Reduced Blower Speed

[euler7]

We have a two-stage 5-ton FHP (AP061) with slinky ground loop. We live in a heating-dominated climate; our system is nicely sized for heating but (unavoidably) oversized for cooling by a good 1.0 to 1.5 tons. I posted here last year asking about strategies for increasing dehumidification when the unit is running during cooling mode and got some excellent feedback.

Other than tweaks to the TSTAT to promote longer Stage 1 run times (which I have done), several folks here also suggested trying to lower the unit's blower speed setting. Our unit's ECM board has an adjustment jumper pin which can reduce CFM by 15%. With cooling season approaching, I was going to try this, but I wanted to ask about a warning I read in the FHP install manual:

NOTE: Do not set the ADJ jumper to the (-) setting when electric heaters are installed. Doing so may cause the heaters to cycle on their thermal overload switches, potentially shortening the life of the switches.

We have strip heating for Emergency Heat on our system. Am I safe to assume the above warning is nothing to worry about if I adjust the blower speed only for cooling mode, then reset it to regular speed next fall before heating season begins?

I read the above warning to mean that reduced CFM could cause the emergency heat to cycle on and off more frequently than normal CFM; hence, possibly burning out the strips faster. Nevertheless, I wasn't sure about the "switches" language or whether there would be something inherent in lower CFM that would be bad for switches. For instance, if CFM is reduced across the coil, would the reduced temperature of air blowing through the unit still mess up the switches even if the unit is in cooling, not heating, mode?

Thank you.

[docjenser]

I don't know the FHPs too well, but you should be fine in cooling mode.
I know other manufacturers have a higher blower setting for Aux heat, and if used for supplement heat (3rd stage) the blower is running at higher speed as well.

[AMI Contracting]

"We have strip heating for Emergency Heat on our system. Am I safe to assume the above warning is nothing to worry about if I adjust the blower speed only for cooling mode, then reset it to regular speed next fall before heating season begins?

I read the above warning to mean that reduced CFM could cause the emergency heat to cycle on and off more frequently than normal CFM; hence, possibly burning out the strips faster. Nevertheless, I wasn't sure about the "switches" language or whether there would be something inherent in lower CFM that would be bad for switches. For instance, if CFM is reduced across the coil, would the reduced temperature of air blowing through the unit still mess up the switches even if the unit is in cooling, not heating, mode?"

Yes restoring speed next fall will be fine.

It warns you about harm to the thermal limits which protect the "strips". They are inexpensive and if overtaxed will ultimately fail. As long as you do not run AUX. heat this is a non-issue.

[euler7]

Thank you, Joe! This is helpful. I'll be sure to reset the jumper pin at the end of cooling season this year.

[docjenser]

For instance, if CFM is reduced across the coil, would the reduced temperature of air blowing through the unit still mess up the switches even if the unit is in cooling, not heating, mode?

Thank you.

If you slow down the blower, the temperature of air actually goes up, since the heat pump makes almost the same amount of BTUs but has lesser air flow to "get rid off" those BTUs.

[euler7]

That makes good sense, Doc. Thank you.

[engineer]

I'm suprised to read that FHP's control board isn't smart enough to disregard the reduced airflow selection while operating strips or while in heat mode

[euler7]

I'm suprised to read that FHP's control board isn't smart enough to disregard the reduced airflow selection while operating strips or while in heat mode

Speaking as only a residential user and not industry professional, I am surprised too. I've read through the ECM program options in the FHP/Bosch, ClimateMaster, and Waterfurnace technical literature, and FHP/Bosch is clearly the least intelligent of the three -- at least if we define intelligence as giving users latitude in controlling CFM. Waterfurnace has the -15% CFM option for cooling in dehumidification mode (CFM for heating is unaffected). ClimateMaster is the most intelligent and has 4 CFM programs each for cooling and heating, and each of these can be further adjusted +/- 15% via another tap setting.

On install, our contractor gave us the option choosing FHP (now Bosch) or CM. They install and service both with almost equal frequency. They said they slightly preferred FHP and their staff tended to install FHP in their own homes over CM, and so I went with that testimony. Because our unit is oversized for cooling to begin with, in hindsight I perhaps would have opted for CM given its impressive ECM programming -- or at least would have more seriously pressed our contractor about the viability of downsizing by a ton (4 instead of 5). But overall we are very pleased with our FHP and can't really complain.

I'll be interested to see if FHP support responds to my email about the explanation of their different cooling and heating ECM programs. The ECM board has 4 tap settings for each mode, but the install manual does not provide any useful info other than to specify which tap setting to use for a particular heat pump model.

[docjenser]

I had 2 severe issues (out of 2) with the quality control of FHP, nor was I very impressed by the design and the controls. While Bosch is known for outstanding engineering and controls, their ownership has not yet transferred into design changes for the GSHPs. Can't wait for that to happen, until then I am shying away from them.