I don't turn the shower up because it's not steamy enough.

I don't boil the kettle for longer because it's not steamy enough*.

I don't add more water to my clothes to dry them for longer because they're not steamy enough.

Therefore there is no additional energy used to increase the RH level in the house via these means.

As for eating more... humans don't regulate their food intake to the degree that a 0.1% increase in their daily energy use results in a 0.1% increase in food consumption. They just get very marginally fatter or thinner.

"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!"

how exactly would more water be evaporating than condenses onto the surfaces? Where does it come from if it hadn't previously condensed on that surface?

Unless you're talking about an extremely vapour impermeable building, much of the vapour that condenses will also migrate through the wall / ceiling anyway, not just continuously condense and evaporate within the building envelope.

There obviously is a net energy of vapourisation that has to come from somewhere, but this isn't new energy, it's energy that was already being spent in the form of showers, drying clothes etc. The choice here is whether to throw the much of this energy away during ventilation as in the case with an MHRV system, or recycle a much greater proportion of it via an EAHP.

David,

As far as I understand your calculations are wrong for all the reasons that I gave in my last post.
I'm not going to convince you of this and if you stick to your current tack you're not going to convince me that your understanding of the energy flows is correct.
Maybe Gavin will be able to put one or both of us right.

Malcolm

Im intersted to know how they get round Building Regs as you just can not replace them with a more inefficient system

Posted By: Gavin_A
"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!"

how exactly would more water be evaporating than condenses onto the surfaces? Where does it come from if it hadn't previously condensed on that surface?

Water doesn't have to condense to get on a surface. The shower cubicle is wet because you spray water at it. The kettle is wet because you fill it with water. The clothes are wet because you wash them. Almost all materials have a film of water adsorbed to the surface all the time - and that usually does arise from condensation. Many materials are hygroscopic - they also have water absorbed within them - soft furnishings are a good example.

If you reduce the humidity, some of it evaporates and that takes energy. Yes there's no specific heating provided. The evaporation causes the temperature of the surface to drop by a small amount and that difference is then made up from the environment and then by the heating system. I say again, the energy has to come from somewhere.

Unless you're talking about an extremely vapour impermeable building, much of the vapour that condenses will also migrate through the wall / ceiling anyway, not just continuously condense and evaporate within the building envelope.

In most buildings, the layer of paint on the surface of walls and ceilings specifically makes them particularly impermeable. Even in a breathable building, most of the movement of moisture is mass transport carried along with the air, not via the fabric of the building.

There obviously is a net energy of vapourisation that has to come from somewhere, but this isn't new energy, it's energy that was already being spent in the form of showers, drying clothes etc.

No, that's the whole point. If you evaporate extra water then there is extra energy expended to do so. Just because its a relatively small and diffuse amount of energy.

I suggest reading some of Tim Padfield's stuff or the papers written by Passivhaus researchers.

Posted By: djh
No, that's the whole point. If you evaporate extra water then there is extra energy expended to do so. Just because its a relatively small and diffuse amount of energy.

Not to any great extent.

in the case of extra steam from showers, this is merely energy in the form of heat that would have been lost down the drain if it wasn't used to create more steam due to the lower RH. Likewise, the water on the shower screen etc would evaporate eventually anyway unless the RH was 100%, it's only the time it takes to evaporate that would change.

A quick bit of research gleans the following vapour sources that are entirely or almost entirely independent of the RH levels, and need no additional energy to evaporate than they would do at higher RH levels.

1kg a day of water vapour per person from respiration and sweating
0.5-1kg a day per person from showers
0.25kg a day dishwashing
0.24kg a day / meal from cooking
3kg a load of washing (assuming its' dried inside, and would have been dried inside regardless)

in a 4 person house with indoor clothes drying, that's probably around 9kg a day of water vapour that's independent of RH levels entirely, or around 5.6kWh a day of latent heat available for recovery.

Granted 5.6kWh a day isn't a vast amount, and this is really why this tech should only be used in extremely well insulated and air tight buildings, and's why the energy consumption levels rocket if people start going much above these sort of levels of energy demand, as at that point all the other arguments raised in this thread start to become valid. I suspect that UK builders simply aren't getting anything close to these sorts of levels of additional heat requirements in their buildings (on top of body heat, heat from electrical appliances, passive solar gain etc).

