Heating and cooling: exhaust air source heat pumps - a cautionary tale

121 to 150 of 170

- CommentAuthorGavin_A
- CommentTimeSep 24th 2012
I'm getting a bit tired now and making my brain hurt, so I'll fully acknowledge there may be some errors in these calcs. The principle is sound though.
- CommentAuthorSteamyTea
- CommentTimeSep 25th 2012
When I made my own MVHR I used the standard formula for heat exchanger efficiency that just uses temperature and it came out at about 50%. As I had put RH sensors in you could see that there was some condensing going on. I did not calculate the energy in that as it did not happen often. What did happen though, was that when the unit switched off the efficiency rocketed to over 1, this was probably the latent heat of evaporation causing it.

Nice to see science in action, what it is all about.
Though a precis of where we are would be helpful
- CommentAuthordavidfreeborough
- CommentTimeSep 25th 2012
Posted By: qeiplDoes the energy represented by 3067W of "Ventilation Heat Loss" leave the house via the extracted air?
Yes & No. It leaves via the extracted air, but only because the air replacing it is at a lower temperature. Its not the gross effect of extraction, its the net effect of extracting & replacing with air of a lower temperature.

Another example to test your theory; what happens when the output of an extractor fan is connected to the fresh air inlet to the house? The inlet air temperature is the same as the extracted air temperature, so no energy is lost; we just consume power running fans.

Now what happens if we take the exhaust of an exhaust heat pump & connect it to the fresh air inlet to the house? The inlet air temperature is the same as the exhaust air temperature & all of the energy recovered from the air flow by the exhaust heat pump directly results in a reduction in temperature in the house. The energy delivered to the thermal store or hot water tank will be equal to the sum of this energy & the power used to run the compressor.

Now what happens if we break that connection between the exhaust heat pump & the fresh air inlet on a day when the outside air temperature is equal to the exhaust heat pump exhaust temperature? The fresh air being drawn into the house is at the same temperature as above, the energy recovered will be the same as above & the energy delivered to the thermal store or hot water tank will be the same as above.

David
- CommentAuthordavidfreeborough
- CommentTimeSep 25th 2012 edited
Posted By: Gavin_Ain winter I'd expect something like a 2.5:1 differential due to the difference in the moisture content.
I've simplified the calculation to get to the bottom of the ventilation heat loss issue. However, for heat pumps like the Ecocent where the exhaust air temperature is similar to that of a 90% efficient MVHR unit, I don't think the difference will be that great. To put it another way, if the temperature at the back-end of an exhaust heat pump is similar to the temperature at the back-end of an MVHR heat exchanger then why would there be significantly more condensation with one than the other?

David
- CommentAuthorqeipl
- CommentTimeSep 25th 2012
Posted By: davidfreeborough
Posted By: qeiplDoes the energy represented by 3067W of "Ventilation Heat Loss" leave the house via the extracted air?
Yes & No. It leaves via the extracted air, but only because the air replacing it is at a lower temperature. Its not the gross effect of extraction, its the net effect of extracting & replacing with air of a lower temperature.

Energy, as far as I understand, can't be in two places at once. It's either inside the house or outside it.
In your calculation the energy represented by 3067W is lost to the outside air via the exhaust from the heat pump. Is that correct?

Another example to test your theory; what happens when the output of an extractor fan is connected to the fresh air inlet to the house? The inlet air temperature is the same as the extracted air temperature, so no energy is lost; we just consume power running fans.

What happens when the energy in the inside air is passed through a heat pump, HW cylinder, UFH, and back to the air? No energy is lost. This is how the EASHP system is warming the incoming air.

Now what happens if we take the exhaust of an exhaust heat pump & connect it to the fresh air inlet to the house? The inlet air temperature is the same as the exhaust air temperature & all of the energy recovered from the air flow by the exhaust heat pump directly results in a reduction in temperature in the house.

No. It results in a reduction in temperature of the air in the house and an increase in temperature of the water in the house. Net temperature difference is zero. No energy is lost from the house.

