Retrofit heating system
We recently bought a second home in the Catskills, which was built in the 1930s. Current heat system is oil furnace forced air with a pretty new high end furnace. We had an energy audit done and found that the house was not well insulated at all and are in the process of takin advantage of energy efficiency programs to upgrade.
The big question is what to do about the heat. Our oil bills last year were astronomical — it was a colder-than-average year, plus as I mentioned, we had no/little insulation. Our energy improvement contractor recommended a air-source heat pump system with electric burner back-up. We initially said yes, until we realized that we would have to a) upgrade our electrical service to 200 amps (we have 100 amps, which is more than adequate for our other needs); and b) require a huge generator if we want a generator – which we do – especially since we aren’t there all the time and worry about pipes freezing). We then consideredd having the oil furnce be the back-up heat source, but that seems pointless, given the average winter temps and the fact that we’d probably need back-up a fair amount.
Now they are recommending a mini-split system with 4-5 heads. The house isn’t large (1,600 sq feet), but it is not an open plan and we need to be able to close bedroom doors. Main questions are: 1) will we have enough warmth in rooms like the bathroom that don’t have a head? 2) how will the cellar, where all the pipes live, stay warm enough to be asfe for said pipes without having any sort of heating mechanism down there? it’s a real cellar — damp and unconditioned, but also where the well pump, washer/rier, hot water heater live.
So, do we stick with what we have even though we hate the fossil fuel source and cost, go with the central heat pump with all the extra costs, or go with the mini-splits?
Very confused….
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Replies
Carole,
The first step is to perform air sealing work and insulation retrofit work to make sure that your house is properly insulated. Only after that work is completed will you be able to accurately calculate your home's design heat loss and come up with a design for a heating system.
You need to find a weatherization contractor or home performance contractor who can perform blower-door-directed air sealing work and insulation retrofit work.
Carole- you'll probably find that increasing your electrical service to 200 amps won't be all that expensive, compared to the air sealing, insulation, etc. One you do it, you'll be glad.
As for a standby generator, the installed cost difference between a 5kw and 10kw generator isn't all that much. Labor, transfer switch, propane tank will be similar, if not identical. I had one installed two years ago and looked into various sizes, but it wasn't worth getting a small one.
Martin: Thanks very much for your response. We are in the process of doing that work. So, you think we should wait on the heat system until after that work is complete? Our understanding is that the heat pump recommendations were based on a computer model that took those changes into account. My husband, on the other hand, thinks we should wait and see what efficiencies we gain just from the sealing, insulating, etc. My concern is whether we can do a second set up uprades under the NYSERDA program, which is financing the work.
Stephen: Thanks very much for your response. We agree that somehwat bigger is better, but what we are finding is that we would need a very large generator (17kw or 20 kw) to handle the amperage required for the back-up heat source! Ouch. We were hoping to buy an 11kw or 14kw generator, but they won't do it.
Carole,
A well insulated 1,660-square-foot house shouldn't require 4-5 minisplit heads.
If your contractor has performed a design heat loss for the house after weatherization work is complete, then please share the design heat loss calculation (in BTU/hour).
With 4-5 ductless heads in a 1600' house each of those heads will be grossly oversized for the individual room loads, especially after you air seal and insulate the place. If an individual room doesn't have a heat load of at least 5000 BTU/hr any ductless head you put in there will be oversized and not operate as efficiently as they should. You end up with the worst of all worlds- more money up front, lower efficiency down the road.
For doored-off floor plans in smaller homes like that you can probably do pretty well with 1-2 min-ducted systems like the Mitubishi SEZ-KDxxNA series (see: http://www.mitsubishielectric.ca/en/hvac/PDF/m-series/Cooling-Heating_SingleIndoorUnit_Spec03.pdf and http://usa.mylinkdrive.com/uploads/documents/4297/document/17_8_SEZ_Ducted_Heat_Pump_Systems.pdf ), splitting the output between a few adjacent/nearby rooms with very short duct runs. The KDxxNA have a rated output -4F (it won't quit, but has no specified rating when it's colder than that) but it falls off pretty quickly below +5F.
Depending on altitude and exact location your 99% outside design temp may be in negative single digits, but that's not a disaster as long as the heat pump can meet the full heating load at +5F. The SUZ/SEZ-KA18NA has a capacity of 10,300 BTU/hr @ +5F, which would probably about half your actual heating load at that temp after you do a decent job on the the weatherization. For the time being you could keep the oil burner as your for when the heat pumps can't keep up, then later add a few small electric baseboards in the rooms you care about as the auxilliary heat. If you set the thermostats for the oil burner or baseboards 5F below where you set the heat pump the heat pump will carry the full load until it can't, but will still be carrying most of the load even when it isn't keeping up.
The max power consumption of the the SUZ/SEZ KA18NA is less than 2000watts per unit, so it won't take a gigawatt of generator to run a couple of them plus a few 300-600watt baseboards.
At least the foundation walls (and maybe the slab) needs to be insulated as part of the weatherization plan. Without the heat leaking off the oi-burner and ducts down there there freezing pipes are a serious risk. It's worth sealing the original ducts too, if you're going to be using them as your Hail Mary backup heat.
Martin -- I do not have the heat loss calculation, but will inquire.
Dana - Wow - thanks - more interesting ideas. Makes me wonder if we might go back to original proposal of Rheem central heat pump with back-up electric burners, but keep oil furnace too for times when we are at the house and need more heat (as opposed to having it set at 50). any efficincies gained there?
