoor temp is above 35-40°F, it could probably keep up. No defrost cycles, no cold blasts. Smart stat will then run 100% oil at lower outdoor temps.

The HP will use LESS amps per ton than your existing AC...bc its more efficient.

You could 'play' with the switchover temp to set it lower and see how low it can carry you and if you care at all about defrost cycles. Lower temps would save more oil.

Given that the heat pump and oil boiler would both last 20+ years, you'd be baking in an extra few hundred tons of CO2 emissions, if you care.

 

[NoGoodAtScreenNames]

Separate from all this, do some reading on 'attic airsealing'... older houses (pre 2000) typically have several square feet (!) of openings between the conditioned space and the attic, and that alone contributes 20-30% to total winter heating demand/bill. I got that fixed when I did my oil to HP swap and it made a HUGE difference on my winter comfort (draftiness). summer comfort (humidity) and bills.

Tells: You say you have R-49 attic insulation, and no air handler up there.
--In the winter, an R-49 insulated attic should have NO snow melting on the roof. That is, the snow on an R-49 roof should melt at basically the same rate as the snow on the ground in the same sun. If yours melts faster than that (or gets a joist pattern showing through) you do not have an 'R-49 attic', you have conditioned air just blowing through all that fiberglass.
--In the summer, if you turn off your AC at noon on a warm sunny day, the upstairs floor in a house with an R-49 attic should not get uncomfortable very fast. Like maybe rise a few degrees by the late afternoon and not get noticeably humid (even when the attic is roaring at 120-130°F). If it is heating up faster, you have superheated and humid attic air getting pulled down into the framing through gaps in the attic.

Attic airsealing is either a large DIY or a small trained pro project. If you have a big open flat attic, it will be relatively easy/cheap to do and have a very fast $$ payback. I'd get it done before installing ducts, tbh.

Yup - did all that. Pulled up 20 sheets of plywood flooring, pulled up each and every bat of old insulation looking for holes (found a ton all over the place). Reinstalled insulation and built a platform higher to add another layer. Also added a radiant barrier on the rafters. Huge difference from before. I was skeptical of the radiant barrier working but it was pretty cheap and works well. Top surface of the insulation is only about 100F on a typical summer day.

 

[EbS-P]

I don’t see tonnage on the AC condenser. My main takeaway from the label is that it’s really old and uses the old refrigerant. So other than switching out capacitors this is one repair away from the junk yard.

Google your model number. I’m guessing the “48” in it is 48kbtus or 4 tons.
Evan

 

[woodgeek]

Google your model number. I’m guessing the “48” in it is 48kbtus or 4 tons.
Evan

NICE. If the AC is that BIG (sized in 1990 for a big drafty house), the ductwork might take a new 4 ton HP as a drop-in to the existing system. That could probably carry the house in winter (now that its airsealed, and better insulated) down to 30°F.

I'm almost wondering if you can patch up the oil boiler, and do a 4 ton HP 'drop in' (no new ductwork or strip wiring or service wiring required).

And you can then decide between replacing the oil boiler with new, or going to all heat pumps NEXT YEAR.

I assume the existing AC air handler is in the basement and putting most if its air into the lower story. Going 'all HP' next year would then require probably a second HP (probably 3 tons) and new (but simple and cheap) ducts in the attic, strip+wiring installation (for BOTH HPs) and $$$ service upgrade.

 

[EbS-P]

I'm almost wondering if you can patch up the oil boiler, and do a 4 ton HP 'drop in' (no new ductwork or strip wiring or service wiring required).

For your use case this really seems like the way to go. They will always want to sell you more. there just is not enough profit in it for them to put any real time in designing the perfect system. To be honest most would not want to pay that extra that extra premium. So that leaves us, the consumer that really cares in the uncomfortable position of telling the contractors what they should be looking at to install.

The second upstairs zone could be a separate system down the road. If you get at minimum a two stage compressor and replace a 4 ton with a 4 maybe 5 ton that runs 3-4 tons on the low stage you could just add the 2nd floor system later and not use the second stage until it gets really cold. Variable speed air handler are nice not sure I posted this but mine has a feature that’s called enhanced mode. Ramps up slow and stays at 80% for first 8 minutes then ramps too 100% when second stage is called. Allows you to run. Higher cfm per ton say 400 maybe 450 but only when it’s really needed.
Look for a thermostat that has 3 stages of heat and 2 cooling. Don’t mess with the Nest smart thermostats. My hvac guy said they have been a huge source of issues.
Evan