Now what happens if we break that connection between the exhaust heat pump & the fresh air inlet on a day when the outside air temperature is equal to the exhaust heat pump exhaust temperature? The fresh air being drawn into the house is at the same temperature as above, the energy recovered will be the same as above & the energy delivered to the thermal store or hot water tank will be the same as above.

No. In the previous example we're circulating the same energy around the air-pump-UFH-air, and air-pump-air circuits.
Now we're introducing additional energy from the outside air. This is the same energy that's available to an external ASHP. Some of this energy is added to the water by the heat pump.
- CommentAuthorqeipl
- CommentTimeSep 25th 2012 edited
Gavin,

Your input is very helpful.

I've revised my calculations, using enthalpy of air at 50%RH inside and outside the house.

I've chosen values of 6C and 21%RH for the heat pump exhaust because they sound plausible and balance the net energy gain with the difference in energy gain to water between the two heat pumps.

I've used your 54% energy efficiency factor for the MVHR unit.

Is this version more realistic or are there too many ifs and buts involved?

[Edit: This version isn't at all realistic. Row24B should be the same value as row16B. Revised version coming soon]

[Edit: table removed because it had multiple errors]

[Edit: see posts below for revised tables]
- CommentAuthordavidfreeborough
- CommentTimeSep 25th 2012 edited
Posted By: qeiplIn your calculation the energy represented by 3067W is lost to the outside air via the exhaust from the heat pump. Is that correct?
No.

3067W appears twice in my calculation for the exhaust heat pump. Once on row 12 where it represents the ventilation heat loss & again on row 16 where it represents the energy recovered from the extracted air by the exhaust heat pump.

The 3067W on row 12 is lost when air at 21°C is extracted from the house & replaced with air at 5°C. What happens to the air extracted isn't relevant to the ventilation heat loss. It will happen whether you use an exhaust heat pump, mechanical extract ventilation or passive stack ventilation.

The 3067W on row 16 is the energy recovered from the extracted air. All of this energy is required to heat the incoming air to room temperature. So the net heat gain for the house, including UFH, DHW cylinder, etc, will be just the compressor input power.

David
- CommentAuthorqeipl
- CommentTimeSep 25th 2012 edited
David,

That's perfectly clear now. Thank you.

You are using "ventilation heat loss" for the energy that is removed from the air in the room at the point that it enters the heat pump intake. This is not the same as the energy lost from the house, which is measured at the heat pump exhaust.
I am interested in the latter because it gives a valid comparison between the two configurations that are under discussion.

You are assuming that all of the energy that's captured from the air by the heat pump is required to warm the incoming air.
This means that the energy we use to run the heat pump does nothing useful - all it does is move energy around the inside of the house.

I'm assuming that the heat pump recycles the energy that's required to heat the incoming air, as well as capturing some of the additional energy that's brought into the house with the fresh air.

This table shows a comparison between two Ecocent systems.
All of the values come from the Ecocent specs. and the online humidity calculator, except for the MVHR efficiency, which comes from Gavin's earlier post.

With the heat pumps running for an hour and the MVHR processing 450m^3 (at flow rate 214m^3/hr) both systems show a net energy loss from the house, but the MVHR configuration performs better to the tune of 0.31kWh.

I will post a version of the table that shows what happens when the heat pumps run for a longer period - long enough to deliver a net energy gain to the house.
- CommentAuthorqeipl
- CommentTimeSep 25th 2012 edited
Here's a version of the table that shows the exhaust heat pump configuration at a steady state - net energy gain to house = zero. The MVHR version is showing a net energy loss of 0.73kWh.
- CommentAuthorqeipl
- CommentTimeSep 25th 2012 edited
And here's a version that shows the MVHR configuration at a steady state - net energy gain to house = zero.
The external heat pump has to run for 1.541 hours to replace the energy lost by the MVHR over a 3.24 hour period.

