Drying clothes indoors - impact on space heating?

[ragg987]

Hi, I have installed a ceiling mounted pole dryer in our plant room to dry clothes in the winter months. I am trying to understand how this affects the energy required for space heating - any thoughts?

 

Plant room contains all our M&E stuff, inc ASHP (internal split unit), 300l DHW, 50l buffer, MVHR. So it is generally quite toasty in here. My thinking is that the evaporation of water from clothes to air removes energy from the house, then this moist air goes to the MVHR which must be able to recover some of this energy through condensation.

 

The parallels I can think of are where a non-condensing drier annual energy required is approx 650kWh. In simple terms, this could be the equivalent of the energy required to remove the moisture from the clothes and into the air. (Ignore drum and motor losses for now). A condensing drier uses 550kWh - perhaps the equivalent of the MVHR scenario - so the condensing in the MVHR could recover approx 100kWh?

 

Any thoughts on how this really works? What do the annual energy consumed published by manufacturers really mean - number of loads per year, for instance.

 

Rajive

[ProDave]

i think you are over thinking this.

 

We routinely dry our washing on the "pulley"  Contrary to popular belief this does not add a noticeable amount of moisture into the house, there is no condensation or mould anywhere. And I cant say it has any effect on the temperature of the small utility room that it is in.  So just do it and don't worry.

 

If you use a non condensing vented tumble dryer, you, like me, will cry at 3KW of heat going out of the house through a big hole in the wall.  If you use a condensing dryer, it will use much the same amount of energy but most of that stays in the room so adds to the heating of the house.  You might detect I am not a fan of tumble dryers. the sole purpose of ours is to make the towels and socks "fluffy" and experimentation has found for that you don't need to dry them in the TD, just partly dry them, then hang them on the pulley to finish off.

 

[TerryE]

If you do the maths, its actually a lot cheaper to hang your wet washing in a MVHR extract room such as a utility room.  (We have a store room configured with extract only for that reason.)  Add a dehumidifier if the moisture is getting too high.  The run rate for one of these is a lot less than a condensing tumble dryer.

[ragg987]

I think this is the best and cheapest option so we will continue to use it. Especially given ashp means that this is not direct electrical hearing like a dryer would be.

 

Question really arises as I notice we require more heat than the as designed PHPP shows, so trying to figure out what might be going on. This is one variable.

 

TerryE, are you able to calculate the heat loss in this scenario?

 

[ragg987]

Done some googling and it seems that the energy to evaporate water is the latent heat - i.e. 2.26MJ / kg. So approx 0.63kWh per kg of water, perhaps 2 or 3kWh per load of washing. Quite small compared to the space heating load on a cold day.

[ProDave]

I think you over estimate how much water is in a load of washing. In our previous house which was cold and poorly insulated (so condensation could be an issue) we dried the washing by shutting it in a cupboard on the pulley with a dehumidifier on. You would not get more than about half a litre of water per load of washing
 

[VIPMan]

PHPP allows you to model this - in my case with a 44% level of residual moisture (from the washing machine specifications) drying indoors as opposed to outside on a washing line uses an extra 400kWh/annum. Haven't put up an outside line yet.

Edited December 14, 2016 by VIPMan

[Bitpipe]

We bought a heat pump condensing tumble drier for our utility - very low energy consumption as it extracts the waste heat from its own condensate.

[ragg987]

On 14/12/2016 at 09:48, VIPMan said:

PHPP allows you to model this - in my case with a 44% level of residual moisture (from the washing machine specifications) drying indoors as opposed to outside on a washing line uses an extra 400kWh/annum. Haven't put up an outside line yet.

Thanks - this answers my question. So not an insignificant amount - approx 15% of the annual space heating demand, something I had not factored into my calculations.

[Temp]

 

On ‎12‎/‎14‎/‎2016 at 09:23, ragg987 said:

Done some googling and it seems that the energy to evaporate water is the latent heat - i.e. 2.26MJ / kg. So approx 0.63kWh per kg of water, perhaps 2 or 3kWh per load of washing.

 

Isn't that similar to an hour in a tumble dryer?

[Ferdinand]

54 minutes ago, ragg987 said:

Thanks - this answers my question. So not an insignificant amount - approx 15% of the annual space heating demand, something I had not factored into my calculations.

 

Or £40 at 10p/hWh.

[ToughButterCup]

On 12/13/2016 at 21:20, TerryE said:

If you do the maths, its actually a lot cheaper to hang your wet washing in a MVHR extract room such as a utility room.  

 

And when I'd visited enough passivhauses and talked to enough owners, (including @VIPMan of this parish) every one of whom said  - you don't need a condensing dryer, the MVHR'll do it for you - I'm just about beginning to believe them.

Well, nearly. @VIPMan could have offered to dry our washing, so when he does that, and the clothes dry by 'magic', I'll believe them all.

[ProDave]

The mvhr will DRY your washing, but it won't make the towels fluffy, Unless anyone knows another way to fluf them, there will still be some need for a TD.

 

Re the energy usage of a TD being roughly equal to the energy a load of drying washing sucks out of your house. That's a simple and usable piece of knowledge.  In the summer, dry your clothes on the pulley and it has a useful cooling effect on the house. In the winter use the TD and it helps heat the house (as long as it's a conensing dryer so the heat stays in the house)
 

[TerryE]

19 minutes ago, ProDave said:

it has a useful cooling effect on the house

 

It did strike me that in terms of the physics, if the moisture is condensed out in the MVHR heat exchanger then the latent heat of evaporation will be returned to the airflow and effectively transferred (mostly) to the incoming air stream.  So you'll only get the net cooling effect if you have the summer bypass on.  This also largely addresses @ragg987's point above.

[jack]

16 hours ago, TerryE said:

It did strike me that in terms of the physics, if the moisture is condensed out in the MVHR heat exchanger then the latent heat of evaporation will be returned to the airflow and effectively transferred (mostly) to the incoming air stream.

 

Yes, although presumably this effect will become less efficient as the external temperature rises (which is fine - it's when it's really cold outside that you want to be recovering as much heat as you can anyway).  

[TerryE]

Correct.  You would only get the heat recovery if the external air was cold enough for the internal air to cool to its dew point on the way out.