We would like to replace our gas furnace of 80,000 BTU capacity with a heat pump for both heating and cooling.
We live in a 2 story townhouse, 1940 sq ft, built in 1977, no basement, attic with old insulation, double pane windows and sharing walls on North and South side. We are in Sunnyvale, California.
Ducting is in attic for upstairs. Down stairs is in a ceiling soffit so it cannot be replaced.
I did a load calculation myself using free online tools:
coolcalc says:
Heating: 29,400 BTU/hr
Cooling: 19,700 BTU/hr
betterbuiltnw says:
Heating: 28,200 BTU/hr
Cooling: 34,400 BTU/hr
Based on my smart thermostat historical data and my current gas furnace using instructions at (search for "How to Find the Right Size Heat Pump For Your Home" at Google) for the coldest hours of the past winter I got:
Heating: 26,667 BTU/hr
All three methods gave a similar load calculation for heating so I feel confident about that value. Not sure about the big difference between the 2 online tools in cooling though. In any case, a 3 ton heat pump seems like a good size for our house.
The rule of thumb based on square footage recommends a 4 ton heat pump. The couple of HVAC installers I talked to also gave quotes for 4 ton heat pumps but without doing a manual J calculation.
Looking at the public dataset at techcleanca most in my area for similar square footage houses went with a 4 ton heat pump. But maybe those were single family houses. My assumption is that since 1/3 of our house (1st floor) doesn't have unconditioned ceiling, and we share walls on both North and South sides we should need a smaller system size than others. Is that accurate?
1. How much should I trust the above methods calculating the size versus what the HVAC installers are recommending? I don't quite trust them since their incentive is to sell more.
Currently we are experiencing a temperature difference between the 2 floors, up to 10 degrees, but I think, at least in the winter, this is because the current gas furnace is oversized and only works for a couple of minutes at a time without a constant fan circulating the air in the house. Even during the coldest hour in the past winter I see on the historical stats of the thermostat that the gas furnace worked 20 minutes with multiple breaks within that hour. Is my thinking right?
Options we got from HVAC installers:
A. regular heat pump (Carrier) and single air handler with existing ducting
B. variable speed heat pump (Mitsubishi) and single air handler with existing ducting.
C. variable speed heat pump (Mitsubishi) and two air handlers. Ducting for upstairs will be replaced.
D. variable speed heat pump (Mitsubishi), one air handler for upstairs with existing upstairs ducting, one mini split unit for downstairs.
E. regular heat pump (Carrier), 2 zone damper system. Ducting for upstairs will be replaced.
2. Could you rank the above options in terms of comfort? Assuming we want the same temperature through out the house, what temperature imbalance between the 2 floors should we expect with each option?
3. Would option B achieve comfortable results if it works constantly at its lowest speed for most of the day circulating air? Will we get an uncomfortable draftiness feeling in the winter if the fan circulates air?
4. If we go with option A or B, could we manually close the existing damper that goes downstairs (or the downstairs registers) in the summer if we only want cooling upstairs? Is the static pressure going to be a problem even for a variable speed system in option B?
5. If we go with option B, could we keep the system and go with option D or E in the future?
We live in a 2 story townhouse, 1940 sq ft, built in 1977, no basement, attic with old insulation, double pane windows and sharing walls on North and South side. We are in Sunnyvale, California.
Ducting is in attic for upstairs. Down stairs is in a ceiling soffit so it cannot be replaced.
I did a load calculation myself using free online tools:
coolcalc says:
Heating: 29,400 BTU/hr
Cooling: 19,700 BTU/hr
betterbuiltnw says:
Heating: 28,200 BTU/hr
Cooling: 34,400 BTU/hr
Based on my smart thermostat historical data and my current gas furnace using instructions at (search for "How to Find the Right Size Heat Pump For Your Home" at Google) for the coldest hours of the past winter I got:
Heating: 26,667 BTU/hr
All three methods gave a similar load calculation for heating so I feel confident about that value. Not sure about the big difference between the 2 online tools in cooling though. In any case, a 3 ton heat pump seems like a good size for our house.
The rule of thumb based on square footage recommends a 4 ton heat pump. The couple of HVAC installers I talked to also gave quotes for 4 ton heat pumps but without doing a manual J calculation.
Looking at the public dataset at techcleanca most in my area for similar square footage houses went with a 4 ton heat pump. But maybe those were single family houses. My assumption is that since 1/3 of our house (1st floor) doesn't have unconditioned ceiling, and we share walls on both North and South sides we should need a smaller system size than others. Is that accurate?
1. How much should I trust the above methods calculating the size versus what the HVAC installers are recommending? I don't quite trust them since their incentive is to sell more.
Currently we are experiencing a temperature difference between the 2 floors, up to 10 degrees, but I think, at least in the winter, this is because the current gas furnace is oversized and only works for a couple of minutes at a time without a constant fan circulating the air in the house. Even during the coldest hour in the past winter I see on the historical stats of the thermostat that the gas furnace worked 20 minutes with multiple breaks within that hour. Is my thinking right?
Options we got from HVAC installers:
A. regular heat pump (Carrier) and single air handler with existing ducting
B. variable speed heat pump (Mitsubishi) and single air handler with existing ducting.
C. variable speed heat pump (Mitsubishi) and two air handlers. Ducting for upstairs will be replaced.
D. variable speed heat pump (Mitsubishi), one air handler for upstairs with existing upstairs ducting, one mini split unit for downstairs.
E. regular heat pump (Carrier), 2 zone damper system. Ducting for upstairs will be replaced.
2. Could you rank the above options in terms of comfort? Assuming we want the same temperature through out the house, what temperature imbalance between the 2 floors should we expect with each option?
3. Would option B achieve comfortable results if it works constantly at its lowest speed for most of the day circulating air? Will we get an uncomfortable draftiness feeling in the winter if the fan circulates air?
4. If we go with option A or B, could we manually close the existing damper that goes downstairs (or the downstairs registers) in the summer if we only want cooling upstairs? Is the static pressure going to be a problem even for a variable speed system in option B?
5. If we go with option B, could we keep the system and go with option D or E in the future?