So what if you can buy modulating furnaces with 3:1 turndown ratios if you're going to oversize it by 3x (re-inventing the single-stage furnace, but for a lot more money), eh?   While I agree modulating furnaces are far more comfortable as hot air furnaces go, unless you right-size for the load much of the benefits of modulation are wasted.

And my point was that while these things exist, their market share is miniscule compared to the single stage and 2-stage units (which are also typically oversized by more than 2x for current code-min houses.)

BTW: I've yet to find the spec on even a modulating furnace with a min-fire under 25K. The popular 60K modulating Goodman has only a 2:1 turn ratio (~28K output.)  The heat load of a new 2400' code-min house in MA is often less than that.  If you know of any with min-fire in the sub-15K range or less, I'd like to read up on it!

Since MA has long been an importer of SO2 from Ohio power plants, I'm well aware of the grid mix there, but there too the grid is changing rapidly with the tapping of the VERY local shale gas resource, and in anticipation of ever more environmental regs.

As of 2009 the OH grid mix might have still been something like 84% coal fired, but as of 2011 it was already down to 78% and falling, according to EIA data. 

That's more than 5% drop in grid share in less than 3 years. That trend didn't just stop- it's accelerating, and you can bet those shiny-new gas fired generators are averaging WAY better than 30% thermal efficiency.  I don't expect 30% thermal-efficiency coal to be even 50% of the mix in 10 years (cood be rong offen am).  With a carbon tax or other dis-incentives, coal could even be all but GONE in 10 years.

I DO expect PV solar to be cheaper than 30% efficiency coal before 2020, and while it's not exactly AZ, they should be able to undercut 30% efficiency gas-fired peakers, lowering the cost of summertime peak power.  If the regulatory environment there is changed up to incentivize PV, and that can happen scary-fast!

Since CO2 lb/kWh vary so much over the country, saying that ASHPs have higher slightly carbon intensity than an efficient NG furnace 'on average' is not that helpful IMO. For some regions the ASHP will have a lower footprint, in others it will be higher.

There are various decent tools for estimating the carbon intensity of your grid electricity. A current one appears to be: http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html

My site on the north side of a ridge is heavily shaded...reducing demand and buying renewable is the best I can do.

Care to repeat your calculation for COP needed for CO2 breakeven versus different lb/kWh supply?

Posted By Dana1 on 28 Dec 2012 05:17 PM
So what if you can buy modulating furnaces with 3:1 turndown ratios if you're going to oversize it by 3x (re-inventing the single-stage furnace, but for a lot more money), eh?   While I agree modulating furnaces are far more comfortable as hot air furnaces go, unless you right-size for the load much of the benefits of modulation are wasted.

Don't be silly. Of course, you would like to size it to the load. However, there is an important factor often left out of the discussion when sizing to the load. Standard procedure is to determine the size based on meeting the requirements at the heating design temperature. This is a necessary, but not always a sufficient condition.

If the owner likes to use temperature setback, then recovery from thermostat set-back can be a comfort factor. For example, if the user uses set-back for business or personal travel and comes home after two days or three days to a cold house, it can be inconvenient to wait for the house to heat up with a "right-sized" furnace based solely on design temperature calculations. Modern thermostats take care of the problem for regualr schedules by pre-heating the house, even using learning algorithms to get it right, but if the schedule is not regular, this can be a minor comfort issue. I have a 60,000 But/hr furnace for a design temperature load of about 14,000 Btu/hr (including ASHRAE recommended ventilation). I specified the smallest unit that I could of the design available from the builder, and would have preferred something smaller. However, I would probably not want something less than 30 or 40kBtu/hr, or the place would be very slow to warm up after set-back, or when coming home after a cold skiing adventure.

So although smaller mod-con natural gas furnaces would be desirable, you might have some dissatified home owners if you installed a 15 kBtu/hr furnace in a house with a 14 kBtu/hr design load. Try it before you recommend it!

Posted By woodgeek68 on 28 Dec 2012 05:36 PM
Care to repeat your calculation for COP needed for CO2 breakeven versus different lb/kWh supply?

What state are you in? What are you natural gas and electric rates? What is your average COP?

