Number crunching

Thanks for the clarification. I read your comment and saw the figure in #133. That's why I made a disclaimer that "I understand that the energy saved may not be a lot". From your figure, 40watts saving for chilling the house 5F down during the setback would be 320Wh (for 8 hours setback). That is about 8 pennies saved each night even with our very high electric rate of $0.24/kWh. Certainly not worth suffering the discomfort on a cold morning. But it is nonetheless saving that small amount of electric energy, isn't it?

 

That article is from 1991, when I believe most heat pumps were just 1 or 2 fixed stages, not continuously variable as are most of today's DHPs. And they were quite inefficient by today's standards. And the "smart" thermostats of that era wouldn't have anything resembling the smarts of today's models.

The backup resistance heater is the first and most obvious reason for heat pumps to not want setbacks. But if it were the only reason today, then the second EnergyStar link in my previous post would most definitely not have called out "variable capacity (as opposed to single-speed) heat pumps or air conditioning" as an important other case.

(1.) No. In fact, I believe most DHPs don't have built-in electric resistance. Even ducted central furnace versions often leave it as an optional feature, you can save purchase cost by leaving it out. Or choose between different backup fuel types.

(2.) A special heat-pump setback thermostat needs to have some idea how much heat power a variable capacity unit is putting out at any given time, plus some idea of how efficient it is at its various settings and conditions. Or some idea how much input power it is consuming vs output heat power. Remember that these units have widely variable efficiencies depending on conditions, and the thermostat must understand or build a map of these differing efficiencies and patterns. For a given pair of input and output temperatures, these systems will have a certain specified efficiency at rated heat power, even higher efficiency if running at reduced heat output (e.g. lower heat demand, or when allowing a gradual cooldown but not going off cold turkey), but lower efficiency when running at maximum heat output (higher than 'rated' or nominal output) when delivering makeup heat when recovering from setback.

A non-heatpump smart thermostat doesn't need to know anything at all about variable efficiency, or usually even variable heat output capacity. Combustion and electric resistance heats normally have a single fixed (or nearly so) efficiency regardless of conditions, and often just a single fixed heat output power too, so their optimization puzzle is much much simpler. That is why this type of smart thermostat, installed on a heat pump, might be counterproductive.

Heat pump smart thermostats also need to understand, or at least not get confused and scrambled by, defrost cycles, something that other thermostats don't need to deal with. A direct data link is helpful here.

That would be true if its heating efficiency is a fixed constant, not variable. But today's inverter-driven units are widely variable. See above.

 

Note that that chart is showing just maximum heat capacities, plus their associated input powers. If you aren't using maximum heat at that temperature, then you aren't using that much input power either, so efficiency will usually be better than shown on that chart.

Note also that this table shows more heat capacity for an indoor 65F than 70F, but the house will actually need less heat at 65F, that is the reason for considering setbacks in the first place. So if the house is hitting that tabulated max heat / input power value at 70F, it won't hit the corresponding 65F tabulated value, so the input power savings will be better. By how much? We don't know from this table. And how much of the savings is burned up on recovery? We don't know that either.

 

There is an additional reason I don't use heat pump setbacks, but this is specific to my local climate and power grid. Other areas have different tradeoffs.

My local electric supply is almost entirely hydroelectric, in a heating-dominated climate zone with huge amounts of legacy electric resistance heat. The highest electric demand here traditionally happens at 7-9 a.m on a cold winter morning, not at 5 p.m. sometime in August as is typical in southern zones. As climate gets warmer and more people get AC, our patterns are shifting, but won't be like the South anytime soon.

Setting back my thermostat at night, recovering in the morning, would mean cutting electric demand during the overnight energy production lull, then boosting demand during the morning peak, amplifying the grid's diurnal power swing. This would increase the flow variability on our hydroelectric-controlled rivers, making conditions even worse for the survival of fish and their egg nests, and increasing riverbank erosion.

Running a constant thermostat setting means I'm pulling more power at night when other loads drop, then not participating in the morning surge. Put another way, I'm subtracting from the morning surge and shifting it to the overnight baseload. This patterns helps reduce that diurnal power grid swing, and thus the artificial diurnal river flow variations.

People drawing energy from a primarily carbon-burning electric grid will have different considerations.

Many BEV enthusiasts recommend that car charging be done overnight, during the overnight baseload lull, to reduce grid stress. Very similar idea.

