Running an Air Handler Continuously
How to efficiently run a Mitsubishi Indoor air handler
My HVAC installer (Diamond Certified) installed a ridiculously oversized system for my house as I discovered later after doing a load calculation using the BetterBuiltNW design tool. That tool revealed that the SVP-KP36NA air handler has more than enough capacity to heat the entire house by itself (without the 3 additional ceiling cassettes he added upstairs). However the unit is “spiky” – I have it on 24/7 but it acts more like my old gas furnace – rather than continuously running keeping a constant temperature it turns on the heat pump (an MXZ-SM48NA) brings the room up to temperature then goes back to running the fan (and the heat pump stops running).
Is there any way to get this running continuously at a low level feeding warm air into the entire house or is the unit incapable of doing that?
I’m in Silicon Valley climate zone 3C.
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… have the tech disable the continuous fan. Run the fan on lowest speed.. not familiar with your particular air handler but most of them come with switchable harness for low/ med/ high fan speed to accommodate various static pressures- have tech plug in/ reconfigure for a lower fan speed range. If using a wall thermostat play with register dampers to avoid heating the thermostat too fast (close a bit the closest to thermostat registers)…. Other than that there isn’t much you can do I believe.
Your indoor model number would be SVZ-KP36NA. The static pressure should be measured, and then set properly via the controller during commissioning. There are three selection, .3" wc, .5" wc, and .8" wc. The installer should check this with a manometer and set the fan accordingly. Definitley check thermostat location. Yes, if it's located too close to a supply it can be problematic. It should be in a central location where the temp response is more gradual, obviously on an interior wall. Also, some thermostats have the option of reading from the return air thermistor, or using an average between the return air and the controller. I prefer to tell the air handler to use just the controller for sensing temp. It simplifies things. I typically set these up on a wireless MHK2 touch screen thermostat. Easy to read and easy to use.
I have an MHK2 and it’s measuring the temp. I’ve also tried experimenting with measuring the return air instead because the MHK2 has a +/- 2 swing which I think is responsible for the spiking. I also tried the PAR440-MMA and didn’t like that one either. The installer set the airflow to the highest number on the advice of an instructor at the Diamond Certified seminar he attended because he installed an AprilAire 313 filter in place of the standard filter.
The temperature swing on on my upstairs ceiling cassettes is better but they have other issues (like the fan shutting off for no apparent reason - not due to defrost mode). My installation has been a disaster I think.
I have a new Mitsubishi air handler and heat pump as well. How did you get the "spikey" information about it turning on and off? The app certainly doesn't supply that info.
Thanks
Nick
I have a Franklin Whole Home solar+battery system. The app shows power usage in real time and graphs it as well. The spikes are obvious on the graphs and when I saw them I thought something was wrong because I had heard that heat pump systems were supposed to run at a roughly slow and steady rate supplying heat rather than turning off and on like my old gas furnace.
Yet again multi split equals unhappy home owner. Hugely oversized multi split equals very unhappy home owner.
Consider replacing the oversized outdoor unit with 2 smaller outdoor units one to run the ducted unit and a second to run the upstairs cassettes.
Your current unit must cycle on and off when its min output exceeds the load from your home. At 47°outdoors that is 27000 BTUs. At 17°outdoors that is 11000 BTUs
https://ashp.neep.org/#!/product/57026/7/25000/95/7500/0///0
If the new outdoor unit were 2.5 tons it would likely not cycle until the load is less than 3000 BTUs.
Walta
Thanks everyone!
I'm in almost the same situation: https://www.greenbuildingadvisor.com/question/advice-on-how-to-alleviate-issues-with-poorly-thought-out-mitsubishi-ducted-heat-pump-installation.
One thing I'm not clear on is if we set static pressure to 0.3 even though it's actually higher, does this trick the unit into running a lower CFM. If so is that *very* bad (overheating?), or the unit self adjusts to this as well (more cycling?)?
The crazy thing is that three diamond certified installers independently speced the exact same system for me, even though if I put some pretty conservative numbers (now knowing what I know about oversizing) at: https://ashp.neep.org/#!/product/57026/7/25000/95/7500/0///0, I get the warning "Oversize Warning: This system has a Percent Heating Design Load Served greater than 150% and will likely experience moderate to severe oversizing effects.".
