4 ms·

Is there a straightforward way to estimate whether a small (24 kbtu), medium (36 kbtu), or large (48 kbtu) unit would be most appropriate for a given home?

cmui 4y ago
Not at the moment, working on a tool like this for the preorder site! There are btu/sq.ft. rules of thumbs that are not appropriate for heat pumps (you can easily oversize by 100%).
pkaye 4y ago
I think the proper method used by HVAC companies is called the manual J method. I found a simplified version below but the real thing involved a whole book. https://www.indeed.com/career-advice/career-development/manual-j-calculation https://www.indeed.com/career-advice/career-development/manu...
- secabeen 4y ago
  This page does the full Manual J, but it is challenging to do and understand. I used it to confirm the estimates that my installer did without a full J workup, and it was helpful. https://loadcalc.net/ https://loadcalc.net/
ta1243 4y ago
For reference for international users, 24 kbtu is 7kW, 36 is 10kW and 48 is 14kW I'm not a heatpump export, but if that's continuous output, with an SCOP of say 3.8 (what I had quoted in the UK for an airsource -> water -- UK heating is typically hot water run from a boiler through radiators in each room rather than pumping hot (or cool) air direct to the romm via ducted which seems common in the US, perhaps because of the historical desire/need for aircon) That would require 1.8/2.6/3.7kW of electricity to run. In the UK at 35p/kWh that would mean 91p to generate 10kWh of heat. For reference my oil boiler at 95% efficency and current prices is about 6p/kWh, but we have had insanely high electricity charges this year. Back in September my oil heating approached 10p/kWh -- about £1 a litre.
- ricardobayes 4y ago
  I hear a lot of horror stories from the UK about this, that people pay upwards of £500 and still cold in their homes. There are a lot of cowboy installers going around but even those who are qualified can't be reached for aftercare/handoff etc. Fiddling with settings at 50p/kWh is not a great experience. On that note I think biopropane will be the solution for Europe.
  - bkor 4y ago
    > I hear a lot of horror stories from the UK about this, that people pay upwards of £500 £500 for what? Total cost for a month? A year? The monthly average of a full year of costs. The monthly average cost thing is common in NL; people often have no clue other than knowing the monthly average cost. And it's often estimated beforehand by the energy company. It's assumed to be correct. Anyway, people need to check the SCOP instead of figuring out if it is efficient (COP) at a certain temperature. > On that note I think biopropane will be the solution for Europe. Oh please no. On UK, I was in Scotland. I thought the homes in NL are often terrible quality. Scotland was another level. Pretty much no insulation, terrible windows. Really drafty. Yet the media (as a result the public as well) hates the "Insulate Britain" movement.
    - ta1243 4y ago
      A great deal of the problem is the inability to understand basic maths. This whole nonsense about a £2000 a year cap or whatever, rather than something nice and simple like a 35p/kWh electric cap, or a 7p/kWh gas cap, is both a symptom and the cause. The number of homes that could have improvements in insulation for tiny costs is astounding, but somehow insulating a home is some lefty woke thing.
    - ricardobayes 4y ago
      Why no? Biopropane is fully renewable from biomass but on the other hand people think heating with electricity is somehow cleaner than other methods. Well, electricity production in Europe is not very clean at the moment and not will be (and can't be) renewable-based unless wind and solar conditions change drastically. Large-scale geothermal production is a pipe dream within our lifetimes. So while most homes in Europe are already connected to a pipeline we are demolishing that and figuring out from scratch how to heat drafty homes with heat pumps. I'm just saying maybe, we shouldn't throw out 50+ years of boiler knowledge just like that.
camel_gopher 4y ago
If you have a Nest thermostat, some of the newer models give a usage display of how many minutes per hour they are running. This will tell you how much of your furnace's capacity is running at. For example, if I have a 40kbtu furnace, and I see a maximum of 15 minutes per hour usage on the furnace during a heating season, that means my usage maxes out at roughly 10kbtu. I might choose my heat pump based on that capacity; if I got a 10kbtu heat pump, I would expect it to be running continuously during the maximum usage time.
friendzis 4y ago
10-20 W/m2 is modern warm house, 30-40 - modernish house or well insulated old house, 60 - barely insulated, 100 - no insulation. Roughly. Assuming they mean kbtu/h continuous and not kbtu, 12kbtu/h is ~3.5kW. So, 7, 10, 14 KW units.

