Re: OT : Plumbing help as well

[Bruno Prior <bruno@xxxxxxx>]

We are currently living in a house with a BAXI ventilation/heat recovery
system. We were also planning to install a BAXI system in the house we 
are currently converting. However, we just got caught by the new 
Building Regs. and the project was so complex that we had to employ an 
environmental consultant for the Part Ls. Surprisingly, he advised us 
that there was little if any environmental or cost benefit to a 
BAXI-style ventilation system.

The reasoning goes something like this. Heat exchange depends on 
temperature gradient. Heat exchange over a small temperature difference
is inefficient. We live in a temperate climate, where the number of days
when there is a large temperature difference between inside and out are
limited. The number of days where a ventilation system offers 
significant energy recovery is also therefore limited. Against this, you
must set the fact that heat exchange ventilation systems require 2 fans,
whereas normal ventilation systems require only one fan. The balance in
our temperate climate is normally that the energy saved by heat exchange
is less than the energy used by the extra fan.

Of course, this is simply one expert's opinion (although our heatpump 
supplier also held this opinion). And it is not hard to imagine how one
might design systems to get round it (allow the system to route round 
the heat exchanger and input fan on warmer days). But you should be 
careful with standard designs, as they are not usually this flexible. 
And if not, not only are you wasting energy and money, but you are also
returning the heat to already hot rooms in summer, when you really don't
want it.

One other observation on the BAXI system we are currently living with. 
It is designed for trickle ventilation - slowly refreshing the air in 
buildings that are intended to be air tight. But trickle ventilation is
useless in bathrooms - you want strong, intermittent extraction, not 
slow, steady extraction. BAXI is wholly inadequate for humid rooms. But
I guess the efficiency of the heat exchanger depends on the flow rate 
across the plates. There may not be a happy medium where you can exhaust
the air from the bathroom as quickly as possible and still recover a 
significant proportion of the energy.

I had hoped to do exactly what you want to do, but I am now persuaded 
that it is a better idea in theory than practice. Instead, we are going
for a (Unico) high pressure/velocity ventilation system without heat 
recovery but with fan coils connected to our heatpump, so we can run the
heatpump in reverse-cycle in summer to provide cold water to the fan 
coils to blow cold air into the building, provide neither hot nor cold 
on the moderate days to provide simple ventilation, and take a hot water
feed from the heat pump to the fan coils on cold days to provide blown 
warm air for a more immediate heating effect than is provided by the 
underfloor heating. Hopefully, this should be environmentally-friendly,
cost-effective and provide a comfortable climate year-round in the
house.

Cheers,

Bruno Prior


Simon Coates wrote:
> I'm going to install a 'home brew' heat recovery system for the
> ventillation.  I like heat recovery systems :o)  They are up
to 70%
> efficient - and work well when extracting warm steamy air from
bathrooms and
> showers etc...  The extract air passes through a heat recovery
chamber (size
> dependant upon flow rate and air speed) which heats up the
replacement
> incoming air.



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