 

I can understand that the variable nature of today's HP can operate more efficiently than the old fixed heating efficiency unit. I admit I don't fully understand all the technology behind the heat pump and how they have improved over the years. But from your comments, it sounds like that with today's HP, it is possible to use more energy by setting back the temperature compared to keeping it constant temperature at a high point. Yes, we don't know how much of the savings is burned up on recovery, but is it possible to use more energy by lowering the thermostat whatever the duration for each day, compared to keeping it constant at the high point all the time with today's variable efficiency HPs?

For my use case, the answer to this question becomes critical. If I don't do anything to the existing central heating system and install a single-zone HP in the central living area, then I am forced to setback HP the temperature setting most likely every night in order to allow the oil boiler system to kick on to heat the rest of the house. Without such intervention, I can see the rooms, especially on the lower level, will freeze, causing major plumbing damage. But if such a setback on HP results in using more electric energy for the HP than keeping the temperature constant, then my saving would be diminished. If the overall saving is not substantial, then it may not be worth installing an HP in the first place. In the worst case, I may just end up using more energy in a different form for no net gain on saving money. The problem is that I don't know how much electricity would be used by the single zone mini-split and how much oil will be saved by using an HP.

 

I think that you should consider your entire house as a system, adding or changing some equipement will influence your energy requirements for the whole system. I suggest that you go for an energy Audit before putting money on the table, enabling you to establish if it is feasable for your application . In my case , for my region , with my house and the price of electricity .the use of a cold weather heatpump reduces my annual electricity bill by 35% . The savings generated will pay the HP in 5years.....one year to go. The machine is garanteed 10 years parts and labor. This is why i drive a Prius Prime

 

I had an energy audit using the blower door test three years ago for our air sealing and insullation. But that was more for an energy assessment for checking the leakiness and heat loss. This audit resulted in the recommendation of additional attic insulation which we did. I didn't receive a specific audit result document, but even if I have such data, I am not sure if one can do any better job estimating energy use for a combination of oil burner central heating with or without the addition of a single-zone heat pump.

The heat pump installer did not perform the blower door test. I can easily see that if our house is currently heated by radiant electricity heaters with a separate thermostat in each room, then there will be the energy and cost-saving benefit of the addition of a singe-zone heat pump to replace the heating function in the largest room of the house. But in our case, it is not as simple as that. I don't see simple energy audit data will give us the prediction of how much energy will be used/saved for the entire house or the individual heating system. That being said, it is probably possible to make some type of prediction based on simulation with very detailed heating data monitoring/collection. I just don't think there is any contractor who would be willing to take up such a project even for a fee.

 

OH well your are missing important data , my audit shows GJoules energy lost per year in different parts of the house.To put it ruffly the average lost of the zone covered by the HP is approx 7500btu/h

 

Yeah, I don't think the auditor who came to our house did a comprehensive heat loss monitoring for each rooms. I can't even remember him going to each and every room nor checked the entire house with a thermography camera. It was a nice fall day, so the heat was not on. The entire house was probably at ambient temperature. What he was interested in was how tight our house was with the all the windows and doors closed. And he fould no major leakyness in our home. But did find our attic insulation was not adaquate. Again, even if he found some difference in leakiness in different areas of the house and such data was presented to me, I don't think that would give me an idea of how much overall energy can be saved with or without installing a single-zone HP.

 

I would say the opposite, that this HP setback-or-not issue I've been replying to, is irrelevant to your case. It really applies only to cases where the HP is the primary heat source for the whole house, or nearly so. And the amount of setback saving in question is comparatively small, peanuts compared to your dual-fuel interoperability issue.

You are looking at a hybrid system where the HP heats a portion of the house, the oil continues to heat the rest. Getting these two sources to satisfactorily function together is a much larger issue. And the potential savings of getting it right, dwarf any normal HP setback-or-not question.

You would definitely need to move the oil thermostat out of the HP zone, to an oil-only zone. And find some way to turn off or throttle or otherwise restrict and control the oil heat radiators in the HP zone, so that the HP can carry the bulk of the heating load there. But not permanently shut off the oil heat to the HP zone, as it may be needed as backup during severe cold snaps. But the oil system unknowns and hidden-inside-the-walls issues you have described seriously get in the way figuring a solution. This whole thing looks like a very major project, much more than the normal cost of adding a 1- or 2-zone DHP.

I do wonder if there is a practical way to put covers over the oil radiators in the planned HP zone, to reduce their heat output enough for the HP to carry the bulk of the load there. Easily removable, for tuning during severe cold. But even this may not be as straight forward as it seems.

========================

Have you looked at SANCO2 (formerly Sanden) heat pump water heaters (HPWH)? This was once a very promising path for domestic hot water (DHW), but other HPWHs have since eaten its lunch in the general market by catching up to its ordinary DHW performance at much lower cost. So these products remain relegated to niche applications.

