Originally Posted By: jmertins This post was automatically imported from our archived forum.
bkelly2 wrote:
Sorry to disagree but I do not do "rules of thumb". I believe if you go down that road you had better apply your "rules" to the entire system. Sizing a HVAC system is Engineering, of which I am not a practitioner.
OMHO
BK
If you came across a 1.5 ton system and the house was 1500 sq feet you would not note, report, or whatever you want to call it your client? Is this what your are saying? Are we not called in to FIND issues with the house? Don't make this a verbage thing with how I worded my "rule of thumb", that is not the point. Please tell me you would let them know in some way, shape or form that this tonage is most likely not correct and a HVAC tech, not a engineer, should look into.
Originally Posted By: bkelly2 This post was automatically imported from our archived forum.
Hi John. Yes we are called out to find things. I think sizing a HVAC system is out of my scope.
That said these are things I note
Type of fuel
Age & location
standard observation ie middlie of design life
Furnace
Vent Pipe
fan
gas valve
conbustion air vents return air comp
evap coil
drain pipe
drip pan
cond coil
disconnect
refridgerant lines
temp splits
thermostat registers
ducts.
As to 1.5 tons not being correct for a 1500 sq/ft home I believe that is a design issue. My house for example is 3000 sq/ft, there are 20 ft ceilings in some areas. It is all single pane windows, not much insulation in the vaulted area, how do I factor all of that in????
Do not get me wrong rules of thumb are fine I just do not use them for my inspection reports.
-- "I used to be disgusted, Now I try to Be amused"-Elvis Costello
Originally Posted By: bkelly2 This post was automatically imported from our archived forum.
jmertins wrote:
So you still would not inform a client that the system may be undersized?
What if the unit/s are oversized do I make that call also?
Not enough information John to make that call. Was everything else ok? How were the splits? What is the cu/ft of the house? What is the R value of the insulation, walls? Does the house have any shade?
Now verbally I might mention something.
A good example here is when a porch is closed in and a window shaker has been installed. I do not test window shakers and I point out that the with the addition of the room it is possible that the heating and cooling may not work adaquetly.
What if the unit/s are oversized do I make that call also?
-- "I used to be disgusted, Now I try to Be amused"-Elvis Costello
Originally Posted By: Steven Brewster This post was automatically imported from our archived forum.
I do use a rule of thumb… for heating and cooling…see code check. If I find a system that is either oversized or undersized for the area served, I make a recommendation that the system be evaluated by a licensed HVAC pro for proper size and function. This recommendation usually goes hand in hand with a function test. Usually a pro in my area will evaluate a system, clean it and check the furnace/exchanger for $50 bucks. Good investment for an older system!!!
I feel it is my job to inform the client of any concerns that I may have regarding the systems within the home. Bottom line, if I have concerns, I recommend a specialists evaluation. CYA 
Originally Posted By: Gary Reecher This post was automatically imported from our archived forum.
dandersen wrote:
You guys are getting closer! 
hint: Q = U A delta T
Close. But once had an a/c that had an add on txv kit installed. Couldn't get there during design conditions. When I did get there next day outdoor temp dropped and was in low 80's. Suction and liquid pressure's and temps checked subcooling right on and cooling coil temp drop ok. Next time they called got there that day. Now subcooling is low and temp drop low. This particluar manufacturer sizes their txv's exactly the tonnage not wanting field techs readjusting the valve. The kit had a valve that was 1/2 ton smaller than what the system was designed for and would cool properly when outdoor temp was low but temp would climb when it was near design temp.
Granted this a fluke but an example of where if you test at low outdoor temps it just might not cool when design outdoor temps are reached.
Originally Posted By: jmertins This post was automatically imported from our archived forum.
Steven Brewster wrote:
I do use a rule of thumb... for heating and cooling...see code check. If I find a system that is either oversized or undersized for the area served, I make a recommendation that the system be evaluated by a licensed HVAC pro for proper size and function. This recommendation usually goes hand in hand with a function test. Usually a pro in my area will evaluate a system, clean it and check the furnace/exchanger for $50 bucks. Good investment for an older system!!!