I agree that there will be a minor increase in energy used for vapourisation of a few small contributors to RH levels, but doubt this is more than 10-15% of the value in most well sealed buildings. The rest is energy that would be expended regardless, and the choice is to recover that energy, or lose it to the outside air.

You're right to an extent about the vapour permiability of the walls, as I'dforgotten we're dealing with highly insulated, air tight houses which will presumably have vapour barriers in walls, ceiling etc. In these cases the vapour would have left via the ventilation, as previously discussed, and this is how the RH equilibrium would be maintained. In older houses a significant proportion of vapour does migrate through the walls, and particularly the ceiling, though most would still leave via drafts etc.

http://www.ornl.gov/sci/buildings/2010/Session%20PDFs/71_New.pdf

just to add, from the above it seems that the energy available is quite well matched to water heating requirements, with minimal left over for space heating.

I'd also think that whether people use gas for cooking, and if they do dry their clothes inside, or use a tumble drier / dry them outside, will make a significant difference to the latent heat energy available. Gas cooking would probably account for another 0.5kg or so of water vapour a day on average.

I just realised I've made a bit of an error last night, and the 5.6kWh figure I came up with only relates to the actual latent heat figure, not for the actual energy contained in the heating / cooling of that mass of water vapour on top of the latent heat figures.

I think I'm right that this only actually makes about 0.1kWh a day difference though, but I'm a bit rusty at these calcs.

This ain't a lot of heat per day, if I'm right that this is basically the only actual source of additional heat available on top of the electricity used directly in the compressor, pump etc.

It'd basically mean that anything over these levels would need to be supplied by other means, and in the case of a house that's 100% heated by these units those other means would be an immersion type heater. The first approximately 7-8 kwh then would be at a very good COP, but anything over that is suddenly at a COP of 1.

'My argument is that this does not prove that exhaust heat pumps should be routinely specified for residential properties, especially those owned by housing associations. If they are specified then an alternative form of heat is needed mid-winter, e.g. external ASHP or gas boiler. '

I'd have to agree with this.

Possibly in a really really well built eco house or something, but I just don't believe standard UK builders are capable at the moment of achieving those levels of air tightness and insulation, no matter what the design specs say, unfortunately.

Posted By: qeiplIf air is at 50%RH does this mean that half of the energy in it is sensible and half latent?

No. It means that the partial pressure of water vapour in the air is half what it would be if the air was in equilibrium with a flat surface of pure water at the same temperature. Here equilibrium means that the rate of evaporation from the surface is equal to the rate of condensation onto it.

In other words, 100% RH corresponds to the equilibrium vapour pressure.

http://en.wikipedia.org/wiki/Relative_humidity

That's also often called the saturation vapour pressure (e.g., in that Wikipedia article). However, that's a very misleading name as it conjures the 18th century notion that the air can “hold” a certain amount of water vapour and condensation happens when that amount is exceeded. That idea breaks down and becomes confusing as soon as the water is not pure or the surface is not flat (e.g., droplets in a cloud or capillaries in materials like wood).

The equilibrium vapour pressure rises very rapidly with temperature. A good rule of thumb to start with is that it doubles for every 10 °C rise. For more accuracy browse from:

http://en.wikipedia.org/wiki/Arden_Buck_equation

To a pretty good approximation the mass of water vapour is proportional to the partial pressure (if the temperature is kept constant) so the RH is also the proportion by mass (to the “equilibrium vapour density”).

Looking at it another way:

Posted By: qeiplRH: the proportion of the available energy in the air that has been used for evaporation.

No, RH has nothing directly to do with energy.

To see this consider an airtight and strong room at 20 °C and 50% RH with the moist air at typical room pressures (say 100 kPa). Suppose you use some sort of molecular sieve to remove all the dry air (nitrogen, oxygen, argon, CO₂, etc) but leave the water vapour behind.

The equilibrium vapour pressure of water vapour does not change much with changes of total pressure (of the order of 1% or so in a case like this) so the relative humidity stays pretty much the same. We still have the same amount of water vapour as before so the latent heat also stays the same.

However, all the sensible heat stored in the dry air has gone through the sieve with it so substantially the same RH but totally different ratio of latent to sensible heat.