[Greenbuilding] Baxi Luna Question

[George J. Nesbitt]

Yes your HVAC contractor is partly right. But it doesn't cost $4000 to 
add an *expansion* (buffer) tank (needs to be installed anyway) and a  
*buffer* (holding) tank (a small electric water heater tank probably 
less than 10 gallons in size). The plumber underbid the job and is 
trying to make up for their lack of experience.

Lets Review how the Baxi works (this also applies to the Quietside.com, 
Monitorproducts.com and system2000.com, 3 similar products that I am 
aware of). When you need heat hot water is circulated through a *closed 
loop* that may have additional loops  (zones) coming off of it. All 
closed loops should have a buffer tank to accommodate the expansion and 
contraction of the water heating and cooling. When Domestic Hot Water is 
called for the hot water is run through an internal flat plate heat 
exchanger to heat the DHW, so their is no mixing of "potable" water. The 
baxi will shut down the heating loop only if it can't heat the DHW, 
other products always give full priority to the DHW. An advantage to a 
closed loop system is that the expensive boiler is protected from water 
quality issues. I hear lots of stories about people going through open 
loop tankless water heaters that may be DHW only or Combined Hydronic 
(DHW & heating). Tankless water heaters aren't inexpensive, and boilers 
are even more expensive.

So every time the unit turns on there is a small delay as the unit 
detects that it must turn on, and to turn on. It take some time for the 
unit to warm up to reach it's "steady state operating efficiency" and to 
burn cleanly. There may be a minimum run time, and there is a post cycle 
shut down. The Baxi has a minimum heat output of 35500 Btu/hr, and if 
your load is less than this (if you are building green you should have 
less than this unless it is a really large house) it can cause the unit 
to short cycle, and this can confuse the electronics and it will 
malfunction. You will also not be spending much operating time at 
"steady state operating efficiency", cause more pollution, and use more 
fuel, increase cost, and maybe reduce service life. The holding tank 
will increase the volume of water that needs to be heated so that the 
unit can fire properly.
Its a small electric water heater probably less than 10 gallons, and all 
that is needed is to plumb the loop into and out of it, thats all, 
shouldn't cost $4000.

Your load (although you did not say what it is) is too low for all the 
products of this type without a buffer tank.

Just think about how you use DHW, and whether is is on/off on/off or 
constant long draws! This is one reason I am not sold on tankless water 
heaters, if you water use is small and infrequent they can save as much 
as 50%, but if you use the "average" its 15% at most, and many people 
end up using more, because now they never run out of hot water! (thats 
not very green).

The Baxi has only a small capacity to heat DHW, 3.2 gpm at a 60 deg F. 
delta T, so you can't have much more than 1ea 2.5 gpm showers at once, 
and a tub takes 5 gpm. The other problem with the Baxi (according to 
Skip Hayden a Canadian gas researcher) is that it doesn't control excess 
air very well. This means that more air is available for combustion than 
needed, it effects efficiency and pollution. A "green" boiler would 
control excess air and be condensing to increase efficiency and reduce 
greenhouse gases. But be carefully, the water returning to the boiler 
must be below 130 deg F. or it won't condense!

So what are you options? You could go to a more traditional ssetup with 
a boiler/heating loop and a storage tank for DHW, you would still have a 
problem due to small load, so you need a Munchkin (Heat Transfer 
Products) boiler that can modulate as low as 10000 Btu/hr. Contractors 
cost for boiler is approximately $2500 and up, a plumbing freak show, or 
now they have some nice preplumbed packages to simplify installation, 
lots of pumps at 85W each! What I don't like is that you have all this 
hot water sitting there for you DHW and you don't utilize it on the 
heating side, but it a closed loop system which is not a bad thing. I'm 
just not sure this is the best for a "green" home, or the most cost 
effective.