I was thinking specifically if you could work out a lb_CO2/MMBTU for a 98% NG furnace, then work out a lb_CO2/MMBTU for ASHPs at different COPs and lb_CO2/kWh. I'll give it a stab....

For NG heat...1 MMBTU (input) requires 10 therms of NG and releases 117 lb of CO2.  For a 98% furnace, lets call it 120 lbs/MMBTU output.

For resistance heat at 1 lb CO2/kWh, that is 10^6/3414 = 293 lb CO2/MMBTU.

So, if my elec is 1.06 lb CO2/kWh, and my SCOP = 2.5 (conventional split ASHP in Philadelphia), then I get 293*1.06/2.5 = 124 lb CO2/ MMBTU delivered.

So, my estimate is that my CO2 with an ASHP with my (conventional) elec is only a couple % higher than with a 98% NG furnace.  If I buy wind power, I win (subject to certain assumptions).

woodgeek68-

Where does the COP number come from?

It looks like the largest source of electric power in Phili is nuclear. That reduces your carbon footprint, but leaves the unsolved problem of what to do with the high-level (and low-level) nuclear waste. It is hard to assign a final value for carbon footprint for that unsolved problem since we don't know what amount of energy might be required for that disposal process, but we will hope that it is small.

One should also consider that electricity used at peak load hours typically pollutes more than electricity generated during off peak hours. So even if you don't have off peak price breaks, it's worth looking for ways to better time loads.

Carbon footprints are very slippery things.  As it is, I buy wind power (which still has an associated but smaller carbon footprint).  But some would still say that I am buying low carbon power that someone else could have bought....so there is no net savings from my choice.

The COP number is my best estimate from modelling my heating load from timing three different heaters with known nominal BTU outputs (a previous oil boiler, the current ASHP and resistance strip), and measuring the power usage of the ASHP with a power monitor, taking defrost cycling and blower energy into account.  At my typical January temp (30°F) and 'dry condition' defrosting, I get something like an all-in COP=2.25.  The seasonal average is tricky, but it looks like I will get 2.5 in a warm year, 2.25 in a typical year and a bit less than 2 in a rather cold year.  Demand reduction is very helpful on this figure as at reduces strip run hours dramatically.

We totally agree that high-eff NG units can provide cheap and low carbon BTUs.  Maybe I'm slow about the message in the original report.  Are we afraid that there are a bunch of people with NG appliances that will scrap them in favor of ASHPs?  Are we afraid that given the choice in new construction, builders will put in expensive mini-splits rather than NG appliances?  Why is the NG versus ASHP the most useful comparison?

It seems to me that NG is hands down the most popular source of space heating in the US, and pretty much anyone on the gas grid uses it, although eff of the units varies widely.  In terms of energy/climate impact, why not build a great $$ and climate case for high eff versus conventional units??  At the current low NG prices, it seems modcon installs are stuck in an 'early adopter/green' niche. 

I think we agree that (unless they decide to run a NG pipe to every house in the US) a future ecotopic society will be using HPs and renewable electricity for a significant share of their space heating needs.  And right now there are far more heating BTUs being created in legacy fuel oil systems, low eff NG units, and electric baseboards than there are coming from compressors.  So, I hope that the future will have both modcons and nice ASHPs, so competing the two against each other, rather than the crappy systems that are still being installed by the millions seems like a bad tactic for constructive change.

I don't worry about the v knowledgeable folks who post on this board, but I DO worry about large numbers of ill informed folks finding this thread in a Google search and taking away the message that ASHPs are not a green choice because they are less green than HE NG in the 'average US case', while in large swaths of the country without NG and relatively green power, ASHPs might be the greenest and cheapest choice they can make.

Posted By woodgeek68 on 29 Dec 2012 07:14 AM
Carbon footprints are very slippery things.  As it is, I buy wind power (which still has an associated but smaller carbon footprint).  But some would still say that I am buying low carbon power that someone else could have bought....so there is no net savings from my choice.

