prog. therm. and heat pump questions

Wow, a troll has tried to set his hook in me. Same MO what a joke

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Reply to
Abby Normal
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All he does is troll and flush toilets.

Reply to
William Souden

Couldnt bullshit its way out of a wet paper bag.

Reply to
Rod Speed

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Couldnt bullshit its way out of a wet paper bag.

Reply to
Rod Speed

I hate to say it, but I have to concur with Rod on this one. His style of argumentation leaves a lot to be desired, but in this case, I agree with his conclusion.

If you are coming off setback and using a secondary heat pump to do only that, it doesn't need duct work. If it just heats the air in one room, the primary system will be running continuously (or nearly) and will be circulating the air around the house anyway. Even if the secondary heat pump heats the air in one room up to 95F in the process, eventually that'll even out as the other system recirculates everything. And heating the air up to such a high temperature is unlikely anyway, since by definition when you are recovering from setback, the temperature is colder than you'd really like it.

However, I do have to admit there could be problems if the secondary heat pump (that's used only during recovery) is putting heat mainly into the room with the thermostat. If the temperature in that room rises faster than in the rest of the house, it could cause the system to think recovery has finished when it hasn't yet gotten to that point. And if the secondary heat pump is too far away from the thermostat, it could cause the system to overshoot the target and heat up the house too much. But, those are control issues rather than duct issues, and they seem solvable.

- Logan

Reply to
Logan Shaw

It isn't going to work. If you raise the temp in one room to a point higher than the desired temp, then you're going to have heat losses through the exterior walls that wouldn't have occurred with a centrally ducted system, thus defeating the purpose, at least to some extent. If you feed it into an interior room then that room will simply get too hot, no question about it. I hate to say it but it's not a very intelligent alternative.

hvacrmedic

Reply to
RP

Logan Shaw wrote

I only use that approach when some prat like this one attempts to bullshit its way out of its predicament with puerile shit like that one attempted.

And that isnt going to happen if it heats up the main area.

It could happen if you were silly enough to put it in a small room.

Nope, not if it heats the main living area.

Not if it heats the main living area and the primary system spreads that heat out to the rest of the rooms. By definition it should have been designed to do that because it needs to do that when not coming back from the setback.

Corse they are solvable and you dont have to use just one thermostat either. One obvious approach is to have a different one for the primary and secondary systems.

Reply to
Rod Speed

Corse it'll work.

That isnt going to happen when the secondary system is JUST used to come off the setback in a reasonable time.

Nope.

Wrong again.

Corse it will work fine. No reason why the secondary system cant have a slightly lower than normal temperature set so it recovers from the setback in a decent time and then stops and leaves the primary system to get the temp back up to normal. Thats going to happen fine since the temp difference is well within its capabilitys.

Reply to
Rod Speed

Supposing that I grant your every point, what would be the point of such a system? Wouldn't it be much easier to simply install a larger staged system?

hvacrmedic

Reply to
RP

It makes setback viable with a heat pump.

Nope, and likely quite a bit cheaper not to too.

If there was not going to be anyone in the house during the day, I'd personally run the setback right thru to the afternoon and have the system come back off setpoint at a time that would ensure that it was back to normal temp by the time everyone came home again and thru the evening. Go back onto setback after everyone has gone to bed.

Reply to
Rod Speed

Why do we need a 2-speed compressor? Abby suggests a high capacity heat pump for winter setbacks will do poorly for summer AC because it will inefficiently short cycle and won't dehumidify well. It seems to me that it can dehumidify well with less indoor airflow and a freezestat that turns off the compressor while the blower keeps running. What are the numbers for this "short cycling?"

When do we have to worry about premature wearout or inefficiency resulting from too many starts? If a 3 ton heat pump runs 100% of the time at the winter design temp, can it do 1 ton of AC in summertime with less indoor airflow? Maybe the answer to this question depends on thermal mass. If the AC runs for

10 minutes out of every half-hour that might be fine. If it has to run for 1 minute every 3 minutes that might be no good. Is there some way to add thermal mass to the indoor coil? Do we need to worry about short cycling on mild winter days?

Nick

Reply to
nicksanspam

Nick, The next time you see an AC or heat pump, Use one of your data loggers to measure the temperature drop across the indoor coil. The indoor coil will take 10 to 15 minutes to reach max coldness. The colder the ID coil gets, the farther below dew point it will be and the more moisture it will take out. With a less cold coil, while it will cool the house, it will not dehumidify well. That is why you want to size it properly. The ID coil does not get cold instantly. A properly sized unit will run longer and dehumidify better without losing effeicieny. By the way, when they test efficiency, they run the systems for an hour before they do the testing. Systems do not read max efficiency till they have run an hour. Gas furnaces are the same way.

If you just lower the air flow, if you go below 350 CFM per ton, your system will loose capacity, while using about the same electricity. Again, use your data loggers to measure capacity and power consumption. So the efficiency goes to hell. If you recall, the original idea of setback was to save money on electricity. Now we are talking about extreeme efforts to make an oversized system work at all, running the electric bills out of sight!

