Posted By: davidfreeborough

Posted By: qeiplWhy would you invest in expensive machinery just to warm incoming air?

Because its cheaper to run & saves CO2 emissions.

David

What do you mean it's cheaper to run?
You don't need any machinery to warm air that comes into the house.
A passive stack extractor will draw fresh air into the building, which will be warmed by the energy in the building.

djh,

I worried about this before I installed my system but the reality is that I don't notice any problem with the air change rate. Maybe because the heat pump runs for half and hour and then sits doing nothing for a while.
The internal volume of the house is (I think) 270m^3 so half an our of Ecocent is equivalent to 0.8ACH. If the pump is off for the next 2.5 hours that's a 3 hour cycle so the ACH is 0.27, albeit not continuous.

The Ecocent isn't designed as an MVHR unit but the way I have it set up gives me MVHR at no extra capital or running costs. It might to meet the design requirements of a dedicated MVHR system but it works.

Malcolm

Stepping back slightly,

did I understand right that an EAHP should cool the outgoing exhaust air, to a temperature much lower than the ambient air that is being drawn in to replace the exhaust air, thus producing a net energy gain which can be used for DHW/UFH?

I saw claims on sales sites that the exhaust air could be discharged at -5degC or even -15degC. If replacement ambient air is drawn into the house at +10degC, then there would be a net gain of about (10-(-5))deg x 1kJ/kg = 15 kJ per kg of air, or 2kW from an airflow of 450m3/h (1kg =~ 1m3), plus or minus latent heat.

Problem would come a) when house needs more net heating than this and b) when ambient temp is lower than this and c) when these claims are not reality

Can anyone with an EAHP and a thermometer, tell us what temperature it is actually discharging the exhaust air at?

Posted By: davidfreeborough

Posted By: qeiplA passive stack extractor will draw fresh air into the building, which will be warmed by the energy in the building.

Cooling the building in exactly the same way as an exhaust heat pump.

David

No! There's no heat loss at the intake end of the ventilation process.
All of the heat loss is at the extraction end.

As the unfortunate owner of a Nibe Exhaust Air Heat Pump (Fighter F470) I can I think, speak with some degree of authority. To summarise, both David and qeipl have made perfectly valid observations. David I think explains very accurately how the Nibes work. My understanding of what qeipl has achieved, is a workable system using an ASHP inside, getting the benefit of solar gain, thermal mass etc.

My unit has ventilation integrated into it, an MVHR if you like. I have no trickle vents in the windows, all ventilation being ducted into the house through the Nibe. There is no transfer of heat between extract and supply air as is the case with normal MVHR. The extract air goes through the heat pump side, the resultant energy used to heat water, which is then used to preheat supply air by way of a 'supply battery' (radiator in the duct) and to provide DHW and radiator or UFH.

In my case the ventilation rate is set at 110m3 per hour. That remains fixed. There is therefore a limited supply of air available from which heat can be extracted.

With the house kept at a constant 19 C, the CoP of the heat pump remains constant. The problem with the Nibe is the size of the compressor. My unit has a 650w compressor. Running at full capacity it operates for 18 hours per day, the remaining time being required for defrost cycles. The running time can be increased but only by increasing the ventilation rate. At a CoP of 3.06 x 650W x 18 hours gives a maximum real output of 35.8KWH per day. If you assume 15kw of that is for DHW (as in our case) that leaves around 20 kwh to provide space heating and pre warming of supply air. We are bringing in 110 m3 per hour of outdoor air. Once building heat loss is taken into account the Nibe heat pump can only supply enough energy to meet DHW and space heating requirements down to an external temp of around 8C. Any colder and the Nibe needs more energy and starts to use immersion to fill the gap, the colder it gets, the more it obviously uses.

As stated by David and qeipl, this is the real problem and why there are so many complaints of high running costs. The literature for these heat pumps is very thin on information regarding how much immersion will actually be used. In my case, this equates to an eye watering 2500kwh of immersion per annum.