When I purchased wind power in the past, the utility that I purchased from was audited to assure that they were purchasing enough wind power to at least match the use by consumers who had contracted for wind power. Therefore, as more folks signed up for wind power, the utility had to add more capacity, increasing the total output of renewable energy on their grid. So I don't think that you are just swapping around somebody else's green energy.

Althernatively, you could look at the electrons coming out of the cord to see if they have little propellers...

The COP number is my best estimate from modelling my heating load from timing three different heaters with known nominal BTU outputs (a previous oil boiler, the current ASHP and resistance strip), and measuring the power usage of the ASHP with a power monitor, taking defrost cycling and blower energy into account.  At my typical January temp (30°F) and 'dry condition' defrosting, I get something like an all-in COP=2.25.  The seasonal average is tricky, but it looks like I will get 2.5 in a warm year, 2.25 in a typical year and a bit less than 2 in a rather cold year.  Demand reduction is very helpful on this figure as at reduces strip run hours dramatically.

So, COP of 2.25 is a good number for a typical year in Phili. (if we ever have one of those), but there is not an easy reference to find typical values.

We totally agree that high-eff NG units can provide cheap and low carbon BTUs.  Maybe I'm slow about the message in the original report.  Are we afraid that there are a bunch of people with NG appliances that will scrap them in favor of ASHPs?  Are we afraid that given the choice in new construction, builders will put in expensive mini-splits rather than NG appliances?  Why is the NG versus ASHP the most useful comparison?

What I have been seeing on this forum is air source heat pumps (ASHP) recommended for all applications. My point in the post was to consider the various options for efficient heating systems including, from simplest to most complicated, passive solar, high-efficiency natural gas furnaces, ASHP, and ground-source heat pumps (geothermal to some). Paying extra for a heat pump system, whether air-source or ground-source to get a "green" solution, instead of buying a high efficiency natural gas furnace and having money left over to buy solar PV, may not be the best choice for everyone.

Lee- I've been installing high efficency gas boilers and furnaces in my houses from 1984 thru four years ago. Last year I installed a water source geothermal heat pump system - worked as well as any gas furnace, but without the combustion issues and at a much higher cost. This year I've installed air source heat pumps. No combustion (except at the power plant; hence the need for photovoltaics), and much lower installed cost than either geothermal or gas. One reason for this is obviously that my minisplits (or, more accurately, my houses) do not require a circulation system - baseboard hot water, radiators, or heated air ducts. And, they (minisplits) are cheaper to run. I don't know about "all applications", but for a superinsulated house, in my area, it makes no sense to use anything else.

Bob,

That is interesting and surprising to me. Some questions:
1. What is a typical zip code?
2. What is a typical heating system energy use over a heating season, and for how many degree days?
3. What is the electricity cost per kWh?
4. What is the gas cost per CCF or therm?
5. At what COP do you estimate that the heat pump is operating in heating mode?
6. What is the installed cost for a minisplit?
7. What is the installed cost for a natural gas furnace, and what is the cost for the ducting?
8. What does the typical insulation package cost?

> my houses) do not require a circulation system

Of course nat gas and geo exchange can be used in a ductless configuration too. I'd like to see more analysis of not using (or minimal) heating ducts and perhaps putting a little baseboard electric in the rooms that don't stay quite warm enough when the door is closed. But one has to have HRV fresh air to each closed room anyway.

Bob,

That is interesting and surprising to me. Some questions:
1. What is a typical zip code? I'm at 03268
2. What is a typical heating system energy use over a heating season, and for how many degree days?
6300 degree days
3. What is the electricity cost per kWh?
about $.18
4. What is the gas cost per CCF or therm? $2.85 per gallon for propane. natural gas only available in the cities.
5. At what COP do you estimate that the heat pump is operating in heating mode? have no idea
6. What is the installed cost for a minisplit? $2500-$3000 for 12,000 BTU; we have three in the 2400 house we're building
7. What is the installed cost for a natural gas furnace, and what is the cost for the ducting?
I've typically paid 12K for a forced hot water baseboard (no AC) and $15-20 for a hot air system with AC. I buy the system installed; I do not purchase components. The three ton geo system purchased last year was $25K installed.
8. What does the typical insulation package cost?

