Show HN: Heat Pumps, Hooray – A heat pump calculator for your home

One thing we've built but not yet included in the site is a way to back into a more precise heating/cooling load for the home using past utility bills. That could be useful even for folks going down the more bottoms-up modeling route as a way to help with the 'garbage in / garbage out' problem of having to estimate insulation, window heat transfer, etc. etc.

And the chart that is shown peaks in winter a couple months early.

Overall, it seems to significantly underestimate my energy costs.

The conclusion is also confusing: it says I would save $89/year or $24K in 15 years. Huh?

$89/year doesn't seem like enough to justify any investment, and $24K is about $1600/year or ~95% of my current costs, which seems too high to believe.

1/ Right now, if you select "No A/C", we still include the cost+emissions of cooling from the heat pump. It's on the backlog to add a "heat vs. heat only" comparison. In part, we're assuming that if you have cooling capability, you'll probably use it (and therefore wanted to include that in the emissions impact), but also that homes without A/C today will have small cooling loads to begin with, so it wouldn't change the answer too dramatically. For now, I'd suggest looking at the monthly costs in the summer on the bar chart and mentally adjusting the headline number for those. Sorry that's not smoother!

2/ Natural gas is indeed way cheaper here -- dramatically so this year, but previously as well. The backtest shown on the site is Feb '21 to Feb '22 -- natural gas prices this coming winter are bound to be much higher than they were last year, so savings numbers will definitely change going forward. It's on our list to (a) make that dynamic clearer on the site, and (b) explore a more forward-looking savings number. Right now, the annual energy costs are "what you would have saved _last year_ vs. your existing equipment", instead of what you might expect, thinking that would be more convincing / less controversial, albeit potentially conservative.

Thrilled to hear you're 6mo into life with a heat pump! Is it air-to-water and still using water/steam radiators, or forced air now?

You're right that our site doesn't yet work for homes outside of the US, unfortunately. The approach would be similar, but some different equipment and home configurations to consider along with different data sets (fuel pricing, weather, grid emissions, etc.) -- I'll keep an eye out for good resources, and would love to help anyone who wants to try to build it!

The best is to have a hybrid system: a heat pump and a gas/oil/wood boiler which will fire up when it is freezing.

That is the amount of heat you require over a year.

Through the magic of heat pumps, you can find the COP value of any model you plan on purchasing, but in essence the COP value tells you how much heat the pump will produce when using 1 kWh of electricity, for instance a COP value of 3.62 means the heat pump will produce 3.62 kWh of heat for 1 kWh of electricity.

So say you have a yearly gas usage of 1600m3, that equals 16880 kWh of heat. Assuming a heat pump with a COP value of 3.62, you can then divide the 16800 by 3.62 to find the amount of electricity you will need with that particular heat pump to produce the same amount of heat, in this case 4663 kWh.

As i said, this is a very crude calculation, and it doesn't take into account that the heat pump efficiency drops as temperature drops to around it's lowest efficiency temperature (for mine it's -25C). When that happens, it has a built in backup heating device that will do regular electric heating, meaning 1 kWh per 1 kWh.

It also doesn't take into account climate control built into many european heat pumps (and maybe american as well ?). Mine has an outdoor sensor that simply shuts off the heatpump whenever outside temperature goes above a certain point.

And then there are all sorts of after market upgrades you can buy, like Tado, who produces a "sensor" for certain heat pump models, which will further reduce consumption based on local weather forecasts, AI and other readings.

If you're choosing 'multi-zone', "Heat pump: Number of units" corresponds to the number of _outdoor_ units, rather than indoor heads. For all other configurations, it maps to both outdoor and indoor (the others are all single zone). So, if you want to go with a multi-zone heat pump, I'd suggest entering 1 or 2 for "Heat pump: Number of units" (or just leaving it untouched, in which case the model will choose automatically based on home size).

> IMO you should just stick to calculate fuel costs for heat with gas, vs heat with electricity, and leave out the installation costs.

Roger that. We started out that way, and the cost-to-install was the number one thing that people asked us in follow-up questions, so we added some high-level guidance. I think the lack of transparency on upfront costs and pricing is a major issue for the transition broadly and a problem that needs to be solved -- that's high on our minds.

> That's really all I want to know: What costs less, heat with gas, or heat with electricity. Or maybe a hybrid.

