Decisions, Decisions...Regarding Oil-Fired Heat

[Mculik5]

Need some help regarding best solution for oil-fired heating for my situation.

Here's my situation:

Live in NJ. Been in this house for a year, and plan to stay here for a LONG time. Heat exchanger on old oil-fired furnace is cracked. Had an independent manual J done, and the heating load is 80K Btu. Current heating system is one zone for the entire house (3200 sqft). It worked well enough last year that I'd stick with one zone if that's the best way to go given my design goals below.

My design goals, in priority order, are:

1. Efficiency - reduce energy consumption as much as possible to save $$$ (especially if/when oil prices go up again)
2. Low total cost of ownership - factoring in initial cost, maintenance costs, fuel costs, etc.
3. Comfort - obviously, but willing to make some sacrifices if necessary to save $$$
4. Longevity - I want a system that will last a LONG time provided I keep up with maintenance (which I will)
5. Low initial cost - I'm a buy once, cry once kind of person; would rather spend more $$$ initially (to a point, of course) to do it right

Here are the options I'm considering:

1. Replace existing furnace with a new oil-fired, right-sized furnace.
2. Same as option 1, but add zoning (whole house is currently one zone).
3. Install an oil-fired boiler, outdoor reset, hydronic furnace for the first floor (in basement, using existing ducts), and hydronic furnace for the second floor (in attic, using existing ducts). Possibly incorporate indirect hot water (vs. electric now).

Which one would you pick, given my design goals?

Notes:
- Sticking with oil as the fuel
- Sticking with forced air as distribution method

Thanks!

 

[WorthFlorida]

You will need someone to look over your heat distribution. Energy saved with a new furnace can be lost do to undersized or leaky air ducts. It sounds like you're asking to install two oil fired furnaces with one in the attic? To get the best cost & efficiency is one furnace with two circulation pumps. Besides, I'm not sure if code or anyone would install an oil fired furnace in an attic space.

 

[Sponsor]

 

[Dana]

Who did the Manual-J? A load of 80K for a 3200 house in NJ reads like an old hack's "25 BTU per foot times 3200 feet gives ya 80K" type rule of thumb. What were the inside and outside design temperatures used? The 99% outside design temps for most of NJ is in the 10-15F range, except for the NW corner counties.

Most reasonably tight, reasonably insulated 2x4 framed 3200' houses with at least clear glass storm windows would come in between 45-55K at those design temperatures. ASHRAE recommends no more than a 1.4x oversize factor, so if the real heat load is ~50K (likely) the burner should be no more than 70K of outbput. With hot air furnaces there isn't a lot of efficiency losses to oversizing, but it does take a hit in comfort.

Since you already have a year's worth of oil bills you can run a fuel use heat load calculation to sanity check the Manual-J. Use only fill-ups in the January-March time frame for better accuracy (less noise from solar gains, higher duty cycle on the boiler delivering something closer to it's nameplate DOE efficiency.)

It's of fundamental importance to get this right. Even if the Manual-J was fraught with errors, if it was spelled out on a room-by-room basis it might still be reasonably proportional, and usable for the duct design, even if it fails on furnace sizing.

The smallest oil burners out there have ~60,000 BTU/hr of output, which pretty much precludes the possibility of right-sizing using two furnaces. What might make better sense is using a right-sized hydronic air handler per zone and an oil fired water heater, or boiler, if the individual air handlers are big enough to keep it from short-cycling.

Ducts are inherently lossy, and add to the actual load by creating pressure differences between rooms, which drives outdoor air infiltration beyond the "natural" infiltration rate. To minimize that a Manual-D duct design, commissioned and verified with manometer readings on the ducts, and room-to-room with doors open/closed is in order. Energy Star housed demand duct design requires no more than 3 pascals (0.012 water inches) of pressure difference from room to room under all operation conditions verified by manometer when commissioned.

If you go with an oil fired water heater (which is inherently self-buffering, due to the thermal mass of the water) you can micro-zone the place using a hydronic fan coil per room rather than duct distribution. That provides maximal control over room temperatures, and does not induce the parasitic room to room pressure differences. There are some reasonably low-profile & quiet fan coils out there now, that can be right-sized for the individual room loads. This would be more expensive than ducted systems, but quieter, and more comfortable.

A hot water heater gives a lot of flexibility on downsizing the air handlers to match the design loads for maximal comfort since the thermal mass of the water prevents short cycling, and it's designed to manage lower water temperatures without flue or heat exchanger condensation. With a boiler solution you're up against balancing the short cycling potential on single zones, but also too low a return water temp that could create a flue condensation or worse, corrosive condensation inside the boiler. With a water heater you can even up-size the burner a bit to be able to provide domestic hot water even with all air handlers or fan coils running (but don't go overboard on that upsizing.)

With any house that you're planning to live in for a long time it's worth upgrading the building envelope with better insulation, higher air tightness, and better windows. Improved building performance adds more to comfort than any heating system can (with the possible exception of radiant floors & ceilings.) If your design heat load really IS 80,000 BTU/hr, there is probably some fairly high return investment to be made on the building envelope front.

 

[Dana]

BTW: Installing an air handler (or furnace) & ducts in an unconditioned attic, above the insulation and out side the pressure boundary of the house also increases the heat load, and makes the system/house as a whole even more sensitive to duct leakage & pressure imbalances. Even if you do the first floor as a hydronic air handler or furnace, using fan coil micro-zones on the upper floor has a lot of merit.

 

[Mculik5]

@Dana - Thanks for that very thorough and helpful answer.

Few things to clarify:

1. The manual J was done by a company called ConsultAir. The report is very detailed, with room-by-room calcs, and the heating load is actually 76,589 Btus, based on a setpoint of 70F and an outdoor air temp of 14F. I rounded it to 80K for this post.