I feel it is my job to inform the client of any concerns that I may have regarding the systems within the home. Bottom line, if I have concerns, I recommend a specialists evaluation. CYA 
Thank you...that was really all I was trying to say.
Originally Posted By: dandersen This post was automatically imported from our archived forum.
jmertins wrote:
bkelly2 wrote:
Sorry to disagree but I do not do "rules of thumb". I believe if you go down that road you had better apply your "rules" to the entire system. Sizing a HVAC system is Engineering, of which I am not a practitioner.
OMHO
BK
If you came across a 1.5 ton system and the house was 1500 sq feet you would not note, report, or whatever you want to call it your client? Is this what your are saying? Are we not called in to FIND issues with the house? Don't make this a verbage thing with how I worded my "rule of thumb", that is not the point. Please tell me you would let them know in some way, shape or form that this tonage is most likely not correct and a HVAC tech, not a engineer, should look into.
Based on the information you provided, absolutely not!
I built a house for MIT in Bedford, MA. that was >1500 sf with < 18,000 btu/hr equipment.
Not all houses (sq. footage) is created equal.
Unless you want to do the math, I recommend keeping away from system evaluation. Throwing up red flags is not better safe than sorry.
Originally Posted By: jmertins This post was automatically imported from our archived forum.
O.k not trying to beat a dead horse. For the sake of a good debate (which we have going here). If the client came back 6 months later and said " ya know you did a great job on my inspection, the house is great but I wish you would have informed me that my unit was undersized/ oversized for my house… is that something you guys generally know?"
What would your response be? Do you feel that we should have a idea, depending on where we live, on the proper sizing of units with relevance to the type of housing we inspect. Obviously frame construction versus concrete, radiant heat, insulatory factors, type of heating mode, etc,etc,etc will determine sizing. Being able to answer these questions would help us make a better recommendation to Joe/Sheila Homebuyer if a HVAC tech should be brought in.
Originally Posted By: bkelly2 This post was automatically imported from our archived forum.
John well said. If I am in the house and it is not getting cold I point that out. If it is not getting hot I point that out. If one room is not getting very much conditioned air I point that out. Commenting on the size of someones unit I am not comfortable with.
– “I used to be disgusted, Now I try to Be amused”-Elvis Costello
Originally Posted By: dandersen This post was automatically imported from our archived forum.
Exactly!
It’s not getting warm/cool.
This has nothing to do with the unit size and sq. ft. numbers.
Also the air temp in and out is not going to tell you if the unit is working.
The wet bulb temp must be dealt with before the dry bulb temp comes down. It does not mean that the unit is not working right. It means that it is working on the latent heat, not the sensible heat that your are measuring.
Originally Posted By: rcooke This post was automatically imported from our archived forum.
dandersen wrote:
Exactly!
It's not getting warm/cool.
This has nothing to do with the unit size and sq. ft. numbers.
Also the air temp in and out is not going to tell you if the unit is working.
The wet bulb temp must be dealt with before the dry bulb temp comes down. It does not mean that the unit is not working right. It means that it is working on the latent heat, not the sensible heat that your are measuring.
I disagree if the air going in is 72 and the air coming out is 70.
Then I would feel it is not working .
If the air going in is 72 and the air coming out is 56 I would say it is working .
If I am not satisfied or feel strange I recommend further exvaluation by qualified person.
It is not my visual inspection job to decide how well it is working again SOP.
I look for the split its ok move on.
I like to do my inspections in about 3 hours .
Any longer and the client starts to loose the information I want them to have.
Originally Posted By: dedwards This post was automatically imported from our archived forum.