I am in the process of engaging with the company that supplied my Nibe as I am very unhappy regarding the high running costs. I will therefore refrain from saying anything else at this stage.

For those who want some figures, have a look at the thread I started in 2010

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=6696

has a lot of data relating to how much energy used / temperatures etc.

Posted By: djhIt's 'really' just the second order term in a power series expansion of the (relativistic) mass of the object.

Nah, it's half mass times velocity squared

Posted By: davidfreeboroughI said that an exhaust heat pump should not be routninely specified for domestic installations because it consumes a lot more energy & produces a lot more CO2 than an MVHR doing the same job, i.e. heating the cold incoming air up to room temperature.

But you're failing to take into account that the exhaust ASHP is only running when it's required for heating water, useful work.
My exhaust ASHP provides efficient ventilation and it adds energy to the house.
MVHR on its own provides efficient ventilation but cannot add any energy to the house.
For a valid comparison with MVHR you have to take account of the energy used by your preferred heating system (gas, external ASHP or whatever).

Posted By: davidfreeboroughThe exhaust heat pump only adds energy to the house because you're using electricity to run the compressor & recover energy from the extracted air.

No. You still haven't got the main point.
I'm not running the heat pump to extract energy from the ventilation air.
I'm running the heat pump to extract energy from the outside air that's being drawn through the house in exactly the same way as an external ASHP. They both use the same air. They both add energy to the house via the water.
The primary purpose of both is the use the captured energy to heat the water.
Ventilation heat recovery is additional - an incidental bonus from running the heat pump inside the house.

However, you can add that energy far more cheaply using an MVHR with a gas boiler or external ASHP.

Only if the external ASHP (or boiler) + MVHR fans use less energy to do the same amount of work (heating water and ventilating) as the exhaust ASHP.

The reverse is more likely to be true.

Malcolm

I'm sure you have a workable system which suits your needs, but this does not prove that an exhaust heat pump should be routinely specified for domestic installations in housing association homes & the like.

You yourself said that "It would 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" & that you use solid fuel for this purpose. Solid fuels are not an option for many & certainly wouldn't be specified for a housing association home. The designer needs to specify something automatic like a gas boiler or external ASHP.

Posted By: qeiplI'm not running the heat pump to extract energy from the ventilation air. I'm running the heat pump to extract energy from the outside air that's being drawn through the house in exactly the same way as an external ASHP. They both use the same air.

If the exhaust heat pump exhaust is at outside air temperature then you've not extracted any energy from the outside air. All the energy you've extracted is coming from the house. If the exhaust heat pump exhaust is below outside air temperature then that proportion of the energy is coming from outside, but you're still paying a lot more than you need to recover the rest of it. You stated earlier that you believed the Ecocent exhaust temperature is within a degree or two of outside air temperature, so in your case, I don't believe you're extracting any energy from the outside air.

However, you can add that energy far more cheaply using an MVHR with a gas boiler or external ASHP.

Only if the external ASHP (or boiler) + MVHR fans use less energy to do the same amount of work (heating water and ventilating) as the exhaust ASHP. The reverse is more likely to be true.

Post some numbers which show how an exhaust heat pump is cheaper to run than an MVHR plus ASHP or gas boiler & I'd be happy to review them.

David

Attached is a comparison of:

1. An exhaust heat pump with a COP of 4;
2. An MVHR with an efficiency of 90% & an external ASHP with a COP of 3.

The results are for an outside air temperature of +5°C with exhaust heat pump exhaust temperatures of +5°C & -5°C. In each case I've assumed that the exhaust heat pump has been sized to provide the required ventilation rate & the external ASHP has been sized to produce the same net heat gain as the exhaust heat pump, so that the overall power consumption figures can be directly compared. In a real system the exhaust heat pump & external ASHP would probably be significantly more powerful & run intermittently, but this doesn't affect the comparison.

At +5°C exhaust temperature, the exhaust air heat pump consumes 571W compared to 281W for the same net heat gain from the MVHR/ASHP combination.

At -5°C exhaust temperature, the exhaust air heat pump consumes 891W compared to 388W for the same net heat gain from the MVHR/ASHP combination.