But this product line still has two advantages that other HPWHs can't (yet) match: it works with very cold ambient air, and can produce a very high water delivery temperature, enough to be useful for space heating applications. You'd probably need more than one unit, and also to keep the oil as backup for severe cold when the house needs more heat. But it might be something to at least glance at to use with your existing radiator system, as an alternative to DHPs.

https://www.eco2waterheater.com/
SANCO2 - Heat Pump Water Heater with exterior condensor - quiet and passivehouse efficient
https://www.smallplanetsupply.com/sanc02

 

Thank you for pointing out this. You are absolutely correct. Yep, the difference in Wh or cost saved with or without the setback is a totally negligible amount in my situation.

I just did some number crunching. Based on our detailed past oil usage record and some measurements I just made, I have come up with the following numbers.

• we use ~600gal/yr of oil for the boiler for both heating and hot water
• of which 411gal/yr is used for the hot water (assuming the hot water usage and heating efficiency stay constant throughout the year)
• remaining 189gal/yr is used for heating during the colder season ~5 mo/yr
• assuming the amount of heat distributed throughout our house is proportional to the length of the hydronic baseboard radiator length in each room, ~52% of the heat is used in the central area where I plan to install an HP
• in best case scenario, assuming that the HP will completely eliminate the need to use the oil for heating the central area where I plan to install an HP and reduce the need for the other rooms on the same floor, the estimated amount of oil saved will be ~98gal/yr
• 98gal of oil at today's rate of $5.30/gal is $520/yr saving

Now, what I need to know is how much electricity cost will be for the HP to heat the area. The 98gal of oil is equivalent to 13426kBTU, but assuming that our 30+ years old oil boiler is running at 60% efficiency, the actual heat used is 8055kBTU. Based on the information found here, the fuel cost per Million Btu for the heat pump is ~$22 at our current electricity rate. Thus for the 8055kBTU, the heat pump will cost ~$177.

So, as a rough calculation, even under the best case scenario, the net annual saving is only $343. At this rate, for ~$6K cost of installation of HP will take 17.5 years to pay back. And this is assuming that the oil price remains as high as today and the electricity rate does not increase for the next 17 years.

No one knows the future direction of the oil and electricity price, but I have a feeling that it is more likely that oil prices will come down but the electricity cost will continue to go up... And if that is the case, then the payback is going to be even longer than 17 years.

Now, compared to the single-zone heat pump installation costing $6K, I already know the cost of replacing our aging oil boiler with the most efficient oil boiler today is going to cost ~$13K. But with 90% efficiency for the newest oil boiler, and as before, assuming our current boiler is operating at 60% efficiency. This switch is an instant 30% saving on the oil used both for heating and hot water. That means our annual oil consumption will drop from ~600gal/yr to 420gal/yr. At today's rate of $5.30/gal for oil, that is a $954/yr saving, and no additional cost for increased electricity. Even with the higher upfront cost of the installation, the payback period is going to be 13.6yrs which is almost 4 years shorter than the single-zone HP installation. But with HP, I will have some state incentive whereas, there is no such thing for an oil boiler. However, there should be a federal tax credit of 26% for both energy efficiency improvements. The cost analysis would be very similar for replacing the current oil boiler with a 97% efficient propane boiler using today's propane and oil rate. But, one thing I have not included in this calculation is the cost of moving the oil thermostat and installing the shut-off valve of sort on the oil heat radiators in the HP zone. Depending on that cost and the state incentives I may qualify for an HP, I still have some wiggle room to adjust as to which option is more cost-effective.

I am still waiting to get a new estimate for the dual-zone HP system. That will reduce the oil use even more, and eliminate the need to relocate the thermostat and to shut off the radiator. I will have to wait a bit longer to make the comparison for this option. Also, it now seems clear that the biggest bang for a buck may not be upgrading the room heating system, but improving the hot water system. I may have to look into several options for the hot water heater efficiency update now.

 

I am holding off the installation of the heat pump at least until I know more about the new Climate bill. If it becomes law as it passed the senate, the new bill includes a lot of incentives for heat pump installation and even the cost of panel upgrade. I don't want to jump in now and lose the potential thousands of dollars I could save if I wait a bit longer.
Climate bill could slash US emissions by 40% after historic Senate vote | Climate crisis | The Guardian

also:
https://www.rewiringamerica.org/policy/IRA-disadvantaged

In the full PDF, you can find details on:

• Non-tax provisions that invest in disadvantaged communities
• Tax provisions that invest in disadvantaged communities
• Tax provisions that offer a just transition for former fossil fuel workers

 

I'm thinking that pump and panel purchases made under the provisions of the 'not-yet-a-law' would be retroactive to 2022, but that's an "I think" rather than I know.
I'd also want to get some deets on any restrictions for non-US sourced green gear.
Despite some sharp discomfiture for Blue Congress Critters in reddening districts, Pelosi is going forward, today, with the House vote as we type....so that means that her lackeys think that they have the votes.

