The energy loss from a house is made up of fabric heat loss, air leakage heat loss & ventilation heat loss.

Yes, but I was talking specifically about the transfer of energy from the cold air to the warm air. In this part of the system there is no energy loss from the house.

.. the exhaust heat pump is taking heat from inside the building using an air supply which is limited by the required ventilation rate ..

No. My exhaust heat pump has an unlimited supply, just the same as an external ASHP.

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.

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?

If the external ASHP achieves a COP of 3 then this compares well with the running cost of a gas boiler.

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.

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.

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.

A significant point seems to have been lost in this thread, in that exhaust air heat pumps should only be specified in situations where mechanical ventilation is required anyway.

In these situations you can obviously fit a MVHR system, but this will still leave the incoming fresh air colder than the outgoing warm air.

An exhaust air heat pump should work on the basis of extracting as much of the remaining heat from hat air as possible and returning it to the house either in the form of hot water, or space heating.

not fitting an exhaust air heat pump would therefore simply just mean that the house was just venting that heated air without recapturing the heat, and would therefore need to supply both that heat (or the difference between that temperature and the external air temperature), via some other means on top of it's basic heating requirements.

Much of the time there is also significant extra energy that can be extracted from the exhaust air over and above simply raising the incoming air temperature to room temperature, which can then be used either for water heating or space heating, or both.

However, the actual amounts of additional heat available from this sources are relatively low, and only suited as the primary heat source to extremely well insulated houses with high levels of passive solar gain.

If fitted to houses that don't meet that spec, then they will need an additional heat source, in the absense of which you end up with a system that's largely reliant on its inbuilt immersion heater back up, a seems to be the case here.

In other words, NIBE are correct in that it's a design, specification, installation and user issue, not a problem with exhaust air heat pumps as such.

Ripping the heat pumps out is nuts, as MVHR system will still be venting all that waste heat from the house, which then won't be recovered. If the house and EAHP aren't coping by themselves, then just add a secondary heat sources, and leave the EAHP to eg heat the hot water, don't rip it out entirely.

'Turning things on and off wastes a huge amount of energy'.... what, like switching the car engine off while you go on holiday, or are at the supermarket?

These have been a sap fiddle since day one!
I know of estates where these were ripped out at vast cost to the HA.

They also do not comply with regs on the ventilation front either.

The hot water takes priority, before the heating kicks in so tenants usually override with immersion.

The were originally designed where a house had a focal fire, even showed this on their literature where it makes more sense (you need a heat source, in order to extract heat to compress etc)

Posted By: qeiplNo. My exhaust heat pump has an unlimited supply, just the same as an external ASHP.

Every cubic metre of air extracted from the house by an exhaust heat pump has to be replaced by a cubic meter of cold outside air. This is not the case with an external ASHP.

If you increase the air flow through an exhaust heat pump the amount of cold air drawn into the house increases proportionately. This is not the case with 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?

This is the trap which concerns me. First of all, its not doing four jobs. The underfloor heating and domestic hot water arrangements do not change depending on whether you have an exhaust heat pump or external ASHP. So the question becomes: why are separate MVHR & external ASHP or MVHR & gas boiler better than an exhaust heat pump?

The answer is running costs & CO2 emissions.

The MVHR I have specified for my build is estimated to recover 9590 kWh per annum at a cost of 525 kWh per annum in electricity. An exhaust heat pump with a COP of 4 would consume 2397kWh to deliver the same amount of energy.

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.

It is not the same source. In one case you're paying to move heat from outside the building to inside. In the other case you're paying to take the energy from the exhaust air & using it to (indirectly) heat incoming air when an MVHR unit would do it for a lot less money & CO2.

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.

So you seem to be agreeing that the exhaust heat pump is not providing useful heating & that other sources are required. So it doesn't in fact replace an MVHR & external ASHP, it just replaces the MVHR while costing more to run.

David

David,

I'm tempted to give up on this but your view of how my system works might be preventing others from taking advantage of its benefits so I'll have one more go. Apologies for the length of the post but the concepts seem to be hard to grasp which pushes me into using lots more words than I'd like...

