A while ago I posted a link to an exhaust air heat recovery device that was an ASHP that used house air to heat water. I couldn't inderstand why one would pay to heat air, then run a device inside the house that took heat from the house air and heated the water with it, because hen you'd have to pay more money to heat more house air, so why not just heat the water with the money you were spending and cut out the losses and inefficiency of using air as a poor heat transfer medium

But I get it now, the intention is that this is an ASHP that is mounted inside an MVHR instead of a heat exchanger.. The ASHP takes heat from the air that is going out to the world and being exahusted, and stores it in water, rather than a simple cross flowing matrix taking heat from the outbound air and storing it in inbound air

I do wonder whether it is more effective, could the ASHP wih the addition of electrical energy establish a steeper delta t and recover more heat from the outgoing air, store it in water, which could then be used to heat more the inbound air via a device something like a car radiator. Of course effective is a question of what you pay and what you get.. Assuming the fan that pushes the air in the MVHR is the same, you then have the added cost of running an ASHP compressor or heat pumping technology, so does it transfer more heat for the extra money? Is it also more flexible in that in summer the hot water generated can be used as hot water for cleaning etc rather than heating house air, when a normal mhr switches to bypass and just loses it

These things are very controversial; it does feel a bit like picking yourself up by your bootstraps.

There are some who get good results from them, e.g., a chap on the Navitron forum at the north end of Skye (no, not Skyewright) but his is a bit of special case as he pre-warms his incoming air by running it along the glazed ridge of his adjacent workshop (if I remember the details correctly).

There have been others (Nibe (sp?)) in housing association-type properties which have been the source of major complaints. It's difficult to know what's really going on, of course, but the response from the manufacturers at least makes sense - AIUI they say that the users aren't using the things properly; that they should be left running continuously rather than being switched on and off like gas central heating as running them at high power off cool inside air is very inefficient.

I'm not sure that trying to extract more energy from the exhaust air of MVHR makes much sense in Britain. An efficient MVHR should be exhausting air which is only a couple of degrees warmer than the outside air so there's little gain to be had in terms of sensible heat. In a dry climate the exhaust air might have significantly more water vapour than outside air and so usable amounts of latent heat but in Britain in the heating season the outside air is likely to be pretty damp anyway.

Dry continental air might be different. Perhaps that's why the Ecocent comes from Austria (I think).

Posted By: YanntoeThe Ecocent unit seems to take warm and possibly humid air from the room in which it sits and extract energy from it which is then stored in (hot) water. The cooler drier air is then released back into the interior of the building.

You could be right but my understanding was that the air from the Ecocent unit is normally exhausted from the building.

The Nibe devices (which are like big Ecocents but feeding to the central heating system rather than directly into a cylinder are intended as the primary heating system for well-insulated houses.

In this situation wouldn't the external humidity be less relevant

Possibly, but the situation which cjard mentioned was extraction of extra energy from MVHR exhaust air. In that case, comparison with extraction of energy from general outdoor air (ie, a normal ASHP) is relevant.

Oh, I've kinda decided on my heating device- I don't really have the budget for anything renewable in the first year so I'm looking at either just an immersion and a big tank (plug solar in later) or a combi boiler (boo hiss)

I'm not immediately motivated to think that a HSHRHP could meet my heating needs.. My father in law is keen on an ASHP, I'm happy to see how his works out (he has the semi on the other side) before fitting one

I have always felt that the Ecocent type units were primarily designed for use in climates that are hot and humid for all or most of the year e.g. Thailand, where they can cheaply satisfy two requirements to produce DHW and provide cooled dehumidified air when they are used with internal feed and exhaust.

I looked at installing something like this when I was building but could never find any proper details on their de-frosting/frost protection capabilities if used with an outside air supply. I have since come across the CTC Ecowater which can operate down to -10ºC and has properly documented frost protection and PV integration. Had I known about it at the time I would most probably have installed it.

My idea was to install about 12m² of home made solar air panels on the inlet to provide some preheating and reduce to risk of needing to defrost.

I think the ultimate solution would be to use Solar air/PV hybrid panels to preheat the supply and power the unit. In summer there is the probability that the pre-heated air would be above the higher limit of what these units are designed for so it might be useful to have the option to have supply and exhaust within the house at those times to provide some useful cooling.

Could you run ducts from wet rooms back to the room/cupboard with the ecocent in it? Therefore using its de-humidification abilities.

Sure, but not everyone wants to fit MVHR nor realise it exists nor have a house capable of running it "efficiently".

If you mean MEV or hole in the wall extractors, well at least by ducting to the Ecocent you are recovering the latent heat in the water vapour?

Ringi, don't all boilers that are highly efficient, have flue gas heat recovery? Surely a boiler is a flue gas heat recoverer.. It burns gas generating massive flue temperatures and then uses it to heat water, recovering heat from the burnt gas in the process.

In terms of boilers, I've been looking at the rinnai hd50 because it drives me nuts that combos cannot operate two outlets at once, so my solution is to go for a bigger combi, that can still modulate down. Rinnai also tout it as being compatible with a cylinder in the circuit, can have preheated water, solar thermal or PV immerrsion etc and sell all the gubbins for it so right now I'm wondering if my house has any use for an eco cent..

Well, the solar panels aren't going to be a feature of the initial completed build I think - i'll add those later. I do have a well ventilated conservatory whose volume is approximately half the house vol. A separate thread arising from an argument between me and my brother as to whether it makes a good air feed for an ASHP can be found here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13459&page=1#Item_18


Overall - the earlier comment about the ESP Ecocent 300 with its heat pump being the same price as a 300 litre akvaterm solar plus, is reasonably accurate.. I'm just not sure if there's any true benefit over getting a cheaper thermal store in a house that has an MVHR (as scavenging 'waste' heat is the MVHR's secondary job and a job a good one should be good at), unless one is looking for a reasonable way to generate hot water in the non-heating season when it could be viewed as a house cooler - if it costs £500 more than an equivalent TS, how many summers of cooling my house (and should the MVHR summer bypass not be enabled?) and generating hot water as it goes, is it going to take to pay that back?