I'm going to ask the installer (talking to them now) to take some responsibility for designing a system that fundamentally doesn't work, maybe with some prodding from Mitsubishi. But I'm guessing it's not going to be easy to convince them to replace a several thousand dollar unit with another several thousand dollar unit. I have the additional problem that my multi zone only serves two units, so I don't think I can leave it in place and have it only serve the wall unit (also this would be a ridiculous 48k to 9k mismatch); I would need to replace my existing unit with two new ones.
Do large installers usually have programs in place to replace equipment without incurring the full cost? Like are they able to reuse/resell the old unit?
I specialized in resolving problem projects in HVAC for residential, commercial, and industrial applications with expertise in ductless, VRF, and ERVs. I will do my best to advise you.
Unfortunately, from what I can glean, both you and your installer failed to follow the steps in HVAC Design. Now is the time to hunker down and start asking better questions. Do not be in a hurry. I recommend that you be pragmatic about the situation and enjoy the process. It can be fun and a good experience. If you don't want to enjoy yourself, then it will be a poor experience and may result in undesirable results.
Take note: Although oversizing a quality variable capacity system like Mitsubishi is possible, it is very difficult to do. Also, I would not recommend using the BetterBuilt HVAC Sizing tool; it is not an ACCA-certified program. I do recommend using CoolCalc.com for simple geometry buildings with a 'normal" amount of glass. If your home has complex geometry, exotic materials, or a large amount of glass, I would recommend KwikDraw 3D, Elite, or Wrightsoft. I am a KwikDraw and Wrightsoft user.
I see people on here giving advice, but in your case, there is just not enough details for me to feel comfortable sharing a well-qualified opinion.
MXZ- The software driving the system operation responds to the settings, temperatures, and other data points as it normally does. The good part is that some of these data points can be adjusted to fit your particular application. This can be achieved by way of the wired remote controller and the outdoor unit dip switches. The bad part is that many of the variables in the design and installation can be causing the issue. Diagnosing the root cause is a multi-step process.
What I would want to know is:
1. Was a heat load/loss calculation completed? If not, this is the first step. If you want to skip this step you may stumble upon a solution in the following steps.
2. Was a Diamond System Builder file completed? This is key to a successful design. I am assuming there are four indoor units connected to the four ton outdoor unit. However, the systems combination ratio may be exceeded. Either way, the three upstairs units may be sharing only one ton of available energy if at max capacity. I would like to review the DSB file to ensure proper design restrictions and limitations are adhered to. The DSB program is free and available on mylinkdrive.com
3. Is the MHK2 sensing from the wall or the duct? The return air sensing location is selectable from either the IDU or the RC.
4. Was the airflow tested? If not, it would put into question the entire installation and duct sizing, GRD (Grille, Register, Diffuser) sizing.
5. Are you certain the correct piping diameters were used?
6. Are you confident in the refrigerant piping and charging?
Following design and installation best practices are essential for a successful outcome. The reality of the "Diamond Certified" dealer is that the Company may qualify, but the individuals do not. Also, it is a symbol for marketing, not quality. A qualified practice for quality can be found using ANSI/ACCA Quality Standards https://www.acca.org/qa
Thanks hvacMENSCH really appreciate the response!
1. I asked for a load calc and they sent back a load estimate for the electrical panel... So, no? :)
2. I'm guessing it was. Is it something I can ask them to provide?
3. I've tried both. I get slightly larger swings with sensing from the wall.
4. Honestly I'm not sure what was tested. It's new ducts but retrofitted on existing registers. Distribution is comfortable except for the spikes though.
5-6. No :)
I noticed today that my MFZ-KJ09NA can continuously draw as low as 1050W. I'm confused by this because I thought that's below the rated min capacity of 27k BTU of the MXZ-SM48NAM. Although I'm still struggling to find reliable wattage/BTU numbers. For example: https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006%5CDB_M-P0855_MXZ-SM_NAM(HZ)-U1_JA22.pdf page 70 doesn't really agree with: https://ashp.neep.org/#!/product/57026/7/25000/95/7500/0///0. Also the former seems to go as far down as 784W, which if I could get my SVZ to draw down to I would be pretty happy. But for some reason it's stuck at ~2000W. Any ideas?