For the same pump run time and the same volume of ventilation air processed, the exhaust heat pump configuration shows a net energy gain of 2.19kWh after 1.541 hours.
- CommentAuthordjh
- CommentTimeSep 25th 2012
Gavin, you're quite right to be concerned about the humidity and latent heat. But just as you have to compensate for lost heat in the exhaust air by accounting for energy used to heat the incoming air, you also have to compensate for latent heat lost in the exhaust air by accounting for energy used to vaporise more water inside the house to maintain the RH. That is why overventilation is a problem, because it drives the RH down to the point where people get itches, dry eyes etc.

Malcolm, the RH in the exhaust air won't be less than the RH in the internal air. It will almost certainly be approx 100%. As the temperature of the outgoing air is reduced by the heat pump, it causes the RH to increase. As it approaches 100% there starts to be condensation and recovery of the latent heat.

There are also ventilation systems that recover the moisture. I believe the Americans call them Energy Recovery Ventilation. They're not usual in domestic systems though.
- CommentAuthorGavin_A
- CommentTimeSep 25th 2012
djh - if the ventilation rate is increased because of the EAHP to the point where RH falls below an acceptable point and extra vapour is deliberately being introduced, then yes I'd agree.

otherwise I'd not agree at all, as the water vapour isn't being added to the air for purpose of driving the EAHP system, it's being added to the air regardless of this, and you can either choose whether to throw this energy away, or utilise it via an EAHP unit.

ie, people will still breath at the same rate, shower at the same rate, cook at the same rate, dry clothes at the same rate etc. regardless of whether they have an EAHP or a MHRV or just open the window / have a leaky house. The choice is whether to recover that energy or not as it leaves the house.

As I think I stated earlier in the thread, I'd only advocate the latter system as being potentially sensible.
- CommentAuthorqeipl
- CommentTimeSep 25th 2012
Posted By: djh
Malcolm, the RH in the exhaust air won't be less than the RH in the internal air. It will almost certainly be approx 100%. As the temperature of the outgoing air is reduced by the heat pump, it causes the RH to increase. As it approaches 100% there starts to be condensation and recovery of the latent heat.

Yes. Thanks for the clarification.
That version of my calcs had other errors as well (in fact I'll remove it so that people don't waste time on it).
- CommentAuthordavidfreeborough
- CommentTimeSep 25th 2012 edited
Posted By: qeiplYou are using "ventilation heat loss" for the energy that is removed from the air in the room at the point that it enters the heat pump intake.
This is not what I said. I'm using it in the conventional sense.
Posted By: qeiplThis is not the same as the energy lost from the house, which is measured at the heat pump exhaust.
I am interested in the latter because it gives a valid comparison between the two configurations that are under discussion.
You cannot ignore ventilation heat loss when comparing ventilation systems. They can only be compared by looking at the energy balance for the house as a whole.
Posted By: qeiplYou are assuming that all of the energy that's captured from the air by the heat pump is required to warm the incoming air.
This means that the energy we use to run the heat pump does nothing useful - all it does is move energy around the inside of the house.
Its not an assumption its a result of considering the overall picture.
Posted By: qeiplThis table shows a comparison between two Ecocent systems.
All of the values come from the Ecocent specs. and the online humidity calculator, except for the MVHR efficiency, which comes from Gavin's earlier post.
The calculations seem to only consider the total energy in the air extracted from the house (sensible & latent) relative to the outside air. For the energy recovered in the MVHR case you seem to have taken the 54% figure from Gavin_A without reference to the performance of any real MVHR unit.

Its not clear how you have calculated the energy recovered in the exhaust heat pump case. What happens on rows 21 & 22? If the Ecocent's exhaust temperature is the same as the MVHR units then I don't understand how it can extract more energy than the MVHR unit (sensible or latent).

It doesn't make sense to increase the air flow of an MVHR unit to increase the amount of energy recovered. Increasing the air flow increases the ventilation heat loss & thereby increases the additional heat which needs to be provided by the external ASHP or other heating system. The air flow should be set to the minimum required for ventilation, i.e. about 0.4 air changes per hour. The fact that your calculations show an improvement in the MVHR case demonstrates that it is not a valid comparison or a true reflection of the energy balance in the house.