The problems with central heat pumps are the long & leaky duct runs, and the pressure differences between rooms created with long (partly outdoors) return paths. It's hard to keep the system efficiency up. With modulating point-source systems like ductless mini-splits the air moving through the ductless head never leaves the room, so there is no air-handler driven infiltration losses.
Other problems are high air handler power due to the duct length/head, and air leakage losses in the ducts themselves.
With mini-duct cassettes the system modulates like a mini-split, and the air is never going further than the adjacent room(s), so the duct losses and air handler driven infiltration are inherently lower, provided you do a reasonable job with the duct design, with balances supplies & returns to each room when dividing the output between 1-3 rooms. It doesn't quite reach best-case mini-split efficiencies, but it's not bad. The Mitsubishi SEZ/SUZ KAxxNA dedicated mini-ducted series have an HSPF of about 10.0, which beats a lot of lesser mini-splits out there. Unlike multi-splits, the control algorithms between the mini-duct casette and outdoor unit are highly tuned for the mini-duct, and it will usually beat a multi-split both on raw HSPF testing as well as in real-world installations. (That goes double if the ductless heads are oversized by more than 1.5x for the room loads.)
In a Catskills location with reasonable and very short duct runs the SEZ/SUZ units would deliver a seasonal average coefficient of performance of 2.8-3.0. That's compared to maybe 3.2-3.3 for a best in-class mini-split, or 1.8-2 for a pretty-good ducted 3 ton central system. Highly efficient modulating ducted systems (eg Carrier GreenSpeed) exist, but below +15F they're pretty much toast capacity-wise. That might be a solution in NJ or Long Island, but not in the Catskills.
At NY style utility rates it's worth going as efficiently as you can on heat pump technology, within some lifecycle-cost bounds.
What model Rheem was being proposed? Were they planning to run new ducts? Are there any ducts in the attic?
What's the ZIP code for this place? (To be able to estimate the average winter temp and the 99% outside design temp).
All good heating solutions start with a room-by-room heat load calculation based on the construction (the "after" picture, if you are upgrading the insulation, windows, air sealing, etc) and the outside design temperatures. Soliciting quotes for systems based on anything else usually results in oversized equipment that costs too much up front and runs less efficiently, and lower comfort levels to boot.
My advice if what you desire is to save money:
To save money you need firstly not to spend a lot on the changes, so...
Good furnace needs very little wattage from a back up generator. Here's the right plan. Use the good furnace to heat home while away. Turn off water while away and drain water if leaving for a month or more. With some weatherization and a 50 degree set point you will use the least amount of money on heat because you won't use that much oil, the price of oil is dropping and you aren't installing four heads of heat pumps. Running the furnace keeps the cellar safe, check check check.
The least expensive item to add to help with heat costs for when you are there might be a nice pellet stove. If not then a single head mini split in the main room blowing toward the hallway and use it with the furnace. You might have to move the tstat for the furnace to the master bedroom...
As to generators I prefer to install the smallest generator that comes with a whole house switching device. Smaller units cost much less and use much much much less fuel. A 20K unit might use $1,000 of propane in less than a week. A furnace does not need even 8k. 8k units come with the switch I like. 8K use less fuel. You are trying to save money.
Air sealing in the attic, at the cellar rim joist, add insulation to both be done.
Best use of the least money to try to save money. If you sell the place in less than 5 years you will not have saved money you will have spent more than if you just did some air sealing and insulation yourself and got a neighbor to help with power outages while you are away and just drain the water for long away periods, set the tstat at 45 degrees if the water is out.
Carole,
AJ's advice makes a lot of sense. If your house has a "pretty new high-end furnace," as you write, then there is no reason to get rid of it. Focus on improving your home's thermal envelope by reducing air leaks, adding insulation, and (if necessary) installing storm windows. You may also need to seal duct leaks and add duct insulation.
Properly done, these measures will drastically reduce your annual energy bill.
Then, as icing on the cake, install one ductless minisplit unit in the largest common room of your house.
Martin, AJ & Dana: Thanks all. Saving money is certainly one of the goals, if not the only. I think we are moving toward the do everything but the heat system this season and see how it goes. Maybe we can use the money we save to install solar (or do one of many other projects).
Dana - what info do you have on the GreenSpeed losing output at +15F? I'm no expert, but what I've seen from AHRI and manufacturer suggests it would be pretty good at that temperature. Ex. AHRI data on 2 ton has 17F output of 24,800 at a COP of 2.5.
Jesse- yes, the GreenSpeed is still just fine at +15F, but that's about the knee in the curve where capacity starts falling off quickly with temperature. That makes it GREAT for places where the 99% outside design temp is +15F (Long Island) or even +10F (New Jersey), but much less so in places where the outside design temp is sub-zero, as it is in most Catskills locations. At 0F the capacity has already taken a ~30% drop from it's +20F output, and at -10F it's more than half.
Click on the "Heating Capacities" tab on this online tool:
http://www.tools.carrier.com/greenspeed/
Play around with the different compressor and air handler options, the story is still about the same in all combinations- the capacity slope falls off proportionally below 15-20F, taking about a 30% hit at 0F.
The -FHxxNA mini-splits will deliver as much at +5F as at +20F, taking only about a 30% hit from that number at -10F, which is a more favorable derating curve if your design temp is 0F or low negative single digits, and doesn't break out of single digits for a week or more during normal cold snaps (or even 0F during a polar vortex event). The -KAxxNA mini-duct units aren't quite as deep-coolth friendly, but they still do OK.