obviously the useage depends on the house. My clients with the geo system paid $315 for electricity to run the heating system for the last twelve months, or 2050 KWH (different electrical utility)

Posted By Bob I on 29 Dec 2012 08:26 PM
Lee- I've been installing high efficency gas boilers and furnaces in my houses from 1984 thru four years ago. Last year I installed a water source geothermal heat pump system - worked as well as any gas furnace, but without the combustion issues and at a much higher cost. This year I've installed air source heat pumps. No combustion (except at the power plant; hence the need for photovoltaics), and much lower installed cost than either geothermal or gas. One reason for this is obviously that my minisplits (or, more accurately, my houses) do not require a circulation system - baseboard hot water, radiators, or heated air ducts. And, they (minisplits) are cheaper to run. I don't know about "all applications", but for a superinsulated house, in my area, it makes no sense to use anything else.

Bob, you did not provide a cost for natural gas, so I looked for values in Manchester, NH, but only found national averages. Found this link for Boston, http://www.bls.gov/ro1/cpibosap.pdf, and it looks like a winter average for natural gas for Boston is about $1.28/CCF. If we allow a 10% upcharge for New Hamshire, that would be $1.41/CCF or about $0.048 per kWh. For a code-min 80% efficient furnace, that would be $0.060/kWh for heat delivered to house, or for a 98% high efficiency furnace, it would be $0.049/kWh.

You quoted an electricity price of $0.18/kWh. If we assume an average COP for a minisplit in southern NH of 2.1, then that would be $0.086/kWh heat delivered to house, or 75% higher than the high efficiency natural gas, or 43% higher than the code-min natural gas furnace.

IF natural gas is available, it would seem to be "in the mix" for ways to heat a house in that area, certainly inexpensive in terms of operating costs. From your cost quotes, it looks like a ducted system of any kind will not compete with a non-ducted system. So to compare with a non-ducted minisplit, perhaps the comparison should be to a wall-mount natural gas furnace like an Empire DV40E High-Efficiency Direct Vent Wall Furnace at (non-installed) $2324.70 (http://www.efireplacestore.com/cui-dv40enat.html). Although called high-efficiency, it looks to be rated at 80%+. So this would be similar or somewhat higher in initial cost compared to the minisplits, although much higher in output capability, so maybe two units instead of three, but not include A/C, and be significantly cheaper to operate. If A/C is desired, that might tilt the decision toward minisplits in spite of the higher operating costs for heating.

If natural gas is not available, then propane is much more expensive per unit energy and is not competitive with the other options in terms of operating costs.

Hi Lee
OK, thanks for your calculations. So we'd need only one furnace (higher BTU load). Does it make a difference in efficency if they are providing heat for a house requiring far less heat than 40,000 btu? I imagine they are modulating furnaces.

Since the load will be small, the difference in cost to run will be probably $100/year or less.
Less than 10% of my projects are on natural gas, so most gas installation are propane. TO use propane, we'd need either a buried tank ($) or stand alone bottles with a buried gas line. You also have to add the installation cost of the piping to the house and cast iron interior gas piping and fittings.

other issues that are important for my clients:
1. A number of them are interested in "getting off fossil fuels" and using cleaner renewable power.
2. Minisplits can run off a PV system so the house could be "energy independent". Of course, most will be grid tied, but the concept is valid even if they don't have a battery system. With PV you have a clean system, with fracking based gas, you don't.
3. the gas units take up valuable floor space, important in these small houses; the minisplits do require any floor space.
4. zero danger of explosion with minisplits or PV; not so with either natural gas or bottles of propane.
thoughts?

By the way, I've had propane for years and discovered a leak in the valve last year. have no idea how much that has cost me in vaporized fuel. On the other hand, I've never had electricity leaks or spills.

You can obtain much lower output natural-gas fueled wall furnaces, but their efficiency may not be specified, making the calculations more difficult for me as I have to guess an efficiency.

The economics for natural-gas and propane are not similar. Natural gas is usually much less expensive than propane (LNG) where it is available.