David
- CommentAuthorqeipl
- CommentTimeSep 25th 2012
Posted By: davidfreeboroughThe calculations seem to only consider the total energy in the air extracted from the house (sensible & latent) relative to the outside air.

Correct. I'm interested in the net energy gain of the house as a whole (air + water).

For the energy recovered in the MVHR case you seem to have taken the 54% figure from Gavin_A without reference to the performance of any real MVHR unit.

Gavin's argument regarding the energy efficiency (rather than temperature efficiency) of MVHR was persuasive, so I've used his 54% efficiency factor. If you can come up with a more persuasive argument that results in a different value then I'll happily change it.

Its not clear how you have calculated the energy recovered in the exhaust heat pump case.

All of the heat pump energy recovery is air to water.
Row16 is the net energy gain to water (total energy to water minus heat pump electricity consumption).
The row16 value is used in row24 to determine the energy lost via the heat pump exhaust (row25).
The term "ventilation heat recovery" is misleading. It should be "ventilation heat loss reduction". There is always a loss.

What happens on rows 21 & 22?

Oops! I edited the table and forgot to correct the row numbers. Should read row19 x row20. Apologies.

If the Ecocent's exhaust temperature is the same as the MVHR units then I don't understand how it can extract more energy than the MVHR unit (sensible or latent)

I would struggle to understand that as well.
We don't know the exhaust temperature of either the MVHR or the Ecocent so exhaust temps do not feature in the calculations
The MVHR energy recovery is determined by its efficiency factor.
The Ecocent's energy recovery is determined by the air to water COP.

It doesn't make sense to increase the air flow of an MVHR unit to increase the amount of energy recovered. Increasing the air flow increases the ventilation heat loss & thereby increases the additional heat which needs to be provided by the external ASHP or other heating system.

Correct. MVHR drains energy from the house.
It would be stupid to increase the flow rate of MVHR beyond what's required for ventilation.
The Ecocent, on the other hand, adds energy to the house. It's a heating device, not a ventilation device.
The ventilation function is incidental to the heating function.
In practice the Ecocent doesn't do more air changes in a day than MVHR, it merely does them periodically rather than continuously. In fact, mine usually does it when it's needed most e,g, after someone has had a shower.
- CommentAuthorGavin_A
- CommentTimeSep 26th 2012
Posted By: davidfreeborough
Posted By: Gavin_Ain winter I'd expect something like a 2.5:1 differential due to the difference in the moisture content.
I've simplified the calculation to get to the bottom of the ventilation heat loss issue.

I'm afraid you've not simplified it, you've made it wrong.

Posted By: davidfreeboroughHowever, for heat pumps like the Ecocent where the exhaust air temperature is similar to that of a 90% efficient MVHR unit, I don't think the difference will be that great. To put it another way, if the temperature at the back-end of an exhaust heat pump is similar to the temperature at the back-end of an MVHR heat exchanger then why would there be significantly more condensation with one than the other?

David

Firstly, you're making some fairly large assumptions here.

secondly you're assuming that the flow rates are identical.

thirdly, a heat pump efficiently extracts heat away from the surface area in direct contact with the outgoing air, meaning the surface temperature can be much lower than with a bit of solid plastic or similar as a solar heat transfer surface in an MHRV unit, so it obviously would be expected to condense out a greater proportion of the vapour than with a MHRV even at the same air temperatures.

As previously explained, there is no mechanism with the laws of physics as they are currently understood by which a MHRV system with the same flow rates and pressures on both sides can extract the same level of energy from the moist outgoing air to drier incoming air as can be extracted via an EAHP. It can not physically warm the incoming air directly to a higher temperature than the outgoing air, which it would need to do if it were to extract the same levels of energy from it as the EAHP. The EAHP on the other hand can extract that additional energy - it's precisely what it's designed to do via the compressor.
- CommentAuthordavidfreeborough
- CommentTimeSep 26th 2012 edited
Gavin_A

My spreadsheet is based on the data Malcolm provided on the exhaust temperature of his own Ecocent unit. If you ignore latent heat then the Ecocent doesn't seem to achieve anything more than a high efficiency MVHR unit & electrical resistance post-heating.

