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Energy cost and use of PTACs vs. WSHPs

asked 2015-07-02 09:49:55 -0600

updated 2016-01-16 11:28:49 -0600

I'm working on an apartment building project for a utility incentive and LEED certification. The baseline model uses PTACs (90.1 PRM System 1) and the proposed model uses WSHPs. Comparing just the HVAC system differences, I haven't found that the WSHP system is much better in terms of energy cost or use. So my question is whether other modelers have found significant differences in energy cost or use when comparing these systems?

The problem I've found is that while the gas usage is less with the WSHP system, there is an electricity penalty for switching from all gas heating in the PTAC system to gas and electric heating in the WSHP system. So while the EUI of the WSHP system is similar or lower than the PTAC system, the energy cost is higher due to the price difference between gas and electricity. I haven't found a "big lever" to pull in the proposed model, such as plant temperatures, that will alleviate this issue. Any thoughts?

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answered 2015-07-02 10:14:47 -0600

We often see this due to switching from a gas heating baseline to a (mostly) electric heating proposed. Depending on rate structures, the electric heating demand in winter will really hurt the cost savings of the WSHPs. We usually see better savings when the client is using really nice high efficiency WSHPs with ECM motors and is also running the condenser loop in a manner that puts more of the heating load on the boiler and less on the heat pumps (ie high condenser water temp during heating).

Hope that helps.

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answered 2015-07-06 05:56:46 -0600

updated 2015-07-06 05:58:06 -0600

A similar question was brought up on the equest mailing list not so long ago: see this thread.

Some info, mostly in the latter posts, is software agnostic and touches on why WLHP might be less efficient than one would think.

I certainly see the point that you'll increase electricity consumption due to the heat pump, pump power, fan power, and that electricity can be significantly more costly than gas. So in terms of costs, sure. In terms of source energy use it could be as well, depending on the energy mix of your location.

Anyways, an ASHRAE article is linked in one of the posts: link. I'm still skeptical until I see it for myself as I generally don't believe articles - especially about new construction modeling - unless I can see the model or I know something about the peer review process.

I'd make sure you have defined all the auxiliaries properly and don't have any major issues in your model before you rule.

Another thing I'd look into is infiltration, and R-value derating by having wall penetrations.

Sure, it's hard to quantify for certain, but having seen a ton of PTAC/PTHP in the NYC area, infiltration is definitely a massive issue. Even if it's installed top notch initially, down the road the caulking and weatherstripping will fail and it'll likely get overlooked.

Here's a (poor) picture of beautifully leaky PTAC, the smoke pen was truly incandescent.

Leaky PTAC highlighted by smoke pen


Another side note, it doesn't apply to you since you seem to have gas PTACs, but another common problem of having PTHPs is in the heating mode in operation (rather that theoretically...) in cold climates. It's generally understood that below a certain OA temperature the PTHP will default to electric resistance (35°F for most models, sometimes you get some mixed mode between 25 and 35°F on some models).

One thing that's typically overlooked is that the PTHPs will often go into "boost mode" turning on the electric resistance full blast when the difference between the setpoint and on the actual indoor temperature is too great (3 to 5°F), even though the OA temperature is high enough to be in heat pump mode. For example, if it's 40°F outside, and when I come home from work and it's 65°F inside, so I turn on the PTHP and set the Tstat at 80°F thinking "it'll get warmer faster!" (in some way it will, but so will the bill...).

Those settings are either preconfigured in the units' control board (and not necesarilly mentioned very well in the documentation) or it's on DIP switches. If you overlook that and you let people have full control of the thermostat, you're in for a potentially nasty surprise if you do M&V.

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Asked: 2015-07-02 09:49:55 -0600

Seen: 2,321 times

Last updated: Jul 06 '15