New Installation Thoughts - Oversizing

Problems related to residential installations.

- New Installation Thoughts - Oversizing

Postby nomadpeo » Tue Feb 01, 2011 1:13 am

it's unfortunate that you did everything right in your research and got someone that did not evaluate your house before the installation. the sad fact is that you may need to move on to another contractor to straighten out the problems.

as the contractor continues to try to bandaid the problems, it gets more bizarre. it's not an isolated story. i hope others will offer comments here. i definitely recommend you bring in another professional.

i will address a couple of issues and ask a couple of questions.

what is the square footage of your house ? this information alone is not enough to size a system. however, if you had a two ton system, more is not always better. in fact, modern technology affords us the ability to scale down sometimes from that approach because the systems are much more efficient.

does the new furnace have a variable speed blower ? if so, it should be able to adapt (somewhat) to the duct static, within reason. the variable speed technology is not designed to compensate for inferior duct design, but it could make the difference in whether it is suitable. usually there are dip switches to select the appropriate cfm of air for the tonnage.

this should be a 2 stage furnace. it could be run on half the heat capacity. combined with a vs blower, this could be a control point. mind you, i am throwing out some thoughts. sight unseen, it's impossible to evaluate.

the issue of heat imbalance between rooms is an airflow balance issue. are there volume dampers installed prior to the branch ducts ? even so, if the furnace is grossly oversized, dampers may contribute to the problem. temperature rise is rated on the name plate. with all dampers open, this reading should be compared with the rating.

based upon the information you have shared, it does not sound like zoning is a solution to your problems. it may only make it worse. if you block off too much air while the furnace is running, it's possible it may shut itself down.

the cfm of air moving across the heat exchanger and the evaporator coil has to be a proper match. of course, the load has to be a match with the equipment or problems like humidity being left behind will result.

i strongly recommend you research and find another company with a well qualified person who can come out and tell you what needs to be done. hope this all works out for you. keep us posted.
nomadpeo
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- New Installation Thoughts - Oversizing

Postby nomadpeo » Wed Feb 02, 2011 1:45 am

800 cfm sounds right for the 2 ton (generally, 350 to 400 cfm per ton is appropriate). with a vs blower, dip switches can be set for the cfm requirement. the unit may be designed to be used on systems ranging from 3 to 5 ton. if you have a digital programmable thermostat, it should also have a configuration menu where you define the system it is controllling (hp/ac/gas,etc) as well as other adjustable parameters such as low and high temp limits, cycle rates, intelligent recovery, etc. make sure they are set right.

cycle frequency will depend primarily upon the location of the thermostat. is your thermostat located near the return air? is your thermostat programmable and if it is an upper end stat, does it have a configuration menu ? if so, number of stages, type of unit (ac/heatpump/gas), cycle limit and other items can be set up. the furnace circuit board will have banks of dip switches to tell it the appropriate cfm for heating and cooling. also, selectable fan ramping algorythms are usually onboard to fit the application. if the duct system is not appropriate, the furnace blower will still ramp up to match the setting it sees to maintain that cfm. still the static in the duct needs to be in the ball park.

do i understand you to say you have a total of 3000 sq ft including the finished basement ? if 3 tons is enough, that is remarkable. if you have an isolated area that warrants its own control because the temperature/humidity is not being sensed there, it is possible that the system cycles off before that area can be conditioned. in that case, since you do have a variable speed blower, it may not be inappropriate to use a separate thermostat to control that area. that would indeed involve a motorized damper controlled by that stat. when that is done, a barometric damper must be added and ducted between the return and supply air in order to maintain the appropriate amount of airflow. it must be sized for the smallest zone. the zones need to be sized as equally the same size as possible. furthermore, the best case scenario is that the outside unit utilizes a 2 stage compressor. if half the heat load goes away, less refrigerant is evaporated and an imbalance occurs. this can be detrimental to the compressor as well as reduce efficiency. a zone controller will receive signals from the thermostat and stage the compressor and indoor fan speeds accordingly. a 2 stage compressor, vs blower and a properly sized and properly installed barometric damper can work quite well in the right application. each zone can actually control the equipment independently for heating and cooling with the zone controller discriminating modes and timing. with the wrong combination and installed improperly, it can be a nightmare. if the thermostat is also sensing humidity, the fan speed can be ramped down 15% during high humidity conditions to provide better dehumidification. these featueres are becoming the standard on upper end equipment.

sounds like a splitter damper between the east and west side may be appropriate, but each starting collar feeding the individual registers should also have dampers which can be adjusted from full open to full closed, manually. these would be installed at the trunk and can be used to balance the air distribution. adjusting air volume at the individual registers is not always effective and can also result in noise.

i'm curious about the layout. is the basement fed from both west and east same as the main floor, or is the basement fed from its own truhk ? also, what is the location and size of your return air grills and what kind of filters are you using ?

get another engineer/contractor to look things over. air movement and properly sized and installed ducts makes or breaks the installation of decent equipment. also, make sure they check the furnace installation for proper venting and drainage.

good luck.
nomadpeo
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