The flow rates are not identical. I'm using 1 air change per hour for the exhaust heat pump because this is the air flow required by the Ecocent unit. I'm using 0.4 air changes per hour for the MVHR unit because this is all that is required for ventilation.

Can you provide data or a specification for a commercially available exhaust heat pump showing the energy recovered from latent heat? Could you add this information to my spreadsheet to show how it changes the overall result?

David
- CommentAuthordavidfreeborough
- CommentTimeSep 26th 2012 edited
Posted By: qeiplI'm interested in the net energy gain of the house as a whole (air + water).
My spreadsheet shows the net energy gain of the house as a whole (air + water) based on typical published COP & efficiency figures. Yours attempts to guess the gross heat recovered by an MVHR unit from exhaust air based on a number proposed by Gavin_A which doesn't match anything I can find on MVHR datasheets.
MVHR drains energy from the house.
It would be stupid to increase the flow rate of MVHR beyond what's required for ventilation.
The Ecocent, on the other hand, adds energy to the house. It's a heating device, not a ventilation device.
The ventilation function is incidental to the heating function.
I agree that an MVHR unit can only lose energy when you look at the net energy balance & that a separate heat souce will always be required. My argument is that an exhaust heat pump will consume more energy to recover the same amount of energy under the same conditions. It is not a heating device with incidental ventilation. It is inherently a ventilation unit because it can only extract heat from the house by over-ventilating it & thereby cooling it.
In practice the Ecocent doesn't do more air changes in a day than MVHR, it merely does them periodically rather than continuously. In fact, mine usually does it when it's needed most e,g, after someone has had a shower.
Time is not relevant if we stick to using power (Watts or Joules per second).

If you want to include the latent heat in the comparison then we also need to allow for the energy required to vapourise the water in the first place. You could argue that the extracted air will already have a high humidity content due to cooking, washing, etc, but it is also true that an MVHR unit will recover some of this & the dry air caused by the exhaust heat pump's over-ventilation will lead to additional evaporation which needs to be paid for through an increased heating load.

David
- CommentAuthorqeipl
- CommentTimeSep 26th 2012
Posted By: davidfreeboroughGavin_A

My spreadsheet is based on the data Malcolm provided on the exhaust temperature of his own Ecocent unit.

I didn't provide any data on exhaust temperatures. I said that when the outside air was less than 5C the exhaust temp felt warmer. I don't have any way of accurately measuring the exhaust temp.
- CommentAuthorqeipl
- CommentTimeSep 26th 2012 edited
Posted By: davidfreeboroughMy spreadsheet shows the net energy gain of the house as a whole (air + water) based on typical published COP & efficiency figures.

Mine are based on specific data from the same model of Ecocent that I am running.

Yours attempts to guess the gross heat recovered by an MVHR unit from exhaust air based on a number proposed by Gavin_A which doesn't match anything I can find on MVHR datasheets.

Gavin's assessment of the energy efficiency of MVHR may not be 100% accurate but neither is the 90% claimed by manufacturers - they're talking about temperature, not energy.

However, if we set the MVHR efficiency to 90% all that means is that it takes a little bit longer for the heat pump to outperform the MVHR, as the chart below shows. After 1.7 hours the exhaust heat pump is adding more energy to the house than the MVHR than the external one.
- CommentAuthorqeipl
- CommentTimeSep 26th 2012
Posted By: davidfreeborough...[an] exhaust heat pump......is inherently a ventilation unit because it can only extract heat from the house by over-ventilating it & thereby cooling it.

No. It's a heating device in exactly the same way that an external heat pump is a heating device. It's extracting energy from outside air and adding this energy to the inside of the house. The external air passes through a warm duct (the house) on the way to the pump. The energy that's used to warm the duct is recycled.