2. The house was built in 1984. Average construction for the time. Lots of windows, and they are all original. Also, three cathedral ceilings, and a bonus room over the garage. Agree with your comments about the building envelope. In fact, we plan to do windows/doors next year, which are 35% of the heating load, according the to the calcs.

3. Poor word choice - "hydronic furnace" - in design option #3. The basement and attic units would indeed be hydronic air handlers connected to the hot water boiler.

4. As I've learned more about this approach, I've realized the boiler will need a buffer tank to prevent short cycling, especially with two zones.

5. Not in a position to change ducts. Static pressures are a bit high, but no issue for an ECM motor. As long as I can get the right combo of CFM and temp rise, I think we'll be OK (though I'm aware of the electricity penalty for the blower pushing against high static pressure). Subjectively, based on last winter, the duct system does a nice job of evenly heating the house.

6. One other thing to note is that the furnace (options #1/#2) and the air handlers (option #3) will have heat pumps on top. On the furnace, the heat pump can be run in the shoulder heating seasons, when the load isn't that high. On the air handlers, my understanding is that the heat pumps can be run in conjunction with the hydronic system so that the heat pump is offsetting some of the fossil fuel load when it gets really cold.

Any comments overall on which system you think will be the most efficient, and which system will have the best total cost of ownership? My gut says the hydronic setup has the potential to be most efficient (if set up correctly, which is a big if, given the complexity), but I'm not sure if it will be sufficiently efficient to overcome the higher initial price over its life.

Thanks.

 

[Dana]

Does the fuel use heat load corroborate the Manual-J? If you have the oil fill up dates and amounts, from November through March, and a ZIP code (to find the nearest weather station data) I can run those numbers for you here on the forum.

The room by room Manual-J is a map, it tells you where to focus on building upgrades, but not always in sufficient detail.

Is it 2x4 or 2x6 framing? With 2x6 framing and low density R19 batts it's usually possible to do a retrofit "dense pack" of cellulose or fiberglass without removing the batts, and it tightens up all the random air leaks in the sheathing & electrical outlets etc by a very large amount. Low density fiberglass of that era was usually poorly installed with gaps and compressions causing it to under perform, and it's also great mouse nesting material. It's harder to dense pack over mid-density R13s in 2x4 framing, but not impossible. An infra-red imaging survey of the house would show where the gaps are, and if accompanied by blower door testing it would also show where the leaks are. You don't necessarily need to do that level of survey to figure out whether you'd get a benefit from a retrofit dense pack to the wall cavities. But one serious round of air sealing is probably a higher priority.

How much was attributed to air infiltration?

Air sealing is far more than just windows & doors. In most houses of that era the foundation sill and band joist crackage is a bigger leak than all window & door leakage combined. With ducts that penetrate the attic floor/ceiling barrier air leakage between the register boots and ceiling gypsum (and the seams of the register boots themselves) and be a HUGE air leak. Ceiling light fixtures (especially recessed cans) are also huge. Utility & chimney chases that run from the basement to the attic are also really common, and gia-normous infiltration leaks (with massive stack effect drive!) . The "right" way to deal with getting the air infiltration load numbers under control is to first fix all the obvious stuff, then do a round of blower-door & infra-red camera directed air sealing by a pro. Blower door directed air sealing is bigger bang/buck than retrofit insulation (and really a prerequisite for problem-free retrofit insuation), and a FAR FAR bigger bang per buck than replacement windows.

How much of the heat load was duct losses?

Is it a finished basement? Are the basement walls insulated? (If so, how?) How much of the load is attributed to the basement? Uninsulated foundation walls can easily run 10-20,000 BTU/hr of load in a house that size at your outside design temperature, even if the basement is allowed to run cold, say 50F at design condition.

Are there any single pane windows without storms? If yes and they're in reasonable shape, a tight low-E storm window over the single pane will bring the performance up to something akin to a current code-min replacement window for half the money. Low-E storms over clear-glass double panes can work too, but can sometimes blow the seals on south or west facing sealed glass units.

If you are planning building-envelope upgrades for later and doing the HVAC stuff now, design the HVAC capacities for the "after upgrades" loads.

What did the Manual-J deliver for a cooling load?

Sizing the heat pump(s) for the cooling load(s) would probably cover at least 2/3 of the heating load. A typical cooling load for a tight house that size is 2.5-3.5 tons. A 4 ton /8 zone fully modulating ductless (or ducted) Mitsubishi can deliver 54,000 BTU/hr @ +5F in heating mode, which covers ~70% of the Manual-J load with the house as-is. It's an expensive but VERY comfortable and quiet solution and it could be the whole solution (TBD) that would allow single room micro-zoning of the bedrooms, etc. If you wanted to use the existing duct system for the main floor, their compatible MVZ air handlers can handle it in sizes ranging from 1 to 3 tons ( and they all have a 3:1 or greater turn-down ratio for max-comfort, max efficiency), so it can be right-sized for the zone load.

If you can peel ~10K off the load with window & door upgrades, and another ~10K of the load disappears with lower air infiltration &/or going ductless (getting rid of the duct losses) you're already within range of 4 ton Mitsubishi @ +14F for heating. It'll be more comfortable (primarily due to the and you can avoid the additional cost of a oil fired heating system as "backup" for the heat pump. If you take the budget for the redundant mechanical system and apply it to air sealing and other envelope up grades it buys more comfort along with lower energy use.

More traditional 1 or 2 stage split system heat pumps are cheaper (but not necessarily if you're buying two of them), but also less efficient, with more rapid capacity fall-off when the temps drop below 25F. You can get them with auxiliary resistance heaters to make up for the lower cool weather capacity, but that usually only makes financial sense in cheap electricity markets.