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Posted By: qeiplDoes the energy represented by 3067W of "Ventilation Heat Loss" leave the house via the extracted air?
Posted By: Gavin_Ain winter I'd expect something like a 2.5:1 differential due to the difference in the moisture content.
Posted By: davidfreeboroughPosted 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.
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.
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?
Posted By: djhMalcolm, 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.
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.
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.
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.
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.
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.
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.
Posted By: davidfreeboroughPosted 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.
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
Posted By: qeiplI'm interested in the net energy gain of the house as a whole (air + water).
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.
Posted By: davidfreeboroughGavin_AMy spreadsheet is based on the data Malcolm provided on the exhaust temperature of his own Ecocent unit.
Posted By: davidfreeboroughMy 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.
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.
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.
Posted By: djhThe amount of vapour breathed out by living organisms - people, dogs, plants etc - depends on the RH of the air
Posted By: Paul in MontrealOtherwise you may as well say cars emit more water on cold dry days
Posted By: davidfreeboroughPosted 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.
Posted By: djhPosted 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.
Posted By: davidfreeboroughMalcolmI'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.Davidhttp://www.greenbuildingforum.co.uk/newforum/?PostBackAction=Download&AttachmentID=3320" >ExhaustHeatPumpComparison4.pdf
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.
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