Within a defined range of temperature differentials there is a continuous energy gain that offsets the energy lost via the heat pump exhaust. Over time, this energy gain accumulates until it balances the energy lost via the exhaust. Thereafter there's a net energy gain to the house.
- CommentAuthordjh
- CommentTimeSep 26th 2012
Posted By: Gavin_Adjh - if the ventilation rate is increased because of the EAHP to the point where RH falls below an acceptable point and extra vapour is deliberately being introduced, then yes I'd agree.

otherwise I'd not agree at all, as the water vapour isn't being added to the air for purpose of driving the EAHP system, it's being added to the air regardless of this, and you can either choose whether to throw this energy away, or utilise it via an EAHP unit.

Though it doesn't actually work like that. The amount of vapour breathed out by living organisms - people, dogs, plants etc - depends on the RH of the air. Ditto for hygroscopic materials. And even, ditto for everything. Specifically any free water surface. If you lower the RH, then the rate of evaporation from everything in the house increases. And the energy to power that evaporation has to come from somewhere. Granted in the case of living organisms, it is biofuel. In the case of condensation on your toilet cistern or whatever, it's the central heating.
- CommentAuthorPaul in Montreal
- CommentTimeSep 26th 2012
Posted By: djhThe amount of vapour breathed out by living organisms - people, dogs, plants etc - depends on the RH of the air

Not really - it depends on how much food you're metabolising as it is effectively a product of combustion. Maybe for plants to an extent, but certainly not mammals. Otherwise you may as well say cars emit more water on cold dry days. If the RH is less than 100% then water can always be evaporated and it doesn't take more energy to do so.

Anyway, as I said before, you guys are way over thinking this whole topic.

Paul in Montreal.
- CommentAuthorSteamyTea
- CommentTimeSep 26th 2012 edited
Posted By: Paul in MontrealOtherwise you may as well say cars emit more water on cold dry days

They may as the charge density is greater. Went to a tyre testing day at Silverstone during the Turbo era, because it was really cold, 1 or 2°C, they engines apparently were getting an extra few horsepower, not as if they needed when the BMW was doing 1000 BHP/Litre. Was a good day out.

I agree about the amount of food calories processed being related to exhaled air moisture content in mammals.
Though we have to be careful here about relative and absolute values.
- CommentAuthorGavin_A
- CommentTimeSep 26th 2012
Posted By: davidfreeborough
Posted By: qeiplI'm interested in the net energy gain of the house as a whole (air + water).
Yours attempts to guess the gross heat recovered by an MVHR unit from exhaust air based on a number proposed by Gavin_A which doesn't match anything I can find on MVHR datasheets.

you're mixing up energy and heat again*. My numbers for the MHRV efficiency relate to energy, not heat.

The manufacturers data relates to heat, not energy, eg 'up to 90% heat recovery', or 'up to 90% thermal efficiency'.

They are not the same when you're comparing 2 volumes of air containing significantly different volumes of moisture, and until you factor this in you will always come up with the wrong answer.

*for the more technica
lly minded, energy should really be referred to as enthalpy, but energy vs heat or the temperature of the air is probably simpler to understand.
- CommentAuthorGavin_A
- CommentTimeSep 26th 2012
Posted By: djh
Posted By: Gavin_Adjh - if the ventilation rate is increased because of the EAHP to the point where RH falls below an acceptable point and extra vapour is deliberately being introduced, then yes I'd agree.

otherwise I'd not agree at all, as the water vapour isn't being added to the air for purpose of driving the EAHP system, it's being added to the air regardless of this, and you can either choose whether to throw this energy away, or utilise it via an EAHP unit.

Though it doesn't actually work like that. The amount of vapour breathed out by living organisms - people, dogs, plants etc - depends on the RH of the air. Ditto for hygroscopic materials. And even, ditto for everything. Specifically any free water surface. If you lower the RH, then the rate of evaporation from everything in the house increases. And the energy to power that evaporation has to come from somewhere. Granted in the case of living organisms, it is biofuel. In the case of condensation on your toilet cistern or whatever, it's the central heating.

well to a small extent, but only in the case of a situation where significant volumes of water are left to just evaporate into the air of their own accord, or someone's deliberately adding vapour to the air to make up for it being too dry will any significant addition of vapour to the air result in an increase in actual energy consumption.

