twh/tex;
Boilers start-ups on a poorly designed system can mean a long wait to get any heat out! It is even possible to stall a boiler indefinitely and it could be literally
raining in your boiler firebox, esp firing nat gas, which will make
8 times the weight of the gas burned as water! If that can't get out of your stack as steam 'cos its too cold....
. The answer is a valve between the hot outlet and the cold inlet ports on the boiler. This bypasses hot water back to the boiler inlet at start. An automatic model will drop out when you hit the target boiler temperature and will respond to load too. Modern lightweight fabricated boilers will corrode out
pdq if run cold for too long- cast iron is more tolerant.
Here's one automatic adjustable bypass valve type:
http://ecc.emea.honeywell.com/products/ecatdata/pg_du145.htmlThese cost about =US$45 in France.
I used a cheap manual bypass-valve, [set at 10% bypass by experiment], but then my boiler is a big iron-lump!
I'm running set-back with no probs. We have to wait 5 hours to get the floor hot from cold [1 time per year, September], but I have also got reversible A/C and a woodstove for backup. Once the 12 tons of concrete floor slab heat up, a 2 or 3 hour am burn raises the slab to give us 20C [68F] for the next 24 hours, but we do live near the Altantic and usually have mild winters, unlike Continental places like Vienna or ND. We target 20C [68F] from 6am through 11pm, then program minima 16C [60F] at night, and these temps are remarkably stable. The boiler has
never fired at night, even at -15C = 0F outside - the slab never cools fast enough.
Underfloor will
never work properly without the investment in insulation. [ min 20cm = 8" Rockwool or equivalent, - I also put 100mm of extruded PS under the slab - and draft sealing, double glazing etc, investments with a payback of 25% or more on capital]. It can't cope with rapid temperature variations or too high a demand. You
must limit the heat requirement to less than 100W per sq meter of floor area, for no underfloor can emit more than that without broiling the dog!
To reduce losses further, I fitted a heat recuperating ventilation system to run 24 7 365. Then I got the first summer's POCO bill after lashing out $300+ on CFLs! A 100W vent fan 24/7 = over 200kwh per quarter with
her indoors having all the doors and windows thrown open! Now it only runs in winter! Live and learn.