The first thing to understand is that all of the air that's processed by an exhaust ASHP comes from outside.
The house is merely a duct through which outside air passes on its way to the heat pump.

Imagine my house with all the windows open so that the temperature and relative humidity of the inside air is exactly the same as the outside air, and my exhaust ASHP is drawing air into the house, capturing energy (sensible and latent) from it before blowing the cool, dry air outside.
There's a continuous flow of air from outside to inside, through the heat pump to outside.
The energy that's captured from this flow of air is delivered to the hot water cylinder.

An external ASHP (air to water) works in exactly the same way, using exactly the same energy source with exactly the same energy content.

Now we close all the windows and turn on the underfloor heating, which runs off the hot water cylinder.
The energy that we've stored in the water is transferred to the floors and the rest of the interior.

The heat pump continues to run, drawing air through the supply ducts (instead of the windows), through the house and into the heat pump in a continuous flow, just as before.
When it enters the house this air has exactly the same energy content as before the windows were closed.

As the air passes through the house it absorbs some of the energy that has been transferred from the water to the interior via the UFH. This additional energy raises the temperature of the air, which reduces its relative humidity, which allows the heat pump to extract more of the latent energy from the air.

The exhaust ASHP is now capturing more of the available energy from the outside air than is possible with an external ASHP, and using this captured energy to heat the house. It's doing the same job as an external ASHP, but doing it more efficiently.

We have used some of the previously stored energy to raise the temperature of the air in order to capture more energy from the air, but most of this previously stored energy is returned to the store in addition to the new energy captured from the new air.

While it's heating the house the exhaust ASHP is also keeping a steady flow of fresh air through the house and keeping most of the energy inside the house. MVHR is an incidental benefit of heating the house.

On warm days there's no need for space heating but still need for hot water and ventilation.
When someone uses hot water (e.g shower) the temperature in the cylinder drops and the heat pump runs.
The warm humid air from the shower room is drawn into the heat pump where its energy is captured and transferred to the water in the cylinder. The stale, dry, cool air is exhausted.
In warm weather the exhaust ASHP does mechanical ventilation and useful heat recovery.

In 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.

When the sun shines (at any time of year), the interior of the house captures solar energy, which is transferred by the heat pump to the water in the cylinder where it can be used for DW or UFH. The house/ASHP does the job of a solar collector, which MVHR can't do.

The 525kWh that your MVHR uses every year just to ventilate your house equates to c.1.44kWh/day, which will run my heat pump for about 1.8 hours. On days when the sun shines that 1.44kWh of electricity will give me at least 4.3kWh of hot water with mechanical ventilation thrown in for free.

Even 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.

For 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.

In cold, dull weather an exhaust ASHP must be provided with an alternative heat source. The problem with the NIBE is that the alternative heat source is an immersion heater, and the heat pump is undersized for most UK houses, so the immersion kicks in too often.

Malcolm

David,

As I've already said, if you have an EASHP don't have trickle vents. Use air supply ducts that incorporate bends to eliminate any reverse air movement and introduce the air into rooms at low level. There are no draughts.

An exhaust ASHP used only for ventilation will of course use more energy than an MVHR unit.
But you'd be daft to use an EASHP for ventilation only.

My exhaust ASHP only runs when I need hot water for UFH or DHW.
It gives me periodic MVHR as an incidental bonus. It uses no additional energy for MVHR.

Your gas boiler or external ASHP uses energy to give you space heating and hot water, but doesn't give you any MVHR.
You have to use additional energy to run your MVHR unit.
All other things being equal, your system will use more energy than mine.

You appear to be persisting with the notion that warming the incoming air uses additional energy. It doesn't.
The house is warm because I want it to be warm. The energy that I've used to warm it has been put to productive use.

I'm paying to move outside air through the heat pump in order to extract new energy from it.
As this air passes through the house it happens to absorb some of the energy that's already in the house but so what?
It doesn't cost me anything and it allows the heat pump to capture more of the energy from the new air.

