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Control Strategy for Supplemental Heat in a Heat Pump using OpenStudio?

asked 2014-10-24 14:04:53 -0500

updated 2017-05-17 12:44:29 -0500

So I added in a number of Packaged Rooftop Heat Pumps to my model, and I was wondering if anyone had any good control strategies for the supplemental electric heater which is included when you bring in the heat pump to your model? My model is using ridiculous amounts of electric heating energy when the coils are a part of each heat pump, and I'm looking for a good control strategy to control that extra electric heating coil.

I am looking to have most of the heating done by the DX heating and cooling coils, with the supplemental electric coil picking up the load on particularly cold days. This is for a reference building, so the coils are all being autosized, located in southern Ontario.

I've tried using a couple of FollowOutdoorAirTemperature setpoint managers before and after the supplemental coil, to no success. I've tried combinations of other setpoint managers too, again with little success.

Ideally, this extra electric coil only is used on the coldest days, when the DX heating heating coil cannot meet the load.

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answered 2014-10-24 14:24:27 -0500

By default OpenStudio has the DX heating coil configured to lockout at -8 C. Are you going below that often? OpenStudio also has the coil configured with 2kW electric defrost. Try eliminating that and measure the impact.

If you don't have any set point managers on the coil outlet nodes, OS will provide mixed air SPMs when it creates the idf file. These will be setup to track the supply outlet node, accounting for the downstream fan heat. Each coil will therefore be trying to reach the same temperature. If the dx heating coil is unable to hit that set point the electric coil is going to be left picking up the slack.

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Below -8 C during the winter months, most definitely. -20C wouldn't be uncommon in the winter. :)

I have tried autosizing the coils, rather than having a specified heater value. I also tried deleting the coils altogether, which was how I found out that it was these coils which were the source of my issue.

Let me try the coil minimum temperature, it seems as though I haven't modified that yet. That may be the solution...

... After trying it out, changing the compressor minimum temperature seems to have no impact.

Benjamin gravatar image Benjamin  ( 2014-10-24 14:44:09 -0500 )edit

The limits are there for a reason. The air source DX coil performance drops off when it gets really cold. I suggest that you eliminate the defrost energy and see what that does. But from there the best way to understand what is consuming energy and if your DX coil is able to hit set point is to look at time series data using the Output:Variable object. OpenStudio's ResultsViewer program is useful to plot these outputs.

Kyle Benne gravatar image Kyle Benne  ( 2014-10-24 15:20:54 -0500 )edit

I did as you suggested and, against my own expectations, it brought the heating levels approximately in line to where I expect the values to be.

It seems counter-intuitive to me that zeroing out the capacity doesn't end up in the same results I got when I simply deleted the supplemental heater. Anyway, thanks Kyle. :) That is one solution I probably would have never tried.

Benjamin gravatar image Benjamin  ( 2014-10-29 16:24:11 -0500 )edit

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Asked: 2014-10-24 14:04:53 -0500

Seen: 344 times

Last updated: Oct 24 '14