Heat Pump Sizing And Modulation Question

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JP2

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Hi I'm Jeremy, and I'm looking for some advice on designing a properly sized heat pump system for home. I've learned a lot already from reading this forum but could use a little more specific advice for my situation. I hope this post is appropriate for the forum.

Here's the background about my house and situation: My house was built in 1996 is 1700 sq ft and is located in Seattle. It has an entirely open first floor living/dining/kitchen and three bedrooms on the second floor. Two adjacent bedrooms are about 110 sq ft and the master is about 250 sq ft with large vaulted ceiling. There is also a large landing/bonus area between the master bedroom and the smaller bedrooms. I calculated a heating load of 21,600 BTU using the calculator at http://hvac.betterbuiltnw.com/, which I read is the most accurate for sizing heat pumps. The house is currently heated by the original gas furnace. I am interested in switching to a heat pump for two reasons: (1) to gain air conditioning and (2) to free up the utility closet on the first floor that currently houses the furnace for other uses.

I had an hvac company out to give an estimate and they recommended a 36000 BTU daikin multizone with a 7k head in the two smaller bedrooms, a 9k head in the larger bedroom, and an 18k head on the first floor. They were not particularly concerned about oversizing because the inverter modulates the load. However, the inverter is limited in how much it can modulate the load.

After doing some research I discovered Neeps cold climate heat pump list. What a fantastic resource. I entered my location and heating design, and it actually looks like a 36000 BTU daikin Aurora --a different model than the hvac company recommended--may be best for my house because of its high turndown ratio: https://ashp.neep.org/#!/product/26570/28/22000/WA/727930/1. It looks like this model can modulate more than smaller models such that it can actually satisfy a greater percentage of the annual load in the modulating range. But since I'm just a curious homeowner, I wanted to get an expert opinion. Does this make sense?

The NEEP website focuses on the outdoor unit's ability to modulate. Will this be affected by the indoor head sizing? In other words, since the indoor heads are all oversized will that cancel out or reduce the benefits of the outdoor unit's large turn down ratio and result in significant short cycling? An alternative option that I have thought up is to put a 7k ducted unit in the unconditioned attic for the two small bedrooms to share, put a 7k ductless unit in the master, and then a 12k ductless for the first floor. That would at least make the demand each head satisfies more tailored to the room it serves. Will more right sizing the heads make a difference for the outdoor unit's ability to modulate and avoid short cycling?

If I can in fact get the performance the NEEP website suggests from the Daikin Aurora that seems like a pretty good option to me. It will more than satisfy the heating needs even if it's unseasonably cold without too much short cycling. But I want to make sure that is in fact realistic before proceeding. Would also love to hear any other ideas for my situation.

Thanks!
 

Fitter30

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Want to know the cooling load. By over sizing the equipment my concern would be shorting the run in cooling even though the compressor will back down might not take enough rh out.
 

JP2

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Estimated cooling load is 17,600 BTU, so even more oversized there. But I anticipate using the cooling only when it's really hot. Heating will be the primary use for the heat pump. I would definitely prefer not to oversize the equipment. What has given me pause are the NEEP charts showing that the oversized Daikin Aurora model might actually short cycle less, at least with respect to heating, than a unit with a rated capacity closer to design load.
 

jadnashua

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One thing to consider with a heat pump is the outlet air temperature. Generally, a heat pump will not provide as warm air as a typical furnace will. As a result, unless the fan speed is lowered, because of the wind chill effect, while the ambient temperature may be where you've set the thermostat, while the fan is running, you may feel cold. This will take some careful positioning and sizing of the air ducts that's easier to do on new construction rather than using existing ducts not originally designed for a heat pump.

A/c needs a fair amount of run time to reduce the humidity levels in a damp location like Seattle. Oversizing can be an issue. You don't want to be calling for the auxiliary heat too often, either in the winter, so you really need to do both a heat and cooling load calculation.
 

Fitter30

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Looking at average temps in winter 28° looks like there a very few days a year it gets that cold. Could supplement the heat pump with electric or lp with either ventless fireplace stand alone heater.
 

JP2

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Thank you. It sounds like the oversizing is still a concern so I'm thinking of going in a different direction. Two outdoor units. For the downstairs I'm thinking a single 12k Mitsubishi H2i floor unit since it's all open plan. The downstairs has an estimated heating load of 11,750 BTU. And for the upstairs with a heating load of approximately 10k BTU a slim ducted Fujitsu 12K with outlets going to the three bedrooms. I read that the Fujitsu 9k and 12k models both modulate down to 3100 BTU. (https://www.greenbuildingadvisor.co...t-duct-sizing-vs-plenum-layout#comment-177905) Does that mean the choice between 9k and 12k is not particularly consequential? If so, I'm leaning towards erring bigger since the ductwork will be in an unconditioned attic.
 

jadnashua

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Ideally, on your design day load, the unit(s) would be running full time. In the summer, that provides maximum dehumidification, and on an exceptionally hot day, even if it couldn't maintain your setpoint, by running constantly, it will provide maximum dehumidification where you tend to feel more comfortable at higher temperatures.
 
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