Sounds like you are all going in the wrong dirrection.

Reply to
Stretch

In a humid environment, running the fan constantly is a bad idea. The reason being, as soon as the compressor shuts off, the indoor coil warms up. The mositure still on the coil will be re-evaporated resulting in elevated indoor RH. 3 steps forward and 2 steps back.

It is even worse with a draw through air handler installed in a horizontal position, the drain pans have a side connection. This means that there is approximately an 1/8 of an inch of standing water tht does not drain. So along with re-evaporating the moisture that did not drain off of the coil, the constant fan makes the air handler into and adibiatic saturator as it dries out the drain pan too.

Ballpark figure, constant fan will keep the indoor RH about 10% higher than fan in "auto" mode. Hard concept to believe in the north but perhaps search and see what Building Science or the Florida Solar Center has to say on this matter.

Well you are the electrical engineer by training so I would assume that you realize that motors are aged every time they are started.

In an environment where in the summer the outdoor dewpoint will not be that high then you could size a heat pump (most likely a ground source heat pump) to the heating load and be grossly oversized for cooling. But with a relatively low ambient dewpoint you would not end up with the high indoor RH.

Typically a heat pump is sized with cooling in mind to avoid the poor humidity control in summer. Therefore in the winter during design cold weather the compressor runs, with incremental auxiliary heat. Would suggest a long run time, compressor on steady, with heat strips cycling on and off. It could be to the point that it is so cold outside that the compressor is off and the home could be on full electric heat even.

Under less than extreme weather the compressor will still have a long run time.

When you want to analyze the system from a temperature and pressure standpoint you should wait 15 minutes for the system to stabalize. Work backwards from 15 minutes and figure out the time constant of the system.

Consider a furnace sized on the money for the heat load. Should be running close to steady when the overnight lows reach and even exceed the design levels. During weather that is cold but not at the design level, they still run long enough to hit steady state and work at their rated efficiency for a period of time, during mild weather they may not run long enough to hit steady state before satisfying the thermostat.

Now consider a furnace that is oversized, runs 20 minutes on the hour under a heating design ambient. Outdoor conditions warm up to a little above the design level and soon not even 15 minute run time. Bulk of the winter, at say half the design temperature differential, it never even warms up to steady state before the thermostat is satisfied. Wastes fuel, and heat exchanger is prematurely aged. HX ages everytime it is heated up and cooled down.

Reply to
Abby Normal

Stretch states:

Got numbers? "Max coldness" may not be important, if it dehumidfies at 50 F but takes 10 minutes to drop from 44 to 43.8 F.

Why not? If we reduce the airflow, the coil becomes colder. In the limit, we dehumidify with little cooling. With lots of airflow, we cool with no dehumidification. And let's not forget hygroscopic house furnishings.

Got numbers? Is this HVAC folklore, or something to do with poor controls in commercial systems? A 40 F coil might dehumidify equally well whether the compressor runs 10% or 50% of the time.

Got numbers? An hour is a long time... to go from 99% of the max efficiency to 99.5%? :-)

Got any numbers for us unbelievers, or simply your word as a God? :-)

Nick

Reply to
nicksanspam

If it rises above the dew point. Sounds like poor control.

Nick

Reply to
nicksanspam

Not following what you are saying Nick. If what rises above dewpoint?

Reply to
Abby Normal

Abby did say "running the fan constantly". There's a difference between running it somewhat more than the compressor and running it 100% of the time. So actually, I think the two of you are talking about different things.

- Logan

Reply to
Logan Shaw

Running the fan when the compressor is off is a bad idea in a humid climate in cooling mode.

Nick mentioned running the fan steady on low speed and cycling the compressor. Lower speed will result in more latent removal, but the coil will warm up quick when the compressor is off either by pressures equalizing in a fixed metering system or just from rapidly picking up heat from the room air.

Even having the blower cycle off 60 seconds after a compressor stops to ring the last sensible BTU out of the thermal mass of the coil is a bad idea with respect to humidity control.

Logan Shaw wrote:

Reply to
Abby Normal

Well Nick never believes anything I say but maybe if some of his cronies say the same thing he will believe them.

I went to a seminar put on by the Florida Solar Education Center, and as a guest speaker, they brought in an aquaintence of Nick. As soon as I heard the guy talk, I knew that he had to know Nick. I ask the guy and sure enough they even collaborated on magazine articles. The seminar was on PV panels, but if these guys say something concerning humidity it surely must be the gospel as far as Nick is concerned.

Maybe check out this link,

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, and look for the heading "Practises That WIll Minimize Mold Growth" with the subheading "Air Conditioner Operation".

Note you hear the same things from HVAC Criminals, except the criminals go on to say that the practise of running the fan constant while the compressor cycles on/off can even dry out the drain pan.

Logan Shaw wrote:

Reply to
Abby Normal

The coil. If it's well-controlled, always between freezing and the dew point, moisture will not re-evaporate.

Nick

Reply to
nicksanspam

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