electric outlet for window AC question

[Joseph Meehan]

You can certainly do that, but I am not sure it is wise. Normally what you suggest will cost more to install and operate (operation cost may be reduced if you carefully zone them). It also can be more noisy.

You may run into some issues with local zoning in some areas.

A well designed central system have few problems, likely fewer than a number of single room systems. If you read here, you will see the results of problem systems. We don't get questions about well designed and installed systems. We get very few question about the room units (as you are describing) but that is because they are not common.

There may well be some situations where the small units may be the best choice, but I would have to believe that would be a small percentage of locations.

[phil-news-nospam]

|> You are being silly. If you have a fixed capacity window unit you can |> size |> it to be "too big" all the time (and play games with fan speed to adjust |> capacity) | | No, as yo already pointed out, it is a fixed capacity, not variable. You | are deluding yourself to thing you can change the capacity. Sort of like | people that keep pushing up the thermostate to get the house heated faster | when the heat is already running. The copressor is either on, or off. | There is no varialble capacity.

It is possible, though not always efficient, to vary the capacity of a heater. Two elements connected in parallel could be reconnected in series to have a lower temperature when the difference between desired and actual temperature of the air is small (1 degree).

Is there anything at all that can make the A/C coils be less cool than they would be at "full tilt"? One reason I want to know is that I see many cases of central A/C units freeze up; the coils are obviously very cold and apparently there is too much water and/or not enough air.

|> or get a unit that's too small some of the time and be downright |> miserable. I will gladly sacrifice a few "perfect" days to avoid a |> single |> "miserable" day | | I said properly sized. If it is too small, it is not properly sized is it? | You will still get reduced humidity in any case.

Having more capacity could, perhaps, reach desired temperature faster. But would an over capacity A/C tend to freeze up more?

[phil-news-nospam]

In alt.engineering.electrical Joseph Meehan wrote: | [e-mail] wrote: |> In alt.engineering.electrical John Gilmer |> wrote: |>

|>| To each his own, but if you have the electrric power and the spare |>| cash when you buy a window unit, go for a larger size. |>| "Efficiency" just ain't all that important if all you are cooling is |>| ONE room. >

|>

|> However, given the constant problems I see with central air |> conditioning, I will be going with individual room air conditioning |> instead in my new house (designing now, build in a few years). There |> will be air ducts for air flow, but they won't be the distribution of |> cooling or heat, they will be for the distribution of fresh air and |> filtering. However, I will not be using window air conditioners. My |> grandfather added a room onto his house back in the 1960's and heated |> and cooled it with a vertical wall unit that was about 6 feet high. |> The exterior portion for expelling heat was right behind it (e.g. it |> probably had to be installed only on an exterior wall). Today's |> market has quite a number of "mini-split" units, which are basically |> a split A/C system of varying sizes down to small rooms. Many |> including heating as an option. And there are still many wall-sleave |> units on the market with heat. | | You can certainly do that, but I am not sure it is wise. Normally what | you suggest will cost more to install and operate (operation cost may be | reduced if you carefully zone them). It also can be more noisy. | | You may run into some issues with local zoning in some areas.

What do you mean? Can you be more specific?

| A well designed central system have few problems, likely fewer than a | number of single room systems. If you read here, you will see the results | of problem systems. We don't get questions about well designed and | installed systems. We get very few question about the room units (as you | are describing) but that is because they are not common.

I would guess that the vast majority of installed central A/C are not properly designed and installed.

How would you design a central A/C/heat system to control each room to it's individual desired temp/RH? Consider each bedroom has individual control, and the kitchen is an excess heat source even if it has the same desired temperature setting as other rooms.

I suspect that zoning by means of controlled air flow (e.g. shut off the air to rooms that are already sufficiently cool) is a culprit in A/C coils freezing up because the zone system sometimes runs too little air past.

But I would never have a zoning system that is based on cutting air flow as the control means. So what else is there?

| There may well be some situations where the small units may be the best | choice, but I would have to believe that would be a small percentage of | locations.

I think my house will be a small enough percentage.

[operator jay]

Defrost heaters.

