Ecohouse 2 by Sue Roaf
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The energy loss from a house is made up of fabric heat loss, air leakage heat loss & ventilation heat loss.
.. the exhaust heat pump is taking heat from inside the building using an air supply which is limited by the required ventilation rate ..
If the exhaust heat pump achieves a COP of 3-4 then I don't think this is particularly good value for moving around heat you've already paid for.
If the external ASHP achieves a COP of 3 then this compares well with the running cost of a gas boiler.
There is a real risk that this will happen if people think that they can replace a gas boiler & MVHR or ASHP & MVHR with a single exhaust heat pump.
Posted By: mike7Whether you regard the air coming in as containing energy or not really depends on what temperature you choose as a datum. Whatever that datum temp is, when one does the sums it is the temperature differences that count.
Posted By: qeiplNo. My exhaust heat pump has an unlimited supply, just the same as an external ASHP.
I don't understand your problem with the pump moving energy around inside the building at the same time as extracting energy from the outside air. It's doing the same job as an external ASHP, a DHW cylinder, UFH, plus MVHR all at the same time. Four jobs all done by one bit of kit at a fraction of the capital cost. Why don't you like this?
So what's the problem with an exhaust ASHP? It's using exactly the same air source as the external ASHP and operates at a higher COP in cold weather than is possible with and external ASHP.
They'd be daft to rely solely on an exhaust ASHP. In winter the house would never get warm.The system needs an alternative source of energy when the sun isn't working.The NIBE uses an immersion heater, which is daft unless you're generating your own electricity, and is probably why EASHPs have a bad name.
Posted By: alecLook at it this way.. a room may be loosing say 400w per hour
Posted By: qeiplIn contrast, in warm weather an MVHR system uses energy to remove from the house all of the energy in humid air. It's doing mechanical ventilation without any useful heat recovery.
Posted By: qeiplFor providing DHW, some space heating, and MVHR an exhaust ASHP (properly installed) will use less energy than a conventional MVHR + gas boiler or external ASHP combination.
Posted By: qeiplEven on dull days the 1.44kWh will capture more than 1.44kWh and store it in the cylinder while providing fresh air for free. On the same day your MVHR will use 1.44kWh of electricity just to provide fresh air.
Posted By: qeiplYou appear to be persisting with the notion that warming the incoming air uses additional energy. It doesn't.
Posted By: davidfreeboroughPosted By: qeiplYou appear to be persisting with the notion that warming the incoming air uses additional energy. It doesn't.This is your fundamental misunderstanding. It does. ...Any energy you remove from the house results in cooling & needs to be replaced by other means. David
Posted By: qeiplThe only air leaving the house is via the heat pump exhaust. Where am I losing energy?
Posted By: Paul in MontrealPosted By: qeiplThe only air leaving the house is via the heat pump exhaust. Where am I losing energy?All the other leaks unless your ACH is 0.0Paul in Montreal.
Posted By: Chris P BaconAny thoughts or details of anyone trying this previously?
Posted By: qeiplGranted, the house isn't 100% airtight but how does running an exhaust ASHP cause them to lose more energy than running an external ASHP + MVHR?
Posted By: mike7 it could be helpful for anyone considering such a system to know more of the circumstances in which satisfied customers use them - level of insulation and airtightness, lifestyle, location, construction, reduced bills, etc.
Posted By: Chris P BaconThey say it needs 450m³/h and the recovery rate is 44 litres per hour at 10C or 84 litres per hour at 35C.
Posted By: davidfreeboroughPosted By: qeiplThe only air leaving the house is via the heat pump exhaust. Where am I losing energy?This air needs to be heated to room temperature & that takes energy. This is what an MVHR heat exchanger is for.
If no energy was lost when warm extracted air was replaced by cold fresh air then what would be the point of an MVHR?
Posted By: qeiplWhy would you invest in expensive machinery just to warm incoming air?
Posted By: davidfreeboroughIf the exhaust heat pump & MVHR are extracting the same amount of air from the house then with the MVHR you just need to run two fans; with the exhaust heat pump you need to run a fan & a compressor. The compressor consumes a lot more energy to do the same job.David
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