On Mon, 11 Dec 2006 13:05:59 GMT someone who may be "Jim Alexander" wrote this:-
Only if one doesn't know what one is doing. Otherwise one increases one or more radiators to suitable sizes.
The fact that the Westminster government ignored this basic design requirement when they came up with various rules about boiler replacements simply tells us that they don't know what they are doing, which is hardly an earth shattering conclusion as they have demonstrated this many times before and since.
For 90% of the year radiators are oversized (or should be). Otherwise turning the boiler stat up reduces the boiler efficiency, but it will still be more efficient than a non-condensing boiler.
You would typically ballance the system to achieve a bigger drop over the rads (about 20 degrees instead of the traditional 11). So it is possible for example to have 70 degree flow, and 50 return. High enough to heat the water to a suitable level, and still have the return level below the dew point for optimum condensation.
I don't dispute your use of the word "condensation" but I do dispute your use of "optimum". Anyway, maybe I want a thermal store next time. I also reject the implication of others that the boiler flow setting can be fiddled with throughout the year and still deliver the parameters you suggest. My demand of a modern system is that should occur automatically. Unfortunately the "bog standard" replacement boilers in the market don't provide this. Probably available at a price. Suggestions anybody?
Jim A
Eco-Hometec, Viessmann.
Or fit a Danfoss Randall BEM 5000 weather compensator to a simpler boiler. It links the flow temp to the outside temp and has a DHW function that will raise the temp to get DHW over 60C. Have a"diverter" valve to give DHW priority. Go to the Danfoss Randall site, they have the instructions and diagrams.
Perfectly good advice which I would incorporate in a new build or a complete system replacement (with a thermal store of course) but most people doing a straightfoward boiler replacement with a mainstream market product are being duped about the "numbers".
Jim A
In message , Jim Alexander writes
This vaillant we have definitely does have an anti- legionnaires disease function via the vrc 400. It's only a box of chips though. The fact that there are some motorised valves and the boiler knowing whether its heating the cylinder or the rads should be enough to allow the act of heating the cylinder to beyond a certain temperature for a short period now and again. If I understand correctly you may have to accept that it isn't condensing when this is going on though.
Thanks again Mark. I fully accept that it needn't be condensing when doing anti-legionaires function. From the manual and VRC 400 brochure taking account where the zone valves are connected and the network connection to the boiler I can't understand why the system can deliver the anti-legionaires function and not separate temperature control of the water circuit which is what I really want. But I asked the question of Vaillant and they said no. On that basis I'm not pursuing the Vaillant option and will keep my 79% efficiency boiler going for a while until an affordable solution appears. The efficiency hit is only costing me £25/year and I'm comfortable maintaining my current boiler. Pity because the Vaillant seems a nice system.
Jim A
If you set the system for the parameters I suggest then it would meet your stated requirement of having the hot water hot enough to kill bugs whilst maintaining good efficency of the boiler.
If you want to vary the flow temp over the year in response to changing wheather then that is a different set of requirements. Ideally you would oversize the rads so this is not a requirement, however an external weather compensator can be used with some boilers to achieve proportional control of the flow temperature in response to changing environmental temperature. You could also implement proportional control within the house as well if the boiler has the capability.
If you plan to change to a thermal store in the future, then that counts as changing the requirements again - so you need to be prepared to change the solution as well.
Indeed. As with all things, if you want advanced control facilities then you need to pay for a more sophisticated boiler. Boilers at the Man Eco hometec level will offer these facilities, as I am sure others will to.
I believe Ravenheat do a system boiler that runs up to maximum boiler temp when the cylinder stat calls for heat. The stat is wired directly into the boiler. That is affordable. They offer 5 year guarantees now.
Thanks, I'll have a think about that. Worcester Greenstars have an optional internal diverter valve which I suspect might work except I think diverter valves are daft (and I'd have to mod my pipework).
Jim A
Sorry, 3 yr guarantee.
Well if you want the boiler to run up to maximum temperature for DHW, and run at a lower user setting and modulating temperature, then a "DHW priority system" has to be used with a quick recovery cylinder (not Part L which is NOT quick recovery) and a "diverter" valve (not mid-position), or two interlocked pumps with non-return valves. If the boiler only comes in system config with integral pump then only a diverter valve will do.
A diverter valve inside the boiler casing is a bonus I would have thought, saving space. It is easy to get at in the W-B.
The best is a simple single rated burner (not modulating) coupled to a thermal store. This is a simple reliable boiler. It throws all it heat at the stores DHW section. When DHW is satisfied it then throws all its heat at the stores CH section which is temp controlled via an outside weather compensator (this promotes condensing efficiency). Have the CH circuit with all TRVs and a Grundfos Alpha pump. The cylinder is a boiler buffer and a CH buffer too, as well as giving instant high pressure DHW. The weather compensator prevents boiler cycling and promotes condensing efficiency.
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