One reason I want to know is that I see

[John Gilmer]

The "usual suspect" is insufficient air flow to keep the coil temperature about freezing. When you can independently vary the speed of the "inside" and "outside" fans, then it's possible to control the humidity by running the fan at a lower speed until the humidity is reduce to a preset level. Any particular coil is limited in the amount of moisture it can remove (rather than the moisture condensing an then blowing back into the living space.)

Under certain circumstances, low refrigerant can cause the problem. When happens is that the pressure in the "low" side (the inside coil" get's so low that the little refrigerant that's left creates a small area in the coil that"s well below the freezing point of water. That part of the coil "freezes up" and insulates the coil there. The point where the refrigerant vaporizes in the coil continues along the junction between the ice and just liquid water. Usually, however, low refrigerant will cause "no cooling." This doens't happen often because the "cooling" capacity of a unit that's very low on refrigerant isn't much and ice does conduct a little heat.

Nope! It's thermostat will just shut off the compressor when the return air temperature falls below the set point. As others have notes, it will tend to short cycle.

>

[ehsjr]

Well, if I understand the limited amount of reading I have done on some of the newer split AC units (like the Mitsubishi "Mr. Slim"), they use a VFD to run the compressor. I *assume* that the coils would be less cool/more cool as the VFD varies. I hope someone who *knows* about these units will chime in with the facts.

Ed

[daestrom]

There are also solid-state proportional heater controls, that use phase-controlled thyristors to vary the heat output over a continuous range. Usually not cost effective for simple space heating, but often used where more precise temperature control is needed.

There is a lot I could go into in this area. The base temperature of the evaporator is determined by the saturation pressure of the refrigerant inside it. Advanced systems use a variety of suction pressure controls to maintain this saturation condition just above 32F. In a high-end system, suction throttle valves (a form of 'back-pressure' regulator) will maintain the saturation temperature in the evaporator at between 35 and 45 F. This is cold enough to condense a lot of the water vapor, but will still leave some in the air. When air is at 100% RH at 40F, and then warmed back to 70 F, it will be about 50% RH.

Another pressure control method is various forms of 'staging' in the compressor, or the use of variable speed compressors. By controlling the amount of pumping the compressor does, the evaporator pressure is kept above freezing. Interestingly, in modern auto A/C, there is a suction pressure switch that cycles the compressor on/off by engaging/disengaging the clutch used to drive it. Of course, cycling a motor driven compressor on/off is not viable as it would have to do this several times a minute, and the repeated high starting currents will burn out the motor.

Contrary to urban myth, the TXV (ThermoeXpansion Valve) is *NOT* used to prevent freezeups. It works by sensing both the pressure and temperature at the evaporator outlet, and is used to control the amount of superheat in the refrigerent as it leaves the coil. A small amount of superheat is desired to prevent the compressor from ingesting liquid (that would damage the compressor). But amateurs will often 'fiddle' with the TXV when either the unit freezes up or doesn't cool properly.

One cause of freeze-ups is too little charge in simple capillary units (no TXV). The lower pressure in the system causes the evaporator to run at lower temperatures.

And finally, despite what I wrote above, there *are* some cheap units that are designed to have the evaporator running below 32F. These cheap units rely on a high air flow to keep the evaporator warm enough to avoid a build up of frost. They don't work well in humid climates. The best you can do with these is to make sure the air flow is unobstruted by dirty filters.

daestrom

[Joseph Meehan]

We might be talking two different zones here. I am talking about local governmental regulations. In some areas they regulate window/thrugh the wall A/C units. Sometimes they only allow them on certain sides of the home or they require certain distance to a property line etc.

Sad to say I agree.

A proper design and if needed zoning can do it. Each situation is different and calls for different solutions. I am only suggesting that anyone would be wise to consult a qualified professional to offer their opinion on the best solution for any given case. Restricting their solutions is counter productive for the home owner. On the other hand if you approach it by asking them to consider independent room equipment I believe that may open up a solution that they might not have considered as I doubt if most people would want it.

In a poorly designed system, yes. In a properly designed system no.

So even if it were properly designed you would reject it?

