Question on the size of Daikin heat pump for new install

yhc · January 16

I am trying to understand the difference between 3 different recommendations for new heat pump/air handler installation for a 2 story, 2055 SF single family house in Bay Area, California. The existing 20+ year old gas furnace is 3.5 ton and AC is 4 ton. Looking at replacing them with new Daikin heat pump and air handler.

The confusing part for me is that 3 estimators recommend 3 different size pump and air handler units. Each of them has convincing reasons.

One contractor recommends for 3 ton DH7VS heat pump and 3 ton Goodman DFVE air handler units based primarily on the “duct capacity”.

The second recommends for 4 ton DH6VS /4 ton DFVE units based on the existing system capacity.

And the third, 4 ton DH7VS /5 ton DFVE based on the existing system capacity and to maximize the efficiency as the system is converted from gas furnace to electric heat pump.

For those who are familiar with heat pump/air handler installation, what are your thoughts?

EBEBRATT-Ed · January 17

. Two things.

There is only one correct way to size a heat pump system. That is a whole house heat loss and heat gain calculation. A lot of contractors will:

guess
2. base it on the size of the existing equipment

Both of those are the wrong way to get started.

That being said the contractor that mentioned duct size has a good point. Putting a liger system in with undersized ducts is a recipe for trouble.

bjohnhy · January 17

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

pecmsg · January 17

Furnaces are measured in BTU/h not Tons.

4-Tons of cooling for 2500 Sq Ft in the bay area is huge.

You need a Manal "J" Heat Load Loss calculation done to know what size heat pump is needed.

The existing duct capacity can be measured with a simple Static Pressure Test………….TESP. Then you'll know how many CFM's they can handle.

pecmsg · January 17

Here's one you can do yourself

HVAC Load Calculation - Maunualj - Whole House Loadcalc

Floheating · January 17

One thing I’d add is that heat pump sizing logic differs from gas furnace replacement.

Sizing strictly off existing equipment often leads to oversizing with heat pumps, especially in Bay Area climates. Oversized systems tend to have short cycles and lose real-world efficiency.

Manual J is the primary method for determining the size of a heating system, and it should be paired with a duct capacity check. In many conversions like this, the correct heat pump size ends up smaller than the existing AC.

yhc · 3:42AM

Thank you all for the replies. I really appreciate it. I tried to run the calculation with those calculators but I don’t have all the insulation factors my parents’ house has. But I will work on it as best as I can with the limited info.

Meanwhile I found this Cliff note version of HVAC load calculator. What do you think of it? If I were to use this calculator, it comes up with 60K BTU and 5 ton recommended equipment capacity

https://www.servicetitan.com/tools/hvac-load-calculator?bt=687921202930&_bk=&_bm=&_bn=g&_bg=160822411874&utm_medium=cpc&utm_source=google&utm_campaign=servicetitan_ga_nb_dsa_content_tools&utm_content=20943357294%7C160822411874%7C687921202930&utm_term=dsa-2272902592075&keyword=&matchtype=&_bn=g&_bt=687921202930&ga_cid=EAIaIQobChMIt73at_yQkgMV3CytBh3hERRVEAAYASAAEgLds_D_BwE&gad_source=1&gad_campaignid=20943357294&gbraid=0AAAAACpko0nS7bHo__iB8wJJo9pL4OA2Q&gclid=EAIaIQobChMIt73at_yQkgMV3CytBh3hERRVEAAYASAAEgLds_D_BwE

DCContrarian · 3:59AM

https://forum.heatinghelp.com/discussion/comment/1882776#Comment_1882776

Any tool that just goes off of square footage, like that one does, is going to be garbage. For my house that tool gave four times the actual heating load.

A few posts up Bjohnny posted a link to a technique using past fuel usage. This is an accurate way of getting a ballpark number. It only takes a few minutes.

Question on the size of Daikin heat pump for new install

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