Resistance Strip Sizing

[hivacjack]

It seems the prevailing "wisdom" is to size the backup heaters to handle the entire design load. WHY? The IRC doesn’t seem to require redundancy in residential HVAC systems. In this particular case 10 degrees F is the DT(2-1/2%) and the load at DT is 44,000BTUH. The HP (scroll compressor, HSPF=9) outputs 19,000BTUH at DT so stageable 5Kw + 5Kw controlled by 2 OD stats seems more than adequate. Am I missing something?

[kuryakin]

When a customer has a dead condensor, he'll be a LOT happier if he has enough electric heat to stay warm until the cavalry arrives!
Given that extra wattage costs hardly anything to install (obviously a BUNCH to operate!), why not?


It seems the prevailing "wisdom" is to size the backup heaters to handle the entire design load. WHY? The IRC doesn’t seem to require redundancy in residential HVAC systems. In this particular case 10 degrees F is the DT(2-1/2%) and the load at DT is 44,000BTUH. The HP (scroll compressor, HSPF=9) outputs 19,000BTUH at DT so stageable 5Kw + 5Kw controlled by 2 OD stats seems more than adequate. Am I missing something?

[beenthere]

As above.

Remember, the back up is also the EMERGENCY heat.
Would you want your house to be 10* below your normal set point for a weekend or longer if a replacement compressor couldn't be gotten and installed right away.

[hivacjack]

When a customer has a dead condensor, he'll be a LOT happier if he has enough electric heat to stay warm until the cavalry arrives!
Given that extra wattage costs hardly anything to install (obviously a BUNCH to operate!), why not?

.....is a dead condenser (at peak load conditions) substantally more probable than a dead furnace at any load? At .125/Kwh the savings over the life of the life of the system will likely pay for it. So is there some HP operational reason? Is operation at 10F going to overstress the condenser? Are there so many defrosts that the COP drops so low that it just ain't worth it anymore?

[hivacjack]

As above.

Remember, the back up is also the EMERGENCY heat.
Would you want your house to be 10* below your normal set point for a weekend or longer if a replacement compressor couldn't be gotten and installed right away.


Fossil fuel fired furnaces outnumber HP's by a substantial margin. What do you
think a customer would say to a proposal which suggests installing TWO gas
fired furnaces in a mannar such that the second automatically comes on when the first furnace fails?

[beenthere]

As kuryakin said, there isn't much of a price difference between installing 10 kw and 15 kw of strip heat.
So your furnace analogy isn't an apple to apples comparison.
We always install enough strip heat to heat the house under design conditions. No one ever complains about it.

Do you change out compressors at 11:00 PM the same as you do inducers.

Also, a house with enough strip heat for design conditions will switch to emergency heat for the night. Saving themselves an after hours service charge that more then makes up for the extra electric they will use for that night.

If your just selling them heat, then just install enough strip to make up what the HP lacks. If your selling them a comfort heating system, then install enough to heat the house under design conditions.






PS: I have never lost a sell because I included enough strip heat for DT conditions.

[Kevin O'Neill]

Here in Myrtle Beach, SC the power company has a program called Good Cents. They do the load calc, and the customer gets a rebate if they go on the program. If it is a retrofit, they loan money at 6.25% for heat pumps, insulation, storm windows and replacement windows. That is in addition to the rebate and the savings from a properly sized high efficiency heat pump.

They allow 1/2 of the calculated heat load to be in strips, if you want the rebate. It has not been a problem. If your compressor fails, the temperature will drift down slightly at night. Then you call your HVAC service company, they find and fix the problem. If you had enough strips to handle the full load at peak conditions, likely you would not know that you had a problem till you got your electric bill. Then there would be wailing and gnashing of teeth :D

The power company does allow an extra 5KW if it is controlled by an emergency heat relay and outdoor thermostat.

This helps keep their peak demand down and helps keep our electric rate at 6.5 cents per KWH.

[kuryakin]

Having enough resistance heating to take care of the ENTIRE LOAD is super cheap to install, but costs no more to operate, simply because the run time will go down as more watts are thrown at it. Total energy costs won't be any different. Remember, it's KW-hours. More KW, fewer run hours for the same delivered heat.

And ANY heat pump is ALWAYS worth running, at any temperature. The COP will always be greater than 1.0, and by a lot. Defrost cycles actually go down when it gets colder because the humidity goes down, calling for fewer defrosts, even with time-temperatur defrosters.