[phil-news-nospam]

In alt.engineering.electrical John Gilmer wrote: | |> Is there anything at all that can make the A/C coils be less cool than |> they would be at "full tilt"? One reason I want to know is that I see |> many cases of central A/C units freeze up; the coils are obviously very |> cold and apparently there is too much water and/or not enough air. | | The "usual suspect" is insufficient air flow to keep the coil temperature | about freezing. When you can independently vary the speed of the "inside" | and "outside" fans, then it's possible to control the humidity by running | the fan at a lower speed until the humidity is reduce to a preset level. | Any particular coil is limited in the amount of moisture it can remove | (rather than the moisture condensing an then blowing back into the living | space.)

If you have a zone control system that shuts off selected air ducts, then you can easily end up with insufficient air flow.

| Under certain circumstances, low refrigerant can cause the problem. When | happens is that the pressure in the "low" side (the inside coil" get's so | low that the little refrigerant that's left creates a small area in the coil | that"s well below the freezing point of water. That part of the coil | "freezes up" and insulates the coil there. The point where the refrigerant | vaporizes in the coil continues along the junction between the ice and just | liquid water. Usually, however, low refrigerant will cause "no cooling." | This doens't happen often because the "cooling" capacity of a unit that's | very low on refrigerant isn't much and ice does conduct a little heat.

I've seen this happen before, too.

[John Gilmer]

"Easily?" Nah!

It can happen but simple stuff like a thermostat in the main distribution duct (before the dampners) that would either shut off the compressor or over ride a "decision" to close a dampner can "solve" that problem.

If ALL zones are satisfied, the compressor should shut down in any case.

>

[phil-news-nospam]

In alt.engineering.electrical Joseph Meehan wrote: | [e-mail] wrote: |> In alt.engineering.electrical Joseph Meehan |> wrote: |>| You may run into some issues with local zoning in some areas. |>

|> What do you mean? Can you be more specific? | | We might be talking two different zones here. I am talking about local | governmental regulations. In some areas they regulate window/thrugh the | wall A/C units. Sometimes they only allow them on certain sides of the home | or they require certain distance to a property line etc.

I'm sure there will be, in some areas, sound abatement rules would apply to any external device, including the unit for a central A/C, or a home generator.

Getting them on the side of the house facing away from the sun as much as possible would be a good idea, but I don't see a particular need for it to be a regulatory issue, other than in California where there is a severe shortage of power generation capacity due to improper planning at the state government level.

|>| A well designed central system have few problems, likely fewer |>| than a number of single room systems. If you read here, you will |>| see the results of problem systems. We don't get questions about |>| well designed and installed systems. We get very few question about |>| the room units (as you are describing) but that is because they are |>| not common. |>

|> I would guess that the vast majority of installed central A/C are not |> properly designed and installed. | | Sad to say I agree.

And how much would it cost to get one to be well designed for 12 zones? This would have to be a design that sustains air flow in all zones.

|> I suspect that zoning by means of controlled air flow (e.g. shut off |> the air to rooms that are already sufficiently cool) is a culprit in |> A/C coils freezing up because the zone system sometimes runs too |> little |> air past. | | In a poorly designed system, yes. In a properly designed system no.

The only zoning systems I have seen do it by shutting off air flow to the zones not to be cooled. And given that I will need at least 8 zones, possibly as many as 12, I suspect this will be a problem.

OTOH, zoning by means of several smaller "central" style A/C systems might still be feasible.

But one issue is that I need to have the air flow in each room whether or not that room needs a temperature change. If I use mini-splits in all the rooms, I'd still have the central air flow ducting in place with a passive blower. But I'd prefer connecting them together somehow if I can find a way, where the room unit has air fed from that central air flow system.

|> But I would never have a zoning system that is based on cutting air |> flow |> as the control means. So what else is there? | | So even if it were properly designed you would reject it?

Not necessarily. I'm just very skeptical that it can really happen. I'm not planning my house design around the prospect of a proper design being feasible.

[phil-news-nospam]

|> If you have a zone control system that shuts off selected air ducts, then |> you can easily end up with insufficient air flow. | | "Easily?" Nah! | | It can happen but simple stuff like a thermostat in the main distribution | duct (before the dampners) that would either shut off the compressor or over | ride a "decision" to close a dampner can "solve" that problem. | | If ALL zones are satisfied, the compressor should shut down in any case.

