Pros/Cons of upgrading to 5 stage heat pump from 1 stage?
My residence currently has a Carrier 1-stage heat pump system. Other HVAC components include: LifeBreath HRV195 balanced airflow ventilation unit; Honeywell DR120 bypass return air DEhumidifier; and a bypass activated carbon & HEPA air filtration unit. Each of those components have their own integral 1-speed air fan. Dealing with the different cfm created by each of those components makes it a challenge for my HVAC service company to achieve “balanced” airflow for the HRV and keeping the HVAC system static pressure within specifications. They have suggested it may be beneficial for me to upgrade from my current 1-stage heat pump to the latest 5-stage heat pump technology. They think the 5-stage may facilitate better overall HVAC system management & operation re equipment differing fan cfm, HRV air balancing, effective reduction of humidity when the HRV is operating in hot humid months, while keeping the overall system static pressure within specifications. (1) are they correct that upgrading to a 5-stage from my 1-stage heat pump will enable more effective HRV air balancing, static pressure & compatibility of differing fan cfms, etc? (2) what are pros/cons of 5-stage heat pump over 1-stage? Note: when I purchased my current 1-stage heat pump, Carrier tech folks advised AGAINST install of the then 2-stage “Infinity” heat pump due to concern that the HRV, dehumidifier & air filter units’ 1-speed fans & intermittent operation of the HRV & dehumidifier would probably screw up the computer-controlled Infinity operation & result in “defaults”. THANKS FOR INPUT!
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Replies
Terry,
I'm a strong advocate for keeping ventilation ducting separate from the ducts used for heating and cooling, for precisely the reason you note. For more information on the benefits of dedicated ventilation ductwork, see Ducting HRVs and ERVs.
I'm also skeptical of the need for a whole-house dehumidifier. In most cases, the cooling system should be able to handle indoor humidity issues. When needed, a $250 stand-alone dehumidifier makes more sense than trying to integrate a whole-house dehumidifier with a heating and cooling system.
Martin, as always, I respect your experience & advice. My experience confirms your advice re separate duct system for ventilation. We considered separate ducting. However, in this 60yr old house, a separate ducting for HRV is just not possible in terms of affordability of retrofit given reconstruction that would involve. Went to bypass dehumid because room dehumidifiers just didnt handle it. While bypass dehumidifier has worked much better, there are still many days when simply have to shut off HRV totally due to high humidity loads.
MY HVAC servicer just added new idea if I dont want to spend mega$$$ upgrading 1-stage heat pump: ADDING A MINI-SPLIT AC/DEHUMID UNIT TO MAIN LIVING RM/KITCHEN AREA with idea that it would (1) dehumidify the main part of house where in-home generated humidity comes from, and (2) keep indoor temps cooler on very hot days when heat pump has trouble keeping main living area <75 system?
Terry,
It sounds like you have a classic example of an expensive and complicated HVAC system that (a) isn't satisfying your needs, and (b) is probably using a lot of energy (especially considering the whole-house dehumidifier).
If adding a ductless minisplit to one of your zones costs less than upgrading to a 5-stage heat pump, then I absolutely support that option. Perhaps it will prove to be the first step on your journey to simplify your HVAC system. Maybe you'll eventually end up with a few ductless minisplits, and you can abandon the ducted system.
What are the pros/cons of upgrading to a 5 stage heat pump instead of sticking with a 1 stage heat pump? Would a 5 stage heat pump have trouble operating as designed, given the return air ducts have (1) bypass air filtration (2) bypass dehumidification and (3) HRV periodic ventilation?
Terry,
It's hard (or impossible) to answer the question as posed.
Part of the problem with systems like yours -- systems that include everything, plus the kitchen sink -- is that they may have been designed without an aggressive Manual J calculation. So if the equipment is oversized to begin with, it's hard to know what is now needed.
Was a Manual J performed? If so, can you share these details?
(a) The number of square feet of your home.
(b) The design heating load shown in the Manual J calculation.
(c) The design cooling load shown in the Manual J calculation.
(d) The nameplate heating and cooling output for your heating equipment and cooling equipment.
A 5 stage system will run most of the time at much lower blower rates, which will interfere less with the balanced ventilation scheme. It's not clear that it'll be "worth it" in the end on either an energy use or air quality point of view. It sounds as if you already have too much (and probably oversized) equipment in the place, and constantly hacking on the kludge probably isn't the best approach.
Short of yarding it all out and starting from scratch, the mini-split band-aid is likely to work just fine, as long as it isn't ridiculously oversized for the residual load that it's carrying. Oversized systems tend to have poor humidity control, as do high SEER systems, and most mini-splits have a fairly high SEER and won't dehumidify well in their "normal" cooling mode, but can be operated in "dehumidify" mode if not keeping up with the latent load. But even in dehumidification mode there will be sensible cooling, and may only be able provide enough dehumidifcation if by cooling the room if the amount of sensible load it's being asked to pick up is low, and the latent load is still high.
One way around that trap is to go with Daikin's Quaternity series, which can dehumidify to an RH% setpoint even without sensible cooling when it needs to. The temperature and RH% setpoints are independent of each other. They're more expensive than most mini-splits, but it's probably your best shot at a reliable band-aid solution.
Whole house dehumidifiers are usually only necessary when the ventilation rates are insanely/unnecessarily high for a residence.
Any mini-split will be an expensive band-aid. Before spending any more time or money on this, hire a qualified credentialed P.E. or a RESNET rater to perform an aggressive room-by-room Manual-J load calculation, and analyze the appropriateness of the existing split AC, and reviewing the duct design. If it's 2-3x oversized (frighteningly common), right-sizing a 1 or 2-stage replacement, mothballing or scrapping the dehumidifier and fixing any duct design/implementation issues may prove to the most satisfactory approach in the end. If you go with a mini-split, go with a Quaternity that's right-sized for the calculated zone load (as if the existing AC system isn't there).
In a 60 year old house, your ventilation rate probably is insanely high. Tightening it up will help with humidity during AC.
It may also mean that operation of the HRV while the AC is running is unnecessary (stack effect alone might provide enough ventilation).
If it's a leaky 60 year old house with unbalanced and ducts up in the attic above the insulation, air handler driven infiltration and could even be the primary problem.
Diagnosis by web-forum isn't always possible, but hiring a competent engineer or energy nerd to figure it out is cheaper than continually buying more equipment. While there are some HVAC contractors capable of figuring it all out, most really aren't up to it. They are in the business of selling & installing equipment, whereas P.E.s make a living off the quality and accuracy of their analysis.