I am the proud owner of a VRC430 which is set as a roomstat in my main room. The boiler is an EcoTec 415 open-vent boiler fitted in a fairly standard Y-plan setup via a VR65. House is small 3 bed semi with cavity insulation etc.
I'm a retired person and very keen to be economical with the gas for financial reasons as well as carbon footprint. I only heat the living room, kitchen and bathroom as a matter of course and usually only to 18C.
I have had ever such a time getting this to work acceptably. I have spent solid weeks measuring, theorising, writing to Vaillant about what they should do to make the boiler work better, modifying the plumbing and tuning. In the end, I have a workable solution with limitations.
I'm now using a heating curve of 1.5 which is down from 2.5 in the early days. I also have the minimum temperature set in the VRC430 to 45C because it doesn't work at all at lower temperatures. With these settings, steady temperatures are well maintained but it is hopelessly unresponsive. I believe that there is no alternative to trickery to get it up to temperature in the morning or to raise the temperature rapidly at other times.
Your questions:
1) Does lowering the heating curve reduce energy consumption?
I have seen graphs showing that there may be up to 8% more efficiency from running at a return temperature of 30C as opposed to 50C. This related to some sort of generic condensing boiler but is an indication that there is something there worth going for. The fact that the boiler runs for longer should not be relevant apart from energy consumption in pumping and any losses in the pipework which don't contribute to the heating of the house. Room temperature should be contained within a narrower range.
2) Why do Vaillant recommend such a high (expensive) heating curve for the UK?
I think it is because they have found that this is more likely to work for their customers so that they get fewer complaints. They would probably say that their boilers are very efficient at high temperatures anyway. My own analysis is that there is a "feature" of the Vaillant boilers operation which is challenging in low temperature operation and this may well be common to other manufacturers. The difficulty is possibly most apparent in smaller and/or better insulated homes. You may have noticed that when the boiler fires up it runs at a fairly constant rate for about the first minute. My boiler is rated to modulate between 5 and 15 kw. In the first minute, it runs at 12kw. Given the minimum acceptable flow rate through the boiler, that gives a rise of about 16C. If the heat requirement of the house falls below 5kw the boiler has to cycle. Once the boiler has switched off, it cannot start again until the return water has fallen to about 15C below the target flow temperature. This is quite quick at high temperatures but it gets increasingly slow at lower temperatures. The details will vary with every installation. In my house, at 70C it takes 10 minutes, at 45C it takes 35 minutes. This is made more difficult because the VRC430 reduces the target temperature as room temperature rises and also if the outdoor temperature rises. This is what happens in the morning. So successful restart is even further delayed. Setting d2 so that the retries (which are time based) have a chance of succeeding is a challenge.
My radiators are on microbore (I upgraded to 10mm for the large living room rad) so a significant proportion of the water going through the boiler goes through the automatic bypass valve and staight back to the boiler. This leads to a bad effect of unsuccessful restarts - they delay the possibility of a successful start while the extra heat is dissipated. I have now worked out how to avoid failed starts - set d2 to 60, d1 to 38, keep the kitchen radiator on and don't try temperatures below 45. I changed the pump to a more powerful 6m head model to get more flow and added a well-insulated 3.6 litre radiator into the bypass circuit to stop the heated water getting back to the boiler until the first minute is up and the boiler is ready to modulate down to 5kw.
All in all, it is much simpler to set a high heating curve. Lower efficiency and lumpy heat delivery are the consequences.
3) What heating curve does any one else (installers/users) recommend?
I would agree with some of your other posters. The lower the better while it continues to keep you warm and is responsive to changes. When I bought this kit , I thought that 5-15kw modulating would mean what it said and I had sized my radiator to work at about 1.2 heating curve. I was horrified by my early experiences but I'm fairly pleased that 1.5 works quite well. I doubt I will notice any effect on my gas bill which has been low throughout because the house and boiler are quite efficient.