The vast majority of additional vapour will come from showers, drying clothes, cooking, boiling kettles, breathing, sweating etc. None of which involve any additional (paid for) energy being used as a result of the RH being artificially lowered by the EAHP.

What it will lead to is a reduction in condensation on walls, windows, ceilings etc which is actually how most houses balance out the RH levels to reach an equilibrium point.
- CommentAuthordavidfreeborough
- CommentTimeSep 26th 2012 edited
Malcolm

I've taken your air flow, input power & COP figures for the Ecocent & used them to calculate the net heat gain in the two cases.

In the exhaust heat pump case there is a net heat LOSS of 1707W & in the MVHR plus external ASHP case there is a net heat GAIN of 2048W. The powers consumed are 810W and 870W respectively. See attached.

David

Edit: See later post for corrected file
- CommentAuthorGavin_A
- CommentTimeSep 26th 2012
david - could you email me a copy of your spreadsheet to save me typing it in from scratch?

I'll rework it then using the actual energy flows incorporating the moisture level impact.
- CommentAuthorqeipl
- CommentTimeSep 26th 2012 edited
Posted By: davidfreeboroughMalcolm

I've taken your air flow, input power & COP figures for the Ecocent & used them to calculate the net heat gain in the two cases.

In the exhaust heat pump case there is a net heat LOSS of 1707W & in the MVHR plus external ASHP case there is a net heat GAIN of 2048W. The powers consumed are 810W and 870W respectively. See attached.

David
- http://www.greenbuildingforum.co.uk/newforum/?PostBackAction=Download&AttachmentID=3320" >ExhaustHeatPumpComparison4.pdf
David,

With reference to your latest spreadsheet..

On the green side:

You are assuming an exhaust air temperature for the heat pump with no evidence to support the assumption.
You then use this assumption to determine the VHL by erroneous means. What you call 'Ventilation Heat Loss' is the energy available to the heat pump from the air, which is the enthalpy of air at 21C at whatever RH, not the enthalpy of the differential temperature (in v. out).

The net heat gain is the VHL minus the power recovered: you have it the wrong way round.

On the blue side:

For a valid comparison you have to run the MVHR for 3.5 hours so that it has provided the same number of air changes as the heat pump, which means the VHL must be the same as the green side.

In terms of enthalpy the MVHR cannot be 90% efficient. The laws of physics don't allow it.

In general:

You don't give a value for the RH of the air, which is required so that we know the energy content of the air.

Ignoring time means that the cumulative energy gain from the exhaust heat pump isn't accounted for.

The calculations do nothing to support your argument.

Malcolm
- CommentAuthordjh
- CommentTimeSep 26th 2012 edited
Posted By: Gavin_AThe vast majority of additional vapour will come from showers, drying clothes, cooking, boiling kettles, breathing, sweating etc. None of which involve any additional (paid for) energy being used as a result of the RH being artificially lowered by the EAHP.

I believe that's the point - they do. The quantity of water evaporated from a wet shower cubicle depends on the surrounding humidity (I was wrong to state RH, for these typically non-organic systems it is absolute humidity/partial pressure that counts). If the air is drier, more water evaporates. That cools the shower glass/tiles slightly more and that energy has to be made up. All of your examples do have to expend extra energy. You can't get around the conservation of energy. And you can't get around the almighty dollar - you have to pay for that energy. For breathing and sweating, you pay for food. For everything else, you pay your electricity or gas supplier.

Think about it. In equilibrium, as much water vapour condenses out onto surfaces as evaporates from them. If you make conditions drier than the equilibrium, more water evaporates than condenses and there is a net energy of vaporisation that has to come from somewhere!

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Heating and cooling: exhaust air source heat pumps - a cautionary tale