difalck said:
I've seen other people mention this topic but still not sure of the answer.
I have a Vaillant Ecotec Pro28 condensing combi boiler. Do I achieve this temperature drop between flow and return at the boiler by shutting down the lockshield valves at the radiators, or is there another way to do it?
1) ANCIENT HISTORY
In the old days, when I were a lad, etc., domestic heating was designed on assumptions of -1 degC outside temperature, and 82 degC flow, 71 degC return, hence the 11 degC temperature difference referred to above by Corgiman.
-1, 82 and 71 degC were used because they were nice round numbers in degrees Farenheit (30, 180 and 160).
You had to keep the return above 60 degC or you got condensation in the flue gases. The condensation, being acidic, would rust the return end out of the cast iron boiler (back end corrosion). We didn't have condensing boilers in the old days. Well, we did, but not intentionally.
2) NOW
Now, with condensing boilers, you want to make the water vapour in the flue gases condense, because then you are extracting the latent heat of condensation from the water vapour in the gases. So you've got to keep the return below 55degC for as much of the time as practical. The less-than-55degC return is the important bit. Maintaining the 20degC differential by balancing the flows is important, because an excessive flow through a radiator(s) will result in an elevated return temperature from that radiator.
3) THE PROBLEM
The problem is that now we are fitting condensing boilers, designed for 70/50 (or 60/40, or 50/30 even) onto heating systems that were designed for 82/71. The radiators won't supply their rated heat output when supplied with water at 70 degC and the rads won’t remove sufficient heat to return the water 20 degrees cooler.
4) SOLUTIONS
You could fit bigger radiators, but as an investment, it’s not very cost effective.
You could fit UFH, so the whole room floor is one huge radiator. It’s a huge PITA in an existing, occupied property, but UFH and condensing boilers are a very effective combination..
The most cost effective solution involves recognizing that the -1 degC outside temperature is a design assumption whilst applies only to the design day, the worst case.
For most of the heating season, it isn’t -1 degC outside, so you don’t need 82 degC flow. You can, for much of the year, reduce the flow temperature and so reduce the return temperatures. How do you do this?
Your boiler has a modulating burner, so it can reduce it’s flow temperature and heat output if used in conjunction with the appropriate room temperature controller. The appropriate controller would be the Vaillant VRT 360. However, you need to study the manufacturer’s instructions and ensure it is wired and set up as an analogue (0 to 100%) device. They can be wired and set up to operate as an ON/ OFF device, i.e., 28kW or 0 kW. They most often are set up as ON/OFF thermostats, usually the installers don‘t understand the controls.
The instructions for both the boiler and controller are available on the Vaillant website.
Do a search for the VRT 360 in the archives here, the question has beeen answered by Agile before.
Also a similar discussion can be found here;
http://www.screwfix.com/talk/thread.jspa?forumID=25&threadID=52309&messageID=547371#547371
