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Advice on Heating Design: Solar Thermal / DHW / Hydronic Radiant Heat / Tankless Heater?

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Posted by rob.lee on April 17, 2007, 6:40 am
 
Hello,

I am in a new construction project and I'm trying to figure out the
best heating design.  I still have a limited understanding though I am
getting my educated through research, but I haven't found similar
setups yet.  I would greatly appreciate some insight and
recommendations.  I tried to design my house to be very green so it
will have:

1) A 30-tube solar thermal system that stores water in a 120-gallon
tank.  The solar thermal guy said this would be for domestic hot water
use.

2) A hydronic radiant heat system.

3) An on-grid 7.1kW Solar Electric System

I've read a lot about tankless heaters for use with DHW and understand
there are good options for both gas and electric tankless heaters with
thermostatic controls so they only heat water if the incoming water
(from the solar thermal storage tank in my case) is below the target
tempature.  I also read about the need for mixing value in case the
solar thermal water is too hot and above some saftely/shutover
temperature.

The solar thermal guy said that using the solar thermal only for DHW
simplifies the system and that it shouldn't be associated with the
radiant floor heating.

Design A
My understanding of the proposed design by the solar thermal guy is to
have a gas boiler maintain the water in the storage tank.  The solar
thermal system will hopefully be doing most of the work here, but on
consecutive cloudy days it will need to do a lot of work it seems.
DHW uses would draw from the tank.   I think the boiler in mind also
can be used on the water circulating in the hydronic radiant heating
system.   The radiant heating guy recommends a high efficiency natural
gas heater vs. electric because he says a 7.1kW system isn't large
enough.  He also says natural gas is much less expensive.   I'm ok
with that as my excess electrical power will just make my meter go
backwards.    My concern with this system is the standby losses of
heating the storage tank on cloudy days since it is a pretty large
tank.   Does this system make sense?

Design B
Would a tankless water heater or heaters be more efficient?   Should
DHW be supplied by a thermostatically controlled tankless water heater
that has the solar thermal system as a pre-heater?  Can the hydronic
radiant heaitng system also draw from the tankless water heater or
should it be a dedicated heater?
My concern with this system is the capacity of the tankless system to
supply enough hot water fast enough on consecutive cloudy days when
there are multiple people showering and the dishwasher is running...
and when the house needs more heating.   Also, is there criteria where
tankless electric is better than natural gas?

Thanks for taking the time to read this and hopefully you have some
recommendations or pointers to other relevant information.

Thanks,
Rob


Posted by DJ on April 17, 2007, 12:30 pm
 
On Apr 17, 2:40 am, rob....@gmail.com wrote:


But not all of them, so choose carefully.


True. With a 120 gallon tank and only 30 tubes, though, unless you are
planning long absences, it isn't likely to happen very often.


That's a bit... odd.


There does need to be a conventional system involved, yes.


Exactly. And good ones will switch preferentially between heating and
domestic use so that when you're in the shower, it doesn't decide to
start heating your basement floor ;-).


Agreed. Alot of the radiant systems use a 6kW or larger heating
element...


That's geographical, not an absolute, as is the greenhouse gas
calculations of natural gas versus electrical, which, as you said you
wanted your system to be "very green".


You don't heat the storage tank at any other time, other than with
solar.


In my opinion, sized properly, yes.


Exactly.


I would draw from the same heater if it was appropriate; some models
are designed for that, some are not.


That's simply a matter of sizing the tankless system.


There is also tankless natural gas, too.

DJ


Posted by Solar Flaire on April 18, 2007, 2:32 am
 Check out Rinnai tankless water heaters. They apparently do not suffer
from many of the design flaws other do. They are available cheaply
from eBay vendors. With this usage the water heater may only last five
to ten years but compared to a stainless steel boiler that may be
cheap at under $K

Make sure well water is heated, at some point, well over 110F.
Research Legionella bacteria in underheated water. It can kill you.



Posted by Jeff Thies on April 18, 2007, 3:40 pm
 rob.lee@gmail.com wrote:


   That seems to e a good size tank for a couple cloudy days of hot water.

  Underfloor radiant heat is well suited to solar temperatures (with
some tempering). However, you'll need a lot more BTUs to heat your
house than your hot water (in winter).

   You know we just had a huge long thread hear on hot water heater
efficiency and I am not sure we answered anything!

  It would seem to me that a convention hot water heater with added
insulation would approach the efficiency of tankless for a lot less money.

   Lets take a tank with 15 square feet of surface insulated to R30.
That is .5 BTU loss per hour per degree F. Say, water was at 140F and
ambient at 40F for a 100F diffence you'd have a loss of 1200 BTU/day.
(Each gallon of that water is ~ 700 BTUs)  That's a bit more than a kWHr
or perhaps a $/month of electricity. More detailed figures in this
groups archives, including an estimation by Nick with thermal time
constants that makes that figure seem high.  It seems to me that a small
well insulated tank heater would do the job. YMMV depending on local
conditions and how much hot water you use.

   You don't have enough collection/storeage for home heating. Seems to
me that you should have enough for nearly all your DHW year round though.

   Flat plate collectors will be cheaper to yield solar energy for home
heating.  The temperatures for radiant floor heating are much lower than
what is need for hot water and can be easily achieved using flat plate
collectors.

   Go get some numbers, you can get weather data from NREL if you are in
the states. Figure how much hot water you use.

<URL: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/  />

   Jeff


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