Return could work, provided the Btus are there AND the air leaving the water-to-air heat exchanger is always above the return air temperature AND below the AHU supply temperature.
If at any time the water is cool, it will cool the return air and make your heat pump or furnace run more. If at any time the water is hot, it will overheat the space while the heat pump sits idle.
Whereas if you put the heat at the heat pump, the builtin controls will take care of all that for you.
Apparently you've never seen an ECM blower on a modern furnace. The controller maintains a constant volume of air, ie X CFM. Put more resistance in the ducts and the motor used MORE energy to move the same amount of air. Moving the same amount of air with a restriction takes more energy because of the increase in pressure. Capiche?
Funny, aren't you the one always ranting about how the UK is way ahead in energy technology? No ECM blowers over there?
Are you trying to tell us you can get more heat out of the same radiator system with less air flowing through it instead of more? How do you maximize the temperature difference if not by using the maximum available air flow? Hmmm?? Air barely moving results in the air being hotter in the heat exchanger and less energy transfer. Air quickly moving results in the air being cooler and more energy transferred. Do they put part of the air through a furnace heat exchanger, or ALL of it? Do they put part of the air through an HVAC evaporator or all of it?
Turbulence does increase the transfer. You think you're gonna get more turbuluence with reduced airflow? Ever hear of a Reynolds number?
He isn't designing the heat exchanger. It's a simple radiator. You want to make it into PHD thesis?
Uhhuh. And one way to get it as high as possible is for the max airflow of the furnace to be going through the radiator. Anything less than that for the same design and he gets less heat transferred.
And heater cores are LOWER RESTRICTION than rads - at least if you get the right one. 2 big heater cores will work inplace of a radiator in many cases - lots of aircraft flying (home-built) with 2 heater cores or 2 air conditioning evaporators instead of a rad.
Well, your experience is different than mine. I've ALWAYS found that a blocked filter lowers the load on the fan.
This is a much more legitimate concern. The blowe motor will handle it - no problem - but the furnace may end up "high limited"
. Which is why I recommended an "air blend" system using a bypass and dampers.
to run constantly - low speed all the time, high when calling for heat.
Years ago, me and my late friend whom I did a lot of HVAC work with, installed a new system equipped with the new ECM fan motor. I hadn't read the literature on the system because I was only peripherally involved with the installation of the new package unit. The customer called complaining something was wrong with the new system because there was little air movement and the blower wasn't working very well. I checked and the fan didn't seem to be blowing very hard and it took it a while to start blowing. A call to the factory rep at the distributor gave me the answer. The darn thing was supposed to do that! The ECM fan motor is supposed to run slowly all the time to keep air circulating in the home. When the system calls for heat or cool, the fan speed ramps up slowly and winds down to a continuous low speed when the set temperature is reached. Darn newfangled energy saving doohickeys. o_O
TDD
Unfortunately it doesn't work that way. You want the water temp of the water leaving the radiator in the air stream to be as low as possible. That way it can pick up the most new heat on it's pass through the solar collector.
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I agree that unless it's a very large solar collector it isn't going to meet the demand. But for it to work at all he needs a radiator of sufficient size. If the water isn't giving up most of the heat collected into the air stream via the radiator, he just winds up moving hot water around and getting little heat out of it.
Apparently the America bashing genius here that rants about how advanced the UK is heating technology isn't familiar with ECM blowers which have been around for over a decade here.
You really think so?
As I recall, the issue was placing another heat exchanger which offers more resistance to air flow in a heating system. That is NOT zero air flow.
No energy efficient ECM blowers in the UK, eh?
Sure. That'll work fine, in the average residential computer controlled environment.
Gonna use PID control on those dampers, or something more sophisticated, one of the new fuzzy logic systems maybe?
Most of what you and the others propose adds complexity and advanced control requirements. I'm trying to suggest going upstream of existing controls to take advantage of what's already there?
Do a google under : Hot water heating coils for residential ductwork . Bear in mind that the added resistance of the heating coil in the supply duct may require a larger blower assembly to deliver enough airflow . Unless he has a huge daily storage of solar hot water...i wouldnt waste the time or the expense .
By the way, even here in North America not all ECM blowers are "variable speed" and controlled by mass air dlow or temperature differential (heat gain). My furnace has an ECM blower and the controller needs to be SET to a particular speed - so it runs like a programmed multispeed fan, not a variable speed. This requires, when setting up the system, that the tech reads the temp and pressure on both sides of the furnace, does some calculations, and sets the speed accordingly. This results in a different speed for AC and Heat.
Mabee we need to get a bit more "scientific" about it. You want to put as many BTUs of heat (using north american (canadian)n terminology) into the air from the water. A BTU is the amoiunt of heat energy involved in cahanging i pound of water 1 degree F. Water weighs 10 lbs per imperial gallon. (works better than 8.35 per US gallon for calculations).
Lets say we have a 40 gallon reservoir of water at 127 degrees F and we pump it through a heat exchanger at a rate that reduces the temperature of the water returning to the reservoir to 87 degrees F.. That means we got 1600 BTUs of heat out of the water, into the air.
To do that in 1 hour would require a water flow of 40 gallons per hour, or .66 gallons per minute.
If the return air is at 68 degrees F, and the furnace fan moves 2000 cfm, what will the air temperature coming out of the heat exchanger be?