cmui 4y ago

Not at the moment, working on a tool like this for the preorder site! There are btu/sq.ft. rules of thumbs that are not appropriate for heat pumps (you can easily oversize by 100%).

pkaye 4y ago

I think the proper method used by HVAC companies is called the manual J method. I found a simplified version below but the real thing involved a whole book. https://www.indeed.com/career-advice/career-development/manual-j-calculation https://www.indeed.com/career-advice/career-development/manu...

secabeen 4y ago
This page does the full Manual J, but it is challenging to do and understand. I used it to confirm the estimates that my installer did without a full J workup, and it was helpful. https://loadcalc.net/ https://loadcalc.net/

ta1243 4y ago

For reference for international users, 24 kbtu is 7kW, 36 is 10kW and 48 is 14kW I'm not a heatpump export, but if that's continuous output, with an SCOP of say 3.8 (what I had quoted in the UK for an airsource -> water -- UK heating is typically hot water run from a boiler through radiators in each room rather than pumping hot (or cool) air direct to the romm via ducted which seems common in the US, perhaps because of the historical desire/need for aircon) That would require 1.8/2.6/3.7kW of electricity to run. In the UK at 35p/kWh that would mean 91p to generate 10kWh of heat. For reference my oil boiler at 95% efficency and current prices is about 6p/kWh, but we have had insanely high electricity charges this year. Back in September my oil heating approached 10p/kWh -- about £1 a litre.

ricardobayes 4y ago
I hear a lot of horror stories from the UK about this, that people pay upwards of £500 and still cold in their homes. There are a lot of cowboy installers going around but even those who are qualified can't be reached for aftercare/handoff etc. Fiddling with settings at 50p/kWh is not a great experience. On that note I think biopropane will be the solution for Europe.
- bkor 4y ago
  > I hear a lot of horror stories from the UK about this, that people pay upwards of £500 £500 for what? Total cost for a month? A year? The monthly average of a full year of costs. The monthly average cost thing is common in NL; people often have no clue other than knowing the monthly average cost. And it's often estimated beforehand by the energy company. It's assumed to be correct. Anyway, people need to check the SCOP instead of figuring out if it is efficient (COP) at a certain temperature. > On that note I think biopropane will be the solution for Europe. Oh please no. On UK, I was in Scotland. I thought the homes in NL are often terrible quality. Scotland was another level. Pretty much no insulation, terrible windows. Really drafty. Yet the media (as a result the public as well) hates the "Insulate Britain" movement.
  - ta1243 4y ago
    A great deal of the problem is the inability to understand basic maths. This whole nonsense about a £2000 a year cap or whatever, rather than something nice and simple like a 35p/kWh electric cap, or a 7p/kWh gas cap, is both a symptom and the cause. The number of homes that could have improvements in insulation for tiny costs is astounding, but somehow insulating a home is some lefty woke thing.
  - ricardobayes 4y ago
    Why no? Biopropane is fully renewable from biomass but on the other hand people think heating with electricity is somehow cleaner than other methods. Well, electricity production in Europe is not very clean at the moment and not will be (and can't be) renewable-based unless wind and solar conditions change drastically. Large-scale geothermal production is a pipe dream within our lifetimes. So while most homes in Europe are already connected to a pipeline we are demolishing that and figuring out from scratch how to heat drafty homes with heat pumps. I'm just saying maybe, we shouldn't throw out 50+ years of boiler knowledge just like that.

camel_gopher 4y ago

If you have a Nest thermostat, some of the newer models give a usage display of how many minutes per hour they are running. This will tell you how much of your furnace's capacity is running at. For example, if I have a 40kbtu furnace, and I see a maximum of 15 minutes per hour usage on the furnace during a heating season, that means my usage maxes out at roughly 10kbtu. I might choose my heat pump based on that capacity; if I got a 10kbtu heat pump, I would expect it to be running continuously during the maximum usage time.

friendzis 4y ago

10-20 W/m2 is modern warm house, 30-40 - modernish house or well insulated old house, 60 - barely insulated, 100 - no insulation. Roughly. Assuming they mean kbtu/h continuous and not kbtu, 12kbtu/h is ~3.5kW. So, 7, 10, 14 KW units.