Helpful! Related to the previous point, we want to make it easier for you, with a proposal from a contractor in hand or existing equipment already installed, to choose that equipment from among what we're analyzing (rather than only seeing cost savings for the most efficient unit that we choose automatically). In your case, you'd then be able to see exactly the cutoff points when your existing fossil fuel equipment is (a) necessary because of reduced heat pump capacity at lower temperatures, or (b) cheaper to run than the heat pump. FWIW, (b) is far rarer than we thought going into this exercise.

Some of the things driving speed:

- Fetching hourly weather data -- we use binned data for the 15-30 years used for equipment sizing and selection, but for the 12 month backtest to get savings and emissions, we're going hour by hour through the last year for the simulation.

- Loading + merging emissions data -- same deal for hourly grid emissions for your local grid, to get emissions from electricity usage (though we do make some assumptions about the grid decarbonizing when we show expected lifetime emissions figures).

- Simulating thermostat behavior -- we do some extra processing before the core simulation to model realistic human behavior on thermostat setting, so that you don't get lots of flipping between heat+cool in shoulder months or the furnace kicking on in June just because it hits 68dF in the middle of the night.

- Heat pump equipment selection -- we're doing a slightly simplified version of the backtest over 15-30 years of historical data for ~2k heat pump units (depending on ducting type) as a bake-off to choose the most efficient-but-properly-sized heat pump for the home. There's more optimization to be done there, but it's about 2-3s of the total time currently.

- Backtest itself -- once we have the equipment chosen and the weather + thermostat scenarios created, we run the simulation itself for the last 12 months, twice (once for the status quo equipment, once for the heat pump configuration). That goes through each hour of the year, simulating heat load on the home (outside weather including cloud cover and solar positioning, humidity, wind speed, temperature, plus internal loads like appliances and the heat occupants give off), simulating equipment performance (for heat pumps, a function of output, outside temp, inside temp, outside humidity, altitude, and layering in backup heat as applicable/needed), plus accounting for surplus/deficit heat that needs to roll over to the next hour based on thermostat settings or insufficient conditioning, etc. etc.. With usage, we then compile and calculate the utility bills monthly (we have support for actual rate plans, TOU plans, etc., but for now we just use statewide marginal retail prices from the EIA for the site version, because those tend to be conservative) and also calculate emissions, which is time intensive for electricity given we're layering in actual emissions from the grid in the simulated hour.

Anyways, there's definitely a lot of optimization we could do to get the processing time down, but hope that gives you a sense of what's going on server-side. Also, the model we're running is more flexible than what's exposed on the site currently (e.g. arbitrary date ranges for the backtest), and if we made a more specialized version of it, we could do a lot more pre-computing.

Efficiency does decrease since they need to spend more time defrosting.

We model decreasing heat pump capacity and efficiency as the outside air temperature declines. Yes, heat pumps do lose capacity and efficiency the colder it gets, but they can still provide heat even when it's very cold out (there's heat energy in the air until absolute zero, or -273, after all!). You can see that declining efficiency if you expand the "Step 2: Choosing your ideal heat pump" section. We'll add a graph of capacity relative to outside air temperature as well.

Backup heat is still often necessary for many homes in cold climates, but it's necessary a lot less often than you might think. Note that in the results you'll see for the site, backup heat is included if necessary for single-zone centrally ducted heat pumps, but not for any other ducting types.

Yes, unfortunately we haven't seen great resources for the rest of the world, either. The same approach and core model here would work elsewhere, but we'll need to source historical data on housing stock, residential energy prices, grid emissions, weather to get it up and running elsewhere. If you know anyone that would be interested in taking that on, definitely let us know!

We've tried to make this a little clearer in the "receipt view", but the major difference is in terms of cost of the hardware.

Furnaces, heat pumps, and A/C units all have a 15-20 year useful lifetime.

While the annual savings will be only a bit less, the hardware cost will be _significantly_ less because you only need a one unit for a heat pump vs two for an A/C + furnace combo.

Does that make more sense? We'll try and clarify in the UI

If you're set on electrifying there, it might make more sense to investigate a ground-source heat pump which leverages the ambient underground temperature for heat exchange.

1. Air-source heat pumps are getting better and better, quickly, so that answer is changing

2. 93% of US homeowners would save money with air-source heat pumps vs. their existing equipment, and only 3% of homes would see an annual utility bill increase above $70. (these from backtesting the national sample of homes from the EIA's RECS survey)

A lot of people think "really really cold" means it hits zero a few times each winter. They don't know what living in Fairbanks is like :), and chances are that heat pumps can keep up with their local weather.