ASHP+gas instead of ccASHP+gas for cold(!) climates ?

Dave Carpentier

If you live where even a ccASHP is going to need back-up for a good chunk of the winter, could it be wiser to use a regular ASHP and just lean on the backup a bit more ?
In either case, full design-temp btu burner is needed.
Presumably, the ccASHP costs significantly more up front, and maybe higher maintenance ?

Hot_water_fan

It’s a fine strategy. You can size perfectly for cooling and get a simple, no thrills furnace which will be inactive most winter.

A cold climate heat pump is more expensive, but I’m not sure it’s the cold climate part or the modulating part. They just are higher end products. 

pecmsg

But do you size for temperature change over or temperature and wind changeover?

Last fall / winter we saw a lot of <40°F rainy days that drove heat pumps nuts!

Hot_water_fan

30-40 degree days are nothing for a heat pump (or at least mine), let’s be real! 

hot_rod

Put some numbers to the number of days you feel you would need back up. Are these 24 hr days or just evening hours.

Look at both COP and reduced capacity when you consider those days.

You can find historical weather data for any weather station in the US to compile numbers.
Like this example for NY.

With a number of years run time on ccA2WHP now we can get some answers to the reality of the backup needs.

SpacePak has another training webinar scheduled for dealers and installers. That will be a good place to get info from the actual installations over the past 3-5 years.

The problem has been always will be, installers and sales people not taking the time to accurately crunch the numbers to get some data to make informed decisions. This is true for boiler installations also, as we see on an almost daily basis here at HH.

NYSERDA has a nice heat pump simulator developed by people with heat pumps installed and data logged fro real time data.

Equipment continues to improve. Compressors can now spin up to 7000 rpm. So you get some additional heat from friction

The Steam Whisperer

Dave Carpentier said:

If you live where even a ccASHP is going to need back-up for a good chunk of the winter, could it be wiser to use a regular ASHP and just lean on the backup a bit more ?
In either case, full design-temp btu burner is needed.
Presumably, the ccASHP costs significantly more up front, and maybe higher maintenance ?

I think what you are talking about may be the true long term solution.... assuming electricity can be generated without pollution. The typical load curve for most of the northern U.S. is you need 60% or less your peak capacity for 90% of the heating season. Size the heat pump for 60% of the peak load and then use gas backup of the last 10%. I'm pretty sure you will want to size the gas for the full load because when record breaking temperatures hit, the heat pump will probably not be able to produce much heat, so you could end up freezing up the building. I. E. design load for Chicago is around 0F, if you size a heat pump to provide design load at 0F outdoor and 70 F indoor, will it be able to keep the indoor temperature above 45F when we hit record lows of -25F.....probably not! I'd design the heat pump to only handle down to about 20F, and then go full gas below that temperature.

hot_rod

If you plan on having the HP and boiler for backup, may as well size the boiler for the design load. Not a lot of choices for small output boilers anyways. 50 K on mod cons, maybe some cast boilers a bit smaller.

The difference in cost between the smallest and the correct sized boiler is not worth the hassle on residential applications, probably.

Your pumps and piping will or should be sized to the largest load regardless of the size of the boiler, or HP.

I want to learn more about the SpacePak concept of not dropping off the HP, but using some boiler power to get the COP back up. Instead of a complete switch from one source to the other.

Again it is all in the numbers, cost of the two fuels, getting COP above the number where is is less than the NG boiler.
I think the simulation programs will make these calculations easier.

Dave Carpentier

hot_rod said:

I want to learn more about the SpacePak concept of not dropping off the HP, but using some boiler power to get the COP back up. Instead of a complete switch from one source to the other.

Like a continuous defrost ?

My design temp is -21f , so this has me curious.
Gas company finally laid a pipe, plan is to retire the oil boiler and go gas-something next summer.

Hot_water_fan

@hot_rod you could have the heat pump on the return right? It heats 110 to 120, then the boiler heats 120 to whatever? 

hot_rod

The way they show the piping at the buffer with backup boiler, both could run together. Maybe the control brings on the boiler at low fire or low SWT for boost, leaving the HP on. I'll ask the tech people.