The Prairie Spy

Alan “Lindy” Linda

There are many types of currently sold heat pumps:

Geothermal–a water-source refrigeration device (RD) that steals either heat or cold  from water under ground and moves it into your home.

Split system–an air-source RD that uses your existing duct system to distribute either heat or cold  stolen from outside air into your home. 

Mini Split– an air-source RD that either moves heat or cold from air outside to a single wall-mounted distribution system inside your home, high up on an inside wall.

Monoblock–not sold yet here in the USA, but coming. More later.

So first, geothermal. Very efficient, because it has water available to it that is circulated underground, where a consistent 40-plus degree temperature is available. “Forty” is a magic number for HPs; it is perfect for adding heat to, or removing heat from, either water (or air). Also, super expensive because of all the underground piping to be dug or drilled in. This unit works with available ducting in the home. And it is super complicated. Lots of opportunities for problems.

Mini split. This RD moves either heat or cold from air outside. These are very efficient, and although they can be run down to below zero temperatures, and can produce some heat at low temperatures, can only heat and cool one room at a time. Much like very efficient automobiles, repairs will kill any savings.

(Note: Once purchase price, installation price, and future repairs of RDs is taken into account–along with the cost of off-peak electrical equipment–nothing compares very well with natural gas. )

Let’s talk air-source heat pumps, which are the most popular. First, the not-so-good points.

The air that comes out of your registers feels cold. FYI– Any air around your skin temp, which is 85 degrees, feels cool like a draft, and a lot of the time, the air coming out of your “heat” registers is very close to that temp. Not comfortable.

Air source heat pumps have some complicated control stuff. Heat pumps heat your house by cooling outside air, stealing the heat from it, and sending it into your furnace. Refrigerators frost up, so do outside coils for HPs. Gadgets have to defrost it. They do that by stealing some of the heat from inside your house. Tricky control stuff.

All the chlorine-based refrigerants used now have some negative environmental impact.

Although they work well when it’s warm outside, they don’t when it’s cold. And when do you need more heat?? 

And of course, they’re more money.

Now the good points.

When air outside is medium temp, there’s a lot of heat that can be stolen and moved into your house. Spring and fall here this far north, these work well. But the cooler it is outside, the less sense these make.

They don’t “make” heat, like most heating units; they “steal” it by cooling the outside air, and transferring that heat into your house, much as a refrigerator takes heat from the food inside it, and sends it out.

This far north, they make the most sense when used with “off-peak” electricity, which is cheaper.

Give me some more time, maybe I’ll have another good point.

So recently, I decided to do some measurements, and some calculations, to figure out just exactly what my air-source HP system is doing. So I measured the temperature of the return (cool) air (RA) going in to the furnace–which is where the exchange coil for the HP is located,  and the warmer temperature of air coming out of the ducts.

HVAC has a formula for this: Btus = Delta T x 1.08 (a constant, too complex to explain here) x CFM (cubic feet per minute, an air quantity measurement). My CFM coming out of the furnace is approximately 1,000 CFM. Do the math, and then some more math from that math, and you get –well, never mind. I’ll just tell you. Please read on.

At 40 degrees Fahrenheit outside air temp,the air temp at my duct outlet is 90 degrees. ((First negative of many about HPs, that air feels cold)) A measurement of the wattage/Btus the HP was consuming, calculated from formulas, as compared to what my 90-percent propane furnace could do, produced a ratio of 50 to 20, approximately. 50 for the HP; 20 for LP. Ah. Looks good. Two-and-a-half more coming out than propane can do.

This is enough to heat my house, at 40 degrees OAT (outside air temp). Let’s drop OAT to 30 degrees: Then the ratio drops to about 35 to 20. Duct temp drops to 87 degrees.  (Below this OAT is when I shut my HP off, because several factors come into play to ruin what looks good otherwise–The outside coil frosts up, and then uses my inside air temp to defrost it. Also now it is running and running and running, plus if the wind blows, it won’t heat my house. Plus an electric heater inside the compressor starts running, to keep the thick oil thin. The 35 to 20 begins to look more like 25 to 20, and I don’t see the benefit of wearing out what is my cooling unit in the summer for some pretty questionable savings in the winter.

NOTE: All this seems more or less good, but only because I am on off peak. I consider off-peak a necessary requirement for air source heat pumps. Other wise? Uh, uh.

(This is a SEER 14 unit. FYI.)

Now for the Monoblock HP, which isn’t here yet, but will be. This unit sets outside your house and uses stolen heat or cold from air to heat a water-based fluid up to 180 degrees. That is circulated to a coil in your ductwork system, then into your house.

180 degrees? That almost seems impossible to those of us who know temperatures and pressures of refrigeration systems. Very possible, because the R290 (propane) used in these can well do this. Plus environmentally very harmless. So: Replace existing boilers, drive baseboard hot water, heat potable hot water–do lots of things with these when they come out.

For those of us who grew up with R12, we miss it. R12 was super friendly, would do anything you asked of it. Unfortunately, it was killing the ozone layer. Yet refrigerators and machinery and vehicles that used it were around a long time, and it became known to us techs that propane would drop right in. I’ve put it in tractors, combines, freezers, refrigerators and milk bulk plants. Works wonderfully.

One problem, right? It burns. So the new monoblock system will have a limited amount in it. Plus it’s outside. These units are going to blow HPs that are here away. Just watch.

I hope this all helps. Sometimes we just wish life was as simple as crank-up windows in cars and wind-up watches. Sigh.