David

Edit: See later post for easier to read spreadsheet

Posted By: davidfreeboroughAttached is a comparison of:

1. An exhaust heat pump with a COP of 4;
2. An MVHR with an efficiency of 90% & an external ASHP with a COP of 3.

The results ...

David,

I'm struggling to understand the calculations. Can you upload a copy of the spreadsheet so that I can see the formulae?

Malcolm

David,
Based on the numbers, you appear to be using a COP of 3 for the exhaust air heat pump, not the 4 specified. Also, not sure why you include fan power there, nor why the ASHP COP doesn't drop with external air temp.

Also not sure what the Net Heat gain bit on the MHVR + ASHP column is - or at least why it's there.

Sorry, been an idiot - I see your using the EAHP power as input to eg hot water and calling it net heat gain, which is also why the COP seems wrong.

A heat pump with a COP of 4 gets three quarters of its power from the air flow & one quarter from the compressor input power.

ASHP output power = (compressor input power) + (energy recovered from air flow)

For an exhaust heat pump:

Net heat gain = (ASHP output power) - (ventilation heat loss)

Net heat gain = (compressor input power) + (energy recovered from air flow) - (ventilation heat loss)

If you set the exhaust air temperature equal to the outside air temperature then all of the energy recovered from the exhaust air flow is required to cover the ventilation heat loss.

If exhaust air temp. = (outside air temp.) then (ventilation heat loss) = (energy recovered from air flow) &

(net heat gain) = (compressor input power)

(power consumed) = (compressor input power) + (fan power)

If you set the outside temperature to 5°C & the exhaust air temperature to 9°C then all of the exhaust heat pump output power is required to cover the ventilation heat loss.

(compressor input power) + (energy recovered from air flow) = (ventilation heat loss) &

(net heat gain) = 0

See attached.


David

David,
Many thanks - guess it depends on what you count as input for the COP,
As it stands, your numbers would indicate that the combination MVHR + ASHP is better than EAHP, although continuing to lower exhaust temperatures does lower the difference, but there is an implicit assumption in the MVHR+ASHP case that the house is airtight. If you add an 'uncontrolled' ventilation rate, then matters change somewhat - for example, adding a leakage rate of 0.4 ach, ie equalling your MVHR, then the numbers for MVHR+ASHP and EAHP come out roughly the same. (the assumption on the EAHP side here is that all air goes through the EAHP, which doesn't seem unreasonable for moderate windspeeds as it causes a reduced internal pressure). Now that's a fairly high leakage rate, but is in the area you might expect if refurbishing an older property, I believe. This leads me to suggest that EAHP wouldn't be the best choice for a new build, where hopefully airtighness is controlled, but may well be good for an older property.

Chris

Posted By: ChrisEnglandMany thanks - guess it depends on what you count as input for the COP

I'm using the standard definition of COP, I'm just starting the calculation in an unusual place because I'm starting from the air flow & the temperature difference between the inside air & the exhaust air. It's important to start from the air flow because with an exhaust heat pump this determines the ventilation heat loss.

Posted By: ChrisEnglandthere is an implicit assumption in the MVHR+ASHP case that the house is airtight. If you add an 'uncontrolled' ventilation rate, then matters change somewhat - for example, adding a leakage rate of 0.4 ach, ie equalling your MVHR, then the numbers for MVHR+ASHP and EAHP come out roughly the same.

The heat loss due to air leakage is additional to that due to deliberate ventilation. It needs to be separately accounted for in both the exhaust heat pump & MVHR + ASHP cases.

An MVHR will be adjusted for balanced flow & so should not pressurise or depressurise the house. This minimises air leakage through the fabric of the building. An exhaust heat pump will tend to depressurise the house leading to increased leakage.

In addition, an exhaust heat pump with trickle vents as air inlets will have some uncontrolled ventilation due to, for example, air coming in the windward facing trickle vents & leaving through the leeward facing trickle vents. I've ignored this as it depends on the particular installation.

David