Latent heat = Humidity (that critical element no one thinks about when they say an AC unit isn’t working) That is why David referred to the Psychrometric chart earlier. It isn’t as easy as saying it is broken if you haven’t done a full “capacity check” on the system. That takes time and training and is well outside the scope of a home inspection and it should be. Trying to size a system based on two or three known figures is risky. I do a lot of things during the course a home inspection that never go onto the report. I do them for ME, so that I know how a system is performing. If it becomes important to share that information I do, if it isn’t critical to my report I do not. I gather data, use that data to draw some conclusions and then write the report to reflect that. I do not attempt to put all that data into a report. I often go into homes that have been either sitting vacant and the AC off or turned up to 80. It is going to take some length of time for that system to run and stabilize plus try to pull gallons of water out of the air, carpeting, walls, etc. Too many times folks want to take a couple of temp readings, get a temp split and come to the conclusion that the system is either good or bad. Check the condensate drain after its been running for a while. Is water being removed? is the drain clogged? How do you tell? We are in the business of observing. You can get a good temp split but find that the ambient air temp inside never goes below say 79 degrees. What is the humidity inside? Working for most realtors is “the fan motor is running and air is coming out of the ductwork” Working for a HI should be is it performing as it was designed to do. If you can see the evap coil, most of the time it is so nasty and clogged due to cheap filters that the air isn’t going through the coil but around it. If the circulated air can’t be dehumidified or cooled it doesn’t matter how big or small the unit is.
Originally Posted By: dandersen This post was automatically imported from our archived forum.
Yes Roy, you said that already.
The point is, the temperature split is no indication of work being performed.
There can be 36,000 BTU is per hour being removed from the air by a three-ton unit and the temperature split will not occur depending on the design of the equipment and the duct system. This is my only point. The temperature split means nothing! However, this seems to be an accepted practice to determine operation. We are assuming that the system was properly installed to deliver a 20? Delta. That is an assumption I do not want to make.
Saying you want to save time, quit taking the temperature readings and wasting your time reporting on something that has no bearing on the operation of the equipment!
I am not telling you that you have to do your inspections any certain way, I am just advising that the way some people are doing it is inaccurate and potentially could be a liability. That's that.
Originally Posted By: dandersen This post was automatically imported from our archived forum.
Let’s see if we can get a little emotion going here!
I’ll bring mold into it!
If I modify an HVAC unit to handle nothing but sensible heat (the heat you measure with a thermometer) I will produce an environment within a house that will promote extensive amounts of mold growth.
So, if you get a 25? split that's better than good, right? That does mean the unit is working, does it not? A twenty five degree split will likely cause the relative humidity within the house to exceed 20%. This condition perpetuates the growth of mold (any of you mold guys can correct me on this figure if this is inaccurate).
The point is, the temperature split may indicate the HVAC system is working however we have overlooked the fact that there is a potential condition (as there always is with other issues associated to mold) that will promote in indoor air quality problem.
In my younger years when I worked for a mechanical contractor, the service manager wanted every service technician to take and record the dry bulb temperatures of the supply and return upon completion of a service call. From my perspective, all this did was track mud across the customers carpet! I told the service manager when he could show me the relativity of the supply and return dry bulb temperatures I would record them. I never recorded them and he never provided an explanation.
Originally Posted By: Steven Brewster This post was automatically imported from our archived forum.
David,
Mold likes humidity > 60%. From my readings interiors should be maintained between 30-60%. Which in turn brings up the question of oversizing a cooling unit here in the south where its hot and humid. My understanding of the effects of an oversized unit is that the system does not operate long enough during cooling cycles to remove moisture from the air which results in an increase in the relative humidity and an environment conducive for mold growth. You stay cool but adverse conditions exist that can precipitate mold growth. If the unit is undersized, inadequate cooling and dehumidification will result. Same results.
Scenario,
Outside temp. 92 with RH 89%
I presume that the systems in the new home I am inspecting has been engineered correctly with proper supply and return pipes and sized according to the structure/orientation. If I perform a temperature differential and say it is 68 out and 75 in a 7degree diff.--what's your take on that, on the flipside, what if the diff is say 28 degrees?
I have been told by several HVAC pros that a good rule of thumb for evaluation of a cooling unit is to measure the temp. diff.. Normal 16-21 degrees. I also was taught this in HI school.
Not trying to be smart but just want to understand your view point on temp diffs.