1 BTU will raise the temperature of 1 cu ft of air 55 degrees F, or 55 cu ft of air 1 degree F.So to absorb 1600 BTUs of heat from the water into the air, we need (1600x55=) 88000 cu ft of air raised one degree F, so at 2000 cfm, or
120000cfh, we would on,y raise the temperature by 0.73 degrees F.If we drop the water temperatre leaving the exchanger at 77 degrees, the air would be raised by 0.91 degrees F.
SO the variables that ALL make a difference are the airflow, the waterflow, the temperature drop across the water side of the exchanger, and the desired outlet air temperature.
If you balance the water flow against the air flow you can make your delta T between the water and the air whatever you want it to be. - which is where a BYPASS system as I explained, allows you to get the results you want by varying the amount of air passing the exchanger and the water flow through the exchanger once you get the basic size of the exchanger close. A bus heater core could be 25000 BTUs.
Many car heaters are about 12000 BTU at 140CFM - but we do not know what the design temperature drop is across the exchanger. The delta T would vary depending on the inside temperature.
A 10X10X2 heater core, at 170CFM, is rated at 20,000BTU. Again, we do not know the design temperature drop, or at what air temperature (delta T)
Depending where he lives, the solar assist could extend the "no heat required" season and reduce the heat required through the winter.
Not likely to help much in places like Halifax or Seatle, but could conseiveably produce pretty good winter results in Regina or Winipeg, or other places with long hours of winter sunlight.
Actually there is a lot of solar energy available in the winter. My Sun room walls are made almost totally of sliding glass doors. Glass is open during warm weather closed during the cold. Jimmie
Putting a standard automotive radiator of say, 6 square feet area, into a 1.5 square foot return duct would have only 1/4 the apparent airflow restriction because of the plenum required to fit it in - and it would reduce the velocity through the rad by a factor of 4.
Using a thicker, smaller, higher resistance heater core in parallel, with it's own blower assist takes the airflow resistance question completely out of the equation.
Harry,
It's funny that you can be snotty about this and be completely wrong.
Yes you want the two media in intimate contact but you then wrongly conclude that means the velocity should be low.... So you think lower CFM through a radiator pulls more heat out of it, obviously wrong. When does your car overheat?
The flaw in your thinking is that you don't consider that there is always more air to replace the air that is moving away and the colder the air in the radiator, the more heat it pulls out.
You are the one confusing heat and temperature.
The usual case for a heat exchanger is is higher CFM = more BTU exchanged and lower temperature.
Take care
Mark
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I have a variable ECM blower too. And guess what? It too has switches that set the target air speed. That says nothing about what happens to power as you restrict the airflow.
And there you go again. Just like Mark said, you are confusing temperature and heat. Let's recap. The OP is proposing to take a heat exchanger similar to a car radiator and place it in the return duct of his furnace. He will pass hot water from solar panels through the radiator.
Is the goal here to get the air as hot as possible? Clearly not. The objective is to get the most heat out of the water and into the air. By moving more cold air through that radiator the temperature differential across the heat exchanger is higher and MORE heat is transferred. Yes, the air coming out or the radiator will not be warmed as much as if there were less air flowing, but there is enough additional air so that it more than make up for the lower temperature and MORE heat is transferred. That you can't grasp this most basic concept just totally discredits you.
Again, you don't understand the difference between heat and temperature. Which takes more energy in the form of heat:
A - Raising the temp of 5000 cubic feet of air 1F
B- Raising the temp of 10 cubic feet of air 10F?
Which air is hotter?
Which of course matters not a wit.
I think it's obvious to all who's confused here. Here's another simple experiment you can try. You have a piece of iron that been heated with a torch. You want to cool it off. Do you get it cooled off faster by applying a very small stream of water to it, in which case the water running off is very hot, or by applying a large stream of water and flooding it as much as possible, in which case the water is just luke warm?
I see. So for example we can take all the coils in the AC systems out there and make them 75% smaller and at the same time reduce the blower speed. That should work great. And you throw rocks at Americans for being stupid?
The last sentence is pretty much the only thing you got right here. And it's also why you want the maximum airflow through that radiator. If you slow the airflow to the limit, ie no air is flowing, the air surrounding the radiator equals the water temp in the radiator. You now have max temp air, but 0 air moving and 0 heat being transferred. Now slowly increase the airflow and while the temp of the air moving through starts to decrease, heat transfer starts because you have cold air coming in contact with the radiator. As you continue to increase the airflow you get more heat out, more air out, but the air goes down in temp. The process continues with a decaying exponential in terms of the additional heat that;s available.
Why the need for variable dampers? Do you disagree that you get the most heat out of the heat exchanger by moving all the furnace air through it?
The only reason I see for using less than all the airflow is because without severly restricting the airflow, the air coming out is going to be almost the same temp as the return air coming in. So, to avoid draft/comfort issues, you might want it to come out warmer. BUT, and this is a big but, if that is the goal, then you'd have to change the blower because even at the lowest blower speed I doubt any system he's gonna put together is going to raise the air temp more than a deg or so. And I don't think you're proposing that he just damper off a huge portion of the existing airflow, leave the blower as is, are you?
That's why I dismissed using just solar from the beginning and I assumed he was talking about using it to supplement the furnace heating only when the furnace was running. If he wants to run the system all day long, using solar, that gets into a whole additional list of potential drawbacks, many of which I believe would make it impractical.
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