Once signed, we'll see what's left of the kickbacks.

 

Slash emissions by 40%???

Not. A. Chance.
At best it might slow the rate of the increase.

It's not going to do much to cut inflation either, because they de-fanged some of the more onerous tax hikes that would have throttled back the economy, resulting in a diminution of inflation.
It's hard to tell because both sides use the fiction that a four-year administration has throttle control over the ten-year's worth of budget assumptions, so it REALLY IS "tax and spend" with most of the spending in the front nine, without the projected income increase on the back nine.

However (comma!) It's probably 'a net good' in the long run.
If nothing else, the tax kickbacks might help the US more than some of our competitors this time, and it almost cannot fail at incentivising some 'green' stuff.

 

I totally agree with you. The key word in the article is "could"... but it's a tabloid newspaper article. The moral of the story is not to put any faith in the credibility of the words printed. As far as the climate change specific law goes, personally, I am with a school of thought who thinks it is too little too late. We have already passed the point of no return. Whatever we do now is like a pebble tossed into a churning sea.

That being said, I will gladly take rebates and incentives to make the expenses easy on my wallet. And to make sure to maximize my saving, I will wait a bit longer for the bills to become law.

 

...and if you can lower your energy/carbon throughput by saving money AND perhaps incentivising a little extra domestic production?

Win-squared.

 

Along the line, I have noticed our oil boiler is running much less than usual this winter. But that's not because we have done any "home improvements". I think I have commented in this thread that we had planned to install multi-zone heat pumps last year as preparation for this winter with very high oil prices. The project was well underway and all I had to do was sign the contract for the loan and installation for the total cost of ~$16K. But I ended up postponing the project in light of the new IRA enactment which includes the new rebate potentially up to $14.5K available if we qualify. But we wouldn't know anything about it until our state implements the programs.

Well, that meant, we have to endure this winter with very high oil prices of ~$5/gal which is over twice the cost compared to last winter. On average, we use ~400 gals of oil over the winter, so that would be close to $2K. Our temporary band-aid fix was to lower the thermostat setting down to 55F and use electric space heaters in strategically placed multiple places and use them as needed. I bought three new 1500W space heaters of various types, and use them in a room when it is needed. Well, the comfort level is nothing like the whole house heating to 65F which is the usual setting in the past. But comfort is something that can be adjusted. Putting on extra layers of clothes and keeping the rechargeable battery-operated hand warmers in both pants pockets has been working for me. The den I spend 8+ hours for my work cools down to 45F in the morning and rarely gets above 60F while I am working, yet I don't feel too uncomfortable with direct heat from the radiant heater under my desk.

We are not quite halfway through the winter yet, but after about a month and a half, we used only ~100 gal of oil. That's about half of our usual usage. And our most recent electricity bill indicates we increase monthly usage by about 30%. Well if we can keep up this rate of usage for both oil and electricity, it will be ~$1K less spending on oil, and ~$200 extra spending on electricity, a net $800 saving for the winter. Now, I am starting to rethink the economic validity of the heat pump installation. Yeah, when this winter is over, I will have to do serious number-crunching to make sure heat pumps make sense or not.

 

Reviving this thread. We still have a few more months of the heating season left in our area, but the coldest months are over now. With the daytime high temperature hitting above the freezing mark, I can now relax that our heating cost will not get much higher than the last few months. I just placed an order for the heating oil delivery and checked the current price of the oil.

Well, the oil price has come down nicely while the electricity rate has gone up substantially. I just did the comparison of the energy cost using the current price and compared it to the data from last year. Last June at the peak of the oil price hike at $5.36/gal, heating with oil would have cost over 70% more than the most efficient heat pump under the ideal condition (290% efficiency). But the same calculation today, even the most efficient heat pump under the ideal condition is going to cost 20% more than heating with oil.



While the oil price can go up again, most predictions I have read are saying it will stabilize more or less around $3/gal. Certainly, the electricity cost can also come down, but at least for this year, the high rate of $0.29/kWh is not going to change. Now, even if we get very generous IRA rebates on the heat pump installation this year, I just don't see it will be cost-effective.