But what if one zone needs cooling?

[Joseph Meehan]

That would depend on the application.

Actually I have seen other systems. However what is possible or more important practical depends on the particular application.

Yes, and maybe best, but always feasible. .

I am curious about why you want air flow to each room all the time. I guess there are good reasons.

Sounds like a good plan. I suggest consulting with the professionals before you finalize things, but there is plenty of time until then. Forming a good idea of what results you want and what problems you have observed elsewhere is a very good start. If it is your home, you should consider all your concerns.

[phil-news-nospam]

|>|>| A well designed central system have few problems, likely fewer |>|>| than a number of single room systems. If you read here, you will |>|>| see the results of problem systems. We don't get questions about |>|>| well designed and installed systems. We get very few question |>|>| about the room units (as you are describing) but that is because |>|>| they are not common. |>|>

|>|> I would guess that the vast majority of installed central A/C are |>|> not properly designed and installed. |>| |>| Sad to say I agree. |>

|> And how much would it cost to get one to be well designed for 12 |> zones? |> This would have to be a design that sustains air flow in all zones. | | That would depend on the application.

A house where:

1. Each major room has its own temperature setting, and if possible, a relative humidity or dew point setting. Major rooms are each bedroom, bathroom, kitchen, dining, living, great room, etc.

1. Each room has selectable air flow that can be sustained even if that room is at desired temperature (e.g. closing a duct damper won't work).

|>|> I suspect that zoning by means of controlled air flow (e.g. shut off |>|> the air to rooms that are already sufficiently cool) is a culprit in |>|> A/C coils freezing up because the zone system sometimes runs too |>|> little |>|> air past. |>| |>| In a poorly designed system, yes. In a properly designed system |>| no. |>

|> The only zoning systems I have seen do it by shutting off air flow to |> the zones not to be cooled. And given that I will need at least 8 |> zones, possibly as many as 12, I suspect this will be a problem. | | Actually I have seen other systems. However what is possible or more | important practical depends on the particular application.

A house.

|> OTOH, zoning by means of several smaller "central" style A/C systems |> might still be feasible | | Yes, and maybe best, but always feasible.

So far I have not found such units small enough.

|> But one issue is that I need to have the air flow in each room whether |> or not that room needs a temperature change. If I use mini-splits in |> all the rooms, I'd still have the central air flow ducting in place |> with a passive blower. But I'd prefer connecting them together |> somehow |> if I can find a way, where the room unit has air fed from that central |> air flow system. |>

| | I am curious about why you want air flow to each room all the time. I | guess there are good reasons.

I don't necessarily want it all the time. I just want to be able to have it without being tied into heating/cooling. A typical one zone central system does have a fan auto/on switch. I often turn the fan on. But if it is a multiple zone system controlled by dampers shutting ducts off when that zone or room reaches desired temperature, then it cuts off the air there as well. I still want to get the air, but without the heat/cool. That would add more complexity to the system since now each zone will need to do a bypass, meaning a 2-way damper, separate blowers, times the number of zones (8-12). It would be a nightmare as a central system.

|>|> But I would never have a zoning system that is based on cutting air |>|> flow |>|> as the control means. So what else is there? |>| |>| So even if it were properly designed you would reject it? |>

|> Not necessarily. I'm just very skeptical that it can really happen. |> I'm not planning my house design around the prospect of a proper |> design being feasible. | | Sounds like a good plan. I suggest consulting with the professionals | before you finalize things, but there is plenty of time until then. Forming | a good idea of what results you want and what problems you have observed | elsewhere is a very good start. If it is your home, you should consider all | your concerns.

The air flow itself does not literally need to be centralized, either. Since the construction will be post-and-beam style, air ducts will need separate consideration. And so I do have to plan where those go as part of the whole layout. Adequate air flow would be less ducting than any central heating/cooling system would need, since it won't really need to go to the central location. Outside mix can be acquired in most rooms. Recirculation within the room would work in many cases. And maybe all I need are some good ceiling fans instead. Much is undecided.