What heat pumps lose at lower temperature more than anything is capacity. Hence the need to auxiliary heat. Two stage condensers push the balance point to a lower temp, where the aux heat needs to kick on.

Efficiency goes down too, but never to the point where it's worthwhile shutting down the condenser when there's electric backup. Alas, this doesn't work with dual fuel systems...


.....is a dead condenser (at peak load conditions) substantally more probable than a dead furnace at any load? At .125/Kwh the savings over the life of the life of the system will likely pay for it. So is there some HP operational reason? Is operation at 10F going to overstress the condenser? Are there so many defrosts that the COP drops so low that it just ain't worth it anymore?

[kuryakin]

I'd know it! When that blue light never turns off, I'll know there's trouble!


Here in Myrtle Beach, SC the power company has a program called Good Cents. They do the load calc, and the customer gets a rebate if they go on the program. If it is a retrofit, they loan money at 6.25% for heat pumps, insulation, storm windows and replacement windows. That is in addition to the rebate and the savings from a properly sized high efficiency heat pump.

They allow 1/2 of the calculated heat load to be in strips, if you want the rebate. It has not been a problem. If your compressor fails, the temperature will drift down slightly at night. Then you call your HVAC service company, they find and fix the problem. If you had enough strips to handle the full load at peak conditions, likely you would not know that you had a problem till you got your electric bill. Then there would be wailing and gnashing of teeth :D

The power company does allow an extra 5KW if it is controlled by an emergency heat relay and outdoor thermostat.

This helps keep their peak demand down and helps keep our electric rate at 6.5 cents per KWH.

[beenthere]

LOL.
The light on the thermostat should get someones attention like kury said.

whats the winter design temp in Myrtle Beach, SC?

[hivacjack]

"Also, a house with enough strip heat for design conditions will switch to emergency heat for the night. Saving themselves an after hours service charge that more then makes up for the extra electric they will use for that night."



......the strip heater is going to operate whether or not it meets the design load. And the point is the unneeded strip heat capacity is going to cost and cost and cost and cost. Correct me on this if I am wrong; but when the strip heat overwhelms the pumped heat it is going to dominate in the response of the thermostat to the energy content of the air. So instead of a heat pump supplemented by resistance heat you move towards a system which is a strip heater with a heat pump operating in the background. A BTUH of resistance heat is 2x to 4x the cost of a BTUH of pumped heat so the cost could easily total several hundred dollars a season even at 4500-5000 degree days.

I suspect that sizing the system to save this kind of money makes for a rather strong selling point. :):):)

[kuryakin]

Not quite true. If you were to install 10KW vs 20KW, the operating cost should be about the same, because the 20KW heater will only run half as much to meet the load as the 10KW heater. The ONLY way a 20KW heater will cost more to run is if the 10KW heater isn't enough to meet the load.

Also, the strip heaters only work when the heat pump alone can't satisfy the load, then they come on only long enough to make up the difference. The heat pump, in nearly all cases, continues to run along with the strips when the backup heat is called on.


The real challenge with system sizing, (which is why I'm a strong proponent of two stage condensers, and eventually inverter condensers) is around these parts, most houses need more BTUs of heat than they do cooling. These parts being the Delmarva penninsula. So, big enough to totally satisfy all heating needs on the heat pump alone, with a single stage condenser, will SURELY mean most of the time it's too large for cooling.



"Also, a house with enough strip heat for design conditions will switch to emergency heat for the night. Saving themselves an after hours service charge that more then makes up for the extra electric they will use for that night."



......the strip heater is going to operate whether or not it meets the design load. And the point is the unneeded strip heat capacity is going to cost and cost and cost and cost. Correct me on this if I am wrong; but when the strip heat overwhelms the pumped heat it is going to dominate in the response of the thermostat to the energy content of the air. So instead of a heat pump supplemented by resistance heat you move towards a system which is a strip heater with a heat pump operating in the background. A BTUH of resistance heat is 2x to 4x the cost of a BTUH of pumped heat so the cost could easily total several hundred dollars a season even at 4500-5000 degree days.

I suspect that sizing the system to save this kind of money makes for a rather strong selling point. :):):)

[beenthere]

Back in the days when mechanical stats were about all we had. The strip heaters wouldn't come on until the ID temp dropped 2* below set point. Weather 10 or 20 KW of strip came on, it took the same amount of heat to come up those 2*. The same as today with cheap digital stats. So just because more strip heaters are on, doesn't mean your using more electric per hour.

With stats such as the th8320 and higher, you can easily lock out the extra bank of strip so it doesn't get energized if its not needed.