Malcolm

Posted 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. Look up ventilation heat loss.

Any energy you remove from the house results in cooling & needs to be replaced by other means. It doesn't matter whether you've "already had use of" the energy, how long the energy has been in the house or what route it takes. The house will get cooler unless you use another heat source to put an equal amount of energy back in to the house. This is basic physics & I thought you conceded this point earlier in your reply to djh.

David

Posted 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 0.0

Paul in Montreal.

Not sure if this will help or hinder .....

Agreed that when heat energy at your 18C is available due to solar gain, then it can be used to heat DHW to say 60C by doing work on it via the heat pump, and we get the original energy uplifted plus the energy of the work done, all at the higher temp. (BTW the theoretical max COP for those temperatures is (60 +273) /(60-18) = 8 and in real life we might hope for 3 or 4 or so.)

So far, so good.

However, if it's cold weather and the energy has to be provided by solid fuel, then it does seem to be going round the houses to have this high-grade-energy source first produce low grade heat energy at 18C and then expend electrical energy lifting that to 60C. The solid fuel could have produced that directly without any electrical power - an infinte COP.

Trying to move on a bit, whatever you make of the thermodynamics, 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 BaconAny thoughts or details of anyone trying this previously?

How much air does the Ecocent 300l need to shift, that is the key number.

Posted By: qeiplThe only air leaving the house is via the heat pump exhaust. Where am I losing energy?

Malcolm

Have you looked up ventilation heat loss? I can't think of any other way to explain it than every litre of warm air removed by the exhaust heat pump will be replaced by cold air drawn in through your fresh air inlet ducts. 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? Why do you think the Passivhaus standard specifies MVHR? Why do you think all the energy balance calculations for low energy buildings include an allowance for heat loss due to air leakage & one for heat loss due to ventilation? Do you think that all these people are missing something?

David

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.

My system works for me. It's been through two very cold winters, one warm wet winter, two normal Hebridean summers and one unusually dry one.

House: 1.5 storey (attached to workshop); 100m^2 floor area; high surface are to volume ratio (stupid shape for energy efficiency).

Level of insulation: walls/floor/roof 0.18; DG 70% 18mm, 30% 12mm

Air tightness: not measured; probably better than my neighbours; definitely worse than PH

Lifestyle: mostly single occupant, often two, occasionally six; work from home office; room stats set to 18C

Location: 57 degrees north, west coast, sea level, relatively sheltered, south facing

Construction: concrete floor slab; timber frame; 200mm EPS in floor/walls/roof; steel cladding/roofing; timber rainscreen on walls

Reduced bills: no direct comparison possible

Typical energy consumption range: 8kWh/day in summer (all electric) to 22kWh/day in winter (electric + solid)

Total energy consumption last year: 6,050kWh

Hope that helps.

Posted By: davidfreeborough

Posted 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.

No it's not. Why would you invest in expensive machinery just to warm incoming air?
When cold air comes into the house it's automatically warmed up by the energy in the house (sun, central heating, bodies, whatever) - you don't need an MVHR unit to do it.

If no energy was lost when warm extracted air was replaced by cold fresh air then what would be the point of an MVHR?

The point of any ventilation heat recovery system is to minimise the energy loss via the extracted air.
This is achieved by capturing some of the energy from the outgoing air and keeping it in the house.

An exhaust air heat pump does the job by transferring the captured energy into the hot water cylinder.
MVHR does the same job by transferring the energy to the incoming air.

The fact that MVHR warms the incoming air is incidental - a by-product of the heat recovery process.
The temperature of the incoming air is irrelevant to the heat recovery process.

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

Yes, the heat pump uses more energy when it's running but it only runs when there's a demand for hot water. I'm having to use the energy anyway. The ventilation heat recovery is an incidental bonus.

If you want to compare the running costs yo have to include MVHR + boiler (or whatever) on your side versus the exhaust ASHP on my side.