Load Calculations for mini split and infloor electric heat

T

trevorm

Member
Location
Central Washington
Occupation
Electrical Contractor
Hi Folks,

I'm doing a load calculation via the optional method for a single family dwelling. The only part of the calculation i'm unsure about is the heating and air conditioning load - 220.82(C)(3) The mini-split heat pump units being added perform both heating and cooling.

This residence currently has:
9900W (3 x 3300W) of infloor electric heat from a central controller
2 x 1700W of infloor heat both on separate thermostats
1 x 2600W of infloor heat on its own thermostat

They want to add:
1 x mini split at MCA of 23A = 5500W
1x mini split at MCA of 14A = 3360W

The wording in 220.82(C)(3) makes me think i can only use 65% of the electric heat and 100% of the heat pump loads if its a "central electric space heating system" How do we treat a situation like this? I ran the calcs with 100% of all the heating an cooling loads and it lands at 187.5A on a 200A service. It would be nice to reduce some of these loads to have a little more wiggle room but also fine if they all need to be included at 100%

Even in the standard calculation I would omit the smaller of heater and air conditioning loads but the mini splits provide heat as well so i assume it would be all heating/cooling loads at 100% in the standard method as well?

Much appreciate any suggestions!
 
I would talk with whomever designed the HVAC system. If the electric heat and heat pumps must all run to satisfy the building in the winter than you have to include all of it.

If the heat pumps are supposed to run summer for cooling and spring and fall for heating and the electric heat takes over in the winter that the electric heat is probably the larger of the two loads and you could ignore the HPs

The designer should have the building heat loss and that can be looked at to decide.

You have to know what it was designed for.

Since heat pumps are more efficient than electric in milder weather and their efficiency drops off in the winter some people and designers intend to use HPs year round except during cold weather
 
Post #2 matches my thinking about how to be realistic in your evaluation. It describes exactly how I use my minisplits: AC in the summer, heat when the temperature is above freezing, but other systems get used when it is really cold.

As far as code is concerned, this is not 'a heat pump with supplemental electric heating'. These are 2 separate systems. You have 4 separate electric space heat systems, and 2 separate heat pump systems.

I _think_ you can take 40% of the resistance heat (since you have 4 separate systems) as one bucket, 100% of the minisplits as the other bucket, and then just use the larger bucket. But I don't regularly do load calcs, so that is just my rough guess.
 
There is no designer here, just a hvac installer suggesting the mini-split units needed for a/c based on their calculations and I’m the electrician for the job.

The existing system is all of the electric floor heat I described in my post above. It can fully heat the house during the winter. The mini splits are being added for A/C in the summer and perhaps backup heat in the winter (for whatever reason the previous owner of this house didn’t install AC in this HOT climate!). So it’s possible all the units can be on at once during the winter but definitely not needed.
 
I just read the Mike Holt page on the optional calculation, and I believe you are kosher using the following approach:

1) Heat pumps:
5500 + 3360 = 8860 VA
2) Space heating, 4 units:
(9900 + 1700 + 1700 + 2600) * 0.4 = 6360VA

Pick the larger of the two, use 8860VA in the rest of your calculation.

I'm not fully sure about the 0.4 factor, not sure if 4 units means 4 separate heating units controlled by 4 separate thermostats, or something else. You might use the 0.65 factor to be on the safe side.

Jonathan
 
Maybe I am missing something but if the HP total 8860 va and the heat totals 15900 I would say you need to use the 15900 and then apply any factor since the electric heat on its own is existing and sized to heat the house on its own and ignore the HP load.
 
The optional calculation divides heating and cooling into different classes, with different diversity factors. You take the largest value after the factors are applied

So you take the heat pumps at 100%, and the electric space heating at 65% (or 40%) and then pick whichever is biggest.
 
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