Vitodens, Low-loss Header, TRVs & Single Pump

Mike T., Swampeast MO

Just placed this quote from Danfoss in another thread regarding using TRVs with a one-pipe system. Rather worth repeating in a different context me thinks...

"In two-pipe installations a small heat load leads to high temperature drops, while a small heat load in a one-pipe installation involves small temperature drop, since all the water is led through the bypass. This means that the flow temperature in a one-pipe installation must generally be as low as possible--depending on the local requirements--since otherwise the return temperature becomes too high, leading to excessive heat loss from the pipes. Regulation of a one-pipe installation thus requires the use of a highly accurate, central regulation valve, which in practice is very difficult because of different time constants. A two-pipe installation, on the other hand, has a positive feature in that if the flow temperature is too high, this simply results in the radiator thermostats regulating even faster and thus even more energy-economically."

This is a VERY good explanation of why you can use only a single, small circulator and don't need the low-loss header with the Vitodens <I>as long as you're using TRVs or FHVs in a two-pipe system</I>.

You merely use your reset curve to manipulate the maintenance flow in the system--this allows you to use only a single circulator of surprisingly low capacity and electricity consumption.

When you do this, something utterly amazing happens (at least it's amazing to me).

Your reset curve may result in a boiler temperature significantly higher than needed in most of the system (due to emission device imbalance or by adding "headroom" for quick setback recovery). BUT the temperature you find in the supply piping will (during maintenance) be significantly lower. Raise the setting of TRV(s) or experience a sudden drop in outside temperature and the temperature of the supply system draws ever-closer to the boiler temperature--once the system agains nears maintenance, the temp difference again widens!

Perhaps it's the high volume and mass of my system that is exaggerating this effect--after all it was originally a gravity system--but I'm starting to believe that this will occur with <I>any</I> system with similar control and a significant radiant output.

The supply temp I measure on the surface of the supply pipe about 3' from the boiler will be quite close to the average (measured in the middle) temperature of the warmest radiator in the house--so close that unless I use the boiler temperature to calculate delta-t (and thus flow) the resulting flow estimate is significantly highter than the built-in pump can possibly produce--even if it were running at 100% rotation speed!