"1. A number of them are interested in 'getting off fossil fuels' and using cleaner renewable power."
I agree with the idea of getting off fossil fuels as long as they include solar PV and generate their own electricity. Simply transferring the problem to the power station does not really get you off fossil fuels, or just as bad in your area, creating high-level nuclear waste that we give to the next generation to figure out how to get rid of.

"2. Minisplits can run off a PV system so the house could be 'energy independent.' Of course, most will be grid tied, but the concept is valid even if they don't have a battery system. With PV you have a clean system, with fracking based gas, you don't."
I agree. What fraction of your clients carry through and actually get solar PV systems that all them to meet net-zero energy?

"4. zero danger of explosion with minisplits or PV; not so with either natural gas or bottles of propane."
I think of propane as particularly dangerous, and like to avoid it. However, the National Fire Protection Association states (http://www.nfpa.org/assets/files/pdf/os.heating.pdf), "Comparisons of different fuel or power options within central heating equipment do not show any specific type to be clearly and consistently better or worse for all types of loss" and "Among space heating equipment, risks are highest for electric-powered devices for all measures of loss except deaths, for which risks are similar for all four types of power or fuel." So, pay your money and take your chances.

Posted By Lee Dodge on 28 Dec 2012 06:23 PM


So although smaller mod-con natural gas furnaces would be desirable, you might have some dissatified home owners if you installed a 15 kBtu/hr furnace in a house with a 14 kBtu/hr design load. Try it before you recommend it!

As if recommending keeping it under 200%  or even 50% oversizing is ANYTHING like a recommendation for sub-7% oversizing...

I happen to live in a house where the heating plant less than 25% oversized for the design condition load, and it's just fine even with the primary zone (more than half the conditioned space) operating under setback control.

I'm not quite sure how you could have so completely misconstrued my posting- I was asking for a furnace that had sub-15K output for it's MIN-fire rating, not a 15K max for low-load houses.  A 30K max with a sub-15K min would be pretty reasonable for these smaller to-mid sized houses with design condition heat loads in the 20-25K range.  I'm sure the house with a 14K load @ design condition would be just fine with that (hypothetical) 15K/30K furnace.  And it would be both more comfortable & efficient than a 60K single stage bang-bang or 60K/30K two-stage.

BTW: If you're going with a wall-furnace fossil burner approach to tiny loads, the modulating ~82% efficiency direct vent Rinnais with ~2/1 to ~3/1 turn-down ratios  beat any bang-bang wall furnace on comfort (and noise).  They're not very expensive either.

The Rinnais are very popular for ski-condos in New England, but I've seen units heated with Mitsubishi H2i mini-splits in the same developments where the propane wall furnaces abound.  (Very few ski areas are on the natural gas grid here.)

Lee: high efficency gas furnaces, unlike minisplits, make lousy air conditioners. So for your efficiency comparison calculations, especially with minisplits, you need to include the cost of A/C.

Posted By Dana1 on 31 Dec 2012 02:29 PM
...snip...
A 30K max with a sub-15K min would be pretty reasonable for these smaller to-mid sized houses with design condition heat loads in the 20-25K range.  I'm sure the house with a 14K load @ design condition would be just fine with that (hypothetical) 15K/30K furnace.  And it would be both more comfortable & efficient than a 60K single stage bang-bang or 60K/30K two-stage.

My natural-gas furnace is a mod-con 60 kBtu/hr max / 21 kBtu/hr min, and I would not mind trying something a little smaller, as a thought experiment anyway. However, I have not seen the efficiency curves versus load, so there is no reason to expect an efficiency improvement with such a change.

I had the temperature set down yesterday since I was out in the snow, and it reached 60 degF last night around 4:30 AM. I noticed that the furnace came on at 4:30 AM this morning to try to reach 67 degF by the 6:15 AM set time. It starts out cranking at full output for about 1/2 of the temperature recovery and then throttles back after that. I suspect that the efficiency is better at the lower firing rates, due to both lower air pressure drops through the ducts and longer residence times in the heat exchanger, but that is only a guess. Going to a higher average firing rate by downsizing the furnace could be counterproductive in trying to increase efficiency. I really need to see (or generate) an efficiency versus output power curve to be able to answer that question.