Heat Pumps for dummies (beginners guide)

[jeff5may]

Originally Posted by Cassie

Continuing, with the heat pumps for dummies theme at what outdoor temperature is it wise to maybe switch to emergency heat option? Last week it was down to minus 15 and it didn't seem that this air to air heat pump was much use. Also what is the thinking behind this nine cycles per hour that the emergency heat was set to.

As described previously, the advantage a heat pump has over straight resistance heating diminishes as outdoor temperature drops. At some point, the advantage is lost. At -15, I would feel safe saying that the heat pump was past that point. Between the diminished capacity and frequent defrosting, the system was probably using more power than the heat it was providing.

To answer your question, the point where heat sources should switch is unique to each installation. It is known as the balance point, and can be calculated. Where the balance point lies depends on the design of the heat pump as well as the backup heat source. To further detail the answer, there are two different balance points that exist: capacity balance and economic balance.

In today's economy, natural gas is very economical, so when your backup heat source is gas, the balance point will be at a higher temperature than when it is a resistance heating element. Gas furnaces also vary in efficiency, so a more efficient gas furnace will have a little bit higher balance point than a less efficient furnace. This is an economic balance point shift based on cost.

On the capacity balance side, the point where the heat load of the house equals the capacity of the heat pump determines the capacity balance point temperature. With an all electric system, it is better not to run it in emergency heat mode at all unless the outdoor unit has failed. Even at Sub-Zero temps, the heat pump is still providing capacity to heat the home. When the outdoor unit takes a long time to satisfy the load, the thermostat kicks on the aux heat strips to help out. In emergency mode, the outdoor unit is shut down, and the heating capacity it provides is lost.

As shown in the graph below, the auxiliary heat strip in a standard heat pump air handler has to carry more of the load as the temperature drops. Even though the outdoor unit is running at diminished capacity and high cost, the heat load is enormous and every little bit counts when you are trying not to freeze to death.



Calculating the balance point of heating a home is not a trivial affair. The more that is known about everything, the better the system will perform.

[dcveem3]

I did mine with Propane. NG doesn't have the market volatility that LP does. Mine can be anywhere between 22* to 41*. This year based on market conditions of electric vs LP, I'm at 40*. Also don't forget system efficiency AND age of he system.

At -15*=like $9/gallon LP, rough guess not knowing particulars.

Don

[MN Renovator]

wait, how much is LP for you? I thought LP has been sky high, did it take a major dip? I remember the HVAC forums recently saying that in most areas going with electric baseboards would be cheaper to run than LP, especially when you use the baseboards in separate zones and heat the occupied areas.

Personally that's what I love about the idea of using mini-split units. If I want to chill in the bedroom/office area of the house for most of the day I could just heat or cool that room and leave the rest of the house at a warmer or cooler temperature within reason, obviously not going to drop it to freezing or not run the AC enough to remove moisture to prevent mold in the basement.

The reality is in my area we have natural gas that is super cheap compared to our electricity costs and we have winters where we usually get more than 10 days that touch with a temperature under -10. BTU for BTU, the cost for electricity is 4.5 times more than from a condensing natural gas furnace. So a conventional split-system heat pump as the only heat method is considered foolish because of the defrost cycles and heat strip usage would be excessive, especially since we don't usually need a huge pile of cooling in our climate but heating BTU needs are higher in the Twin Cities, Minnesota area. My house is 2100 sq ft and needs 20,000 BTU at design conditions -13f and needs to remove 15,000 BTU at 83 degrees. A 2 ton central heat pump would probably manage at around 15f degrees here if it didn't need to defrost, but it will need to. The economic balance point here would probably be between 30 and 40f, which means that it won't be used often enough to justify the extra cost, especially with the way a natural gas or propane furnace handles defrost cycles. When a heat pump goes into defrost, the furnace will take over until the entire cycle is satisfied and then the system shuts down and the heat pump will take over for the next defrost cycle, which basically means most of the heat ends up being fossil fuel heating below 20f even if I decided to run it below the economic balance point here.

Mini-splits are better than conventional systems by a ton, but when they start to ramp up below freezing and also start needing defrost cycles, they really don't seem to compete with the cheap natural gas costs in most places. I think localized heating would be the exception and in the summer the super SEER ratings would be the true benefit.

Of course if natural gas wasn't an option here, I'd probably put mini-splits in nearly every room and plan for backup heat coming from cheap electric baseboards or something. I can't think of any other option that would be cheaper and I don't think coal or wood are good options because they generally can't be unattended and require physical work to operate.

[dcveem3]

Originally Posted by MN Renovator

wait, how much is LP for you? I thought LP has been sky high, did it take a major dip? I remember the HVAC forums recently saying that in most areas going with electric baseboards would be cheaper to run than LP, especially when you use the baseboards in separate zones and heat the occupied areas.

Personally that's what I love about the idea of using mini-split units. If I want to chill in the bedroom/office area of the house for most of the day I could just heat or cool that room and leave the rest of the house at a warmer or cooler temperature within reason, obviously not going to drop it to freezing or not run the AC enough to remove moisture to prevent mold in the basement.

The reality is in my area we have natural gas that is super cheap compared to our electricity costs and we have winters where we usually get more than 10 days that touch with a temperature under -10. BTU for BTU, the cost for electricity is 4.5 times more than from a condensing natural gas furnace. So a conventional split-system heat pump as the only heat method is considered foolish because of the defrost cycles and heat strip usage would be excessive, especially since we don't usually need a huge pile of cooling in our climate but heating BTU needs are higher in the Twin Cities, Minnesota area. My house is 2100 sq ft and needs 20,000 BTU at design conditions -13f and needs to remove 15,000 BTU at 83 degrees. A 2 ton central heat pump would probably manage at around 15f degrees here if it didn't need to defrost, but it will need to. The economic balance point here would probably be between 30 and 40f, which means that it won't be used often enough to justify the extra cost, especially with the way a natural gas or propane furnace handles defrost cycles. When a heat pump goes into defrost, the furnace will take over until the entire cycle is satisfied and then the system shuts down and the heat pump will take over for the next defrost cycle, which basically means most of the heat ends up being fossil fuel heating below 20f even if I decided to run it below the economic balance point here.

Mini-splits are better than conventional systems by a ton, but when they start to ramp up below freezing and also start needing defrost cycles, they really don't seem to compete with the cheap natural gas costs in most places. I think localized heating would be the exception and in the summer the super SEER ratings would be the true benefit.

Of course if natural gas wasn't an option here, I'd probably put mini-splits in nearly every room and plan for backup heat coming from cheap electric baseboards or something. I can't think of any other option that would be cheaper and I don't think coal or wood are good options because they generally can't be unattended and require physical work to operate.

For me? I bought 1000 gallons of propane this past summer for 1.10/gal. I've bought it for less than that. I just bought 600 gallons in the dead of winter for 1.60/gal.

I'd LOVE to have NG!! Had it before where I lived. Pretty stable pricing so you can justify expenditures on it. You can also pretty much count on always having it too! LP is a bit different. For example this winter some area's of the mid-west couldn't even get LP.

Sure my dual-fuel propane over HP systems will still keep my house 66* at 20deg outside running off the heat pump side of the system...but it's costing more per btu with electric than LP(today) to do so.


Don

[MN Renovator]

That's a great price, it was $4/gallon this summer in my area.

[dcveem3]

Originally Posted by MN Renovator

That's a great price, it was $4/gallon this summer in my area.

Man...that must be with company owned tanks? EIA says prices in your area are $1.89/gal right now.

Don