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First off you must understand that your house required a certain amount of cooling in the summer and a certain amount of heat in the cooler winter months. By adding a zone that was not included
Understanding this can help make your choice a bit more complex. Some would argue that it is doubtful that you will need all zones running at the same time thus the likelihood of you not having enough sizing to do the job is a mute point. However this is not always the case. So that is something you will need to think about.
To get around this issue there are things that you can do to literately over size your system for the house. And that is air sealing. You must understand that most HVAC systems are over sized for the house. Assuming it was NOT an energy star home. The professional that installed the equipment knew that the house was going to be constructed poorly with lots of air leaks and poor insulation quality, If you go through the steps to make the house more efficient you will find that your HVAC system if your successful in doing it right, will be over sized for the house. This benefit will allow you to use the extra new found overage to heat and cool the house. It is not uncommon for a system to be over sized assuming the house was done correctly by as much as one ton of AC and 40 thousand BTU of heat. In fact many are even worse than that.
OK, lets assume you did some basic energy saving work and you gained enough extra capacity to heat and or cool the basement addition your thinking about. Now what?
Well, duct sizing has a lot to do with proper operation of the system. As it is easy to zone a new HVAC system as the ducts can be sized properly and most new equipment has modulating controls that will increase or decrease the amount of cooling or heating that is produced based upon the amount of air flowing through the system. This is not the case for most older equipment. What this does in a sense is cause the heating or cooling system to run at full capacity while you only use a small part of the effect that one or the other is producing. It is sort of like driving a car and putting the gas pedal to the floor and controlling the speed with the break pedal. Not such a good idea, Not only will you waste fuel, you will wear out the breaks in the process and put additional strain on the engine. The same thing happens when you try to zone a single stage heating or cooling system. It still runs at full speed, but your using the ducts to act like the breaks in the car. Something will end up costing you over time.
There are controls however that can help take the strain out of the system when you retrofit the system you have to a zone system, but those controls are costly, and can break down costing you even more money.
Lets say you go with the zone system with dampers, There is still another issue to contend with. The cooling side of the system. Like described above, the cooling system has no way to know if your using the breaks or not, it simply runs at full capacity until it is told to turn off. The bigger issue is however is the moisture removal and lack of proper air flow over the cooling coil located in the furnace. For each ton or 12,000 BTU of cooling you have, you need 400 CFM of air flowing over the coil for it to properly evaporate the refrigerant in the coil. Less air means a cooler coil. The result is freezing up. This in effect damages the cooling system lowering its life expectancy. Again there are controls for that also, but expensive and if they break can mess up your system overall.
Now lets talk zoning. We now know that each duct is sized for each room. You have a maser duct that runs through the house and as it does, take off taps are put into it and those run to rooms located in both levels of the house. To damper that, you need to either put one damper in each take off, expensive, or take those that are running to just the 2nd floor and put them on their own main duct., Then the remaining would need to be installed onto another new duct sized for just those runs the the rooms. Again expensive.
The bottom line is, yes you can install a damper system and zone the house to three or as many as you wish. But there are drawbacks in doing this and most translate to more money. Most HVAC, not all contractors will not tell you about these and many other pitfalls that you will encounter if you move ahead and rework the system into three zones.
My advice is to install another single zone or multi zone heat pump system to take care of the basement work alone and leave the rest of the house as is. Over all this may cost about the same as doing the zone dampers and all the fixes that just happen to show up after they tear out your current system. And you will gain the much higher efficiency that these units have to offer. Sanyo is one of the many manufactures of these mini-split systems. Can be installed in one day with little or no mess what so ever.
I have two HVAC systems. One for the 3rd level only and one for the Main/basement. I already upgrade to a 4 ton 95% as told because I knew I was going to finish off the basement. So the heating
But I am a bit confused, You say you have two systems. One serving the 2nd floor (bedroom area) and another system serving the 1st floor level and in future the basement. Correct?
The unit on the 2nd floor bedroom area is what size?
The Unit on the 1st floor and possibly basement level is what size?
Which one is new? upper level or 1st floor level?
My suggestion would be to install another unit sized just for the basement alone. However if you do not want to go that route, then install a 2nd duct system to the HVAC system that services the 1st floor and do not install any auto dampers at all. If the ducts are correctly sized, and the furnace blower has the capacity to handle the additional addition to the system as far as outlets, then you should be fine. Not ideal, but it will work.
Also do not forget about a return duct in the basement, you must have the same amount of air returning as you are delivering to the rooms. If you do not, your comfort levels will be reduced as well.
Lastly, HVAC contractors tend to over size systems to take into effect leaky homes. Most homes today and in the past are way over sized to take into effect that type of poor construction. As homes are constructed even better the past few years the systems installed by still many are over sized because the contractors feel from past experience that they always put that size system in that size home in the past.
The manufactures and design pros now use systems that in some cases are almost to small these days. The efficiency of the newer systemsare so good that even if they ran all day, they still cost less to run the older units only part time. The longer the units run, the better the comfort over all, this translates into cleaner air, lower humidity in the home during the summer and less drafts. The difficult part is to get the HVAC contractor to understand that they must take into account how the house is built and that they must do a manual J and D to determine exactly how much cooling and heating is needed and exactly how much air is required in each room so the duct sizes can be properly designed.
If the contractors who have come to the house have not suggested doing a load profile, or duct design profile, then they should not be considered unless they are willing to do this. Not having that valuable information in their hands, they are simply guessing on what needs to be done.
I know this all sounds confusing, but without writing a book on the subject it can be difficult to understand.
The bottom line is. The heating and cooling plant that is currently installed is designed to have a set amount of air flowing through it in order for it to work properly. If you change anything that makes this set amount of air to change the result can be a early failure of the equipement, discomfort, and wasted money.
My suggestion is to install a small mini-split heat pump in the basement to service just that area. And when the time comes to upgrade the older system to install a 90%+ furnace and high end efficency cooling system. Both with have variable speed blowers and compressors that will run longer and more efficenctly then the older equipment you currently have installed.
You can tap into the older unit, and reblance the system and live with that however until you can afford to redesign the entire HVAC system on the 1st floor level. But all the ducts wil require some changes so the air will properly flow when zoned with dampers. You simply cannot, effectivly and correctly install a duct damper sytem and have it work succesfully in an older duct sytem without some major modifications to the ducts themselves and the addition of special controls to the furnace and AC system. All which will become usless once you upgrade the furnace and AC system. It is throwing good money into bad.
the 3rd level(bedrooms only) have a 3 ton furnance and a 3 ton A/C(which is new) unit on one thermostat its about 1200 sq
The main and basement have a 4.0 ton (new 95%) Hi EFF and 3 ton A/C(its about 5 yrs old) on one thermostat
The total footage is about 3600 for all 3 levels. I have a double garage/walk out basement and 2 story family/kitchen which kills a lot of the 3rd level bedroom space
this started when looking to finish the basement and realizing my duct work was to big and I couldn't do a drywalled ceiling. So I have contractors coming in to tell me what my options where.
So all said keep my current duct work same size just (tighten/restrap rerun two). One contractor said new AC system with 4 registers one return same thermostat, other two say 3 registers manual damper one return new thermostat 3.0 unit. Other says 4 registers, one return leave AC as is 3 ton or go to a 3.5. but auto damper Zone it
Of course none said anything about doing a load profile or Manual D duct design to determine what you really need. correct?