You could go to a condensing tank water heater, contractor cost $3000 
and up. American, A.O. Smith, Bradford White, Heat Transfer Products 
have these. HTP has it with an internal heat exchanger so you heating 
loop would be closed loop, with only one pump. The others you will have 
to have a heat exchanger and 2 pumps if you need a closed loop system. 
One way to minimize the potential problem of an open loop system is to 
have the pump turn on and circulate water for a short time ever so 
often. Of course water can stagnate in both the hot and cold DHW lines 
also (I think my whole family got sick drink some water from a faucet in 
Elk City Idaho 5 or 6 years ago from a cold line on a house that had 
been vacant for several weeks). At standard test conditions these tanks 
use only 50 therms less natural gas than a low efficiency tank water 
heater. They have very high Recovery Efficiencies 94-99% your heating 
AFUE will be somewhat lower, and a DHW Energy Factor of .80 =/-. The 
GAMA directory is a good place to go to learn about the efficiency of 
some equipment. www.gamanet.org

You could use a standard tank water heater, I would recommend a sealed 
combustion power vent model. You can get to about  .81 AFUE and .64 EF. 
Bradford White has an internal heat exchanger model. Of course you have 
lost any benefit from going to a condensing water heater. Contractor 
price $800+.

There are a lot of advantages to combined hydronic systems, there are 
some downsides too. I would prefer to use the storage tank for both DHW 
and heating.

Combined Solar Hydronic. Of course we would love to use solar for as 
much as we can. For my house I am considering a SDHW system, flat 
panels, with either glycol or a drain back, single pump (PV) storage 
tank with built in heat exchanger. I will heat my house with a fan coil 
(forced air hydronic), I have not decided whether it will be open or 
closed loop, I would prefer closed to protect the heat exchanger in the 
fan coil, but only if I can do it with one pump. I will use a condensing 
boiler in a closed loop to backup the storage tank, when needed. 
Combined Solar Hydronic is not the best match, because when we need the 
most heat is when we have no sun (night) or not much (cloudy, rain, 
snow). But with a low energy house and the right control strategies 
could contribute significantly.

Radiant Floor Heating has sure gain wide acceptance as "green" but I 
would highly recommend that anyone doing it on slab on grade 
construction insulate under the slab, in addition to the perimeter. I 
haven't seen any evidence that it is inherently more efficient than 
forced air. I think that if done right they will be about the same, but 
the forced air will be less expensive, and can serve cooling and 
ventilation (HVAC) also.Of course most forced air systems greatly under 
perform and probably worse than hydronic systems?. Environmental 
Building News did an excellent article on the issue in Volume11, #1, 
January 2002

Anyway just a few of my thoughts.

George J. Nesbitt
Environmental Design/Build
Building Performance Contractor
HERS Rater
Oakland, CA
geoedb at idiom.com


Eggert, Thomas L wrote:

>We were to put a Baxi Luna combined boiler into a new house that we are
>building.  It was to supply both the domestic hot water and a
>multi-floor radiant system.
> 
>The plumber that our builder found had no experience with the Baxi Luna
>system and gave us a bid for about $3000.  He now wants an additional
>$4000 because he is telling us that the system has too much capacity for
>our uses and that he would need to install both a buffer tank and a
>holding tank.  It would seem that he wants to convert a tankless system
>to a tanked system.  It does not make any sense.
> 
>The radiant system would service about 1000 square feet of floor.  In
>addition, we would provide the hot water for an upstairs bathroom.  Does
>it make sense that the Baxi Luna system has too much capacity for this,
>and that the only alternative is to install these storage tanks?
> 
>If we would need to give up on the Baxi Luna system, does anyone have
>any recommendations of very high efficiency systems that would work for
>both domestic hot water and a radiant system?
> 
>Tom Eggert
>
>Madison, WI
>
>"If you want something really important to be done, you must not merely
>satisfy the reason; you must move the heart also"
>Mahatma Ghandi
>
> 
>
> 
>
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