Planning to replace my old combi with a new boiler. I plan to use a Vaillant Ecomax 635e system boiler hooked up to a
250litre unvented cylinder. Im thinking about using a weather compensator:
I can't comment on that boiler specifically, but I do have one (MAN Micromat) with a weather compensator based on the same principle of monitoring the outside temperature and adjusting the boiler output to match.
It works very well in terms of maintaining the set temperatures in the house without needing to cycle the boiler on and off. For example, I noticed this afternoon that there was a drop in temperature of about 2 degrees over quite a short time. The boiler had been poodling along at about 5kW output across the middle of the day, but began increasing output as it became colder.
The room thermostat is also "analogue" in behaviour in that it will indicate temperature changes to the boiler, again modulating it rather than cycling it. This results in very constant temperatures - typically under half a degree of variation. I notice from the brochure that the Vaillant one has this feature as well. Again I'd say worthwhile.
I also have the same thing for the hot water cylinder - i.e. an analogue sensor rather than strap on thermostat. This fits into a pocket in the cylinder. With this, the boiler switches to hot water mode when the temperature is 5 degrees below the set point (adjustable) and then modulates down at the end of the burn to avoid overshoot and higher temperature than the set point. This is nice, but I think not as important as the other two.
One comment that I would make is that if you are going to go for a large cylinder and 35kW boiler, do make sure that the cylinder is a fast recovery type - not just a part L1 model. Otherwise you will find that during HW recovery, the boiler may well modulate down to half to 2/3 output if the heat isn't transferred fast enough. Most of the major manufacturers have these - e.g. Albion cylinders. etc.
In message , Andy Hall writes
RMC Ultrasteel; I believe they are fast recovery.
In message , mark writes
:)
OK so no opinions on the boiler then :)
More food for thought. rcm say that the cylinder will reheat in 28 minutes given 21kw of input. My calculations indicate I need about 14kw, sedbuk site says 22kw (!!!) I plan to have a y plan type diverter and I'd put up with the odd occasion that I'd need the cylinder re heated from scratch for 28 minutes.
Does this mean I could go for a smaller boiler? I could use a 622e as this pumps out 22.4 kw or is this too small a margin?
How did you do the sums for the heating? Did you take into account air changes?
If you mean that SEDBUK says 22kW for a house of a given type, that's pretty meaningless. It's about as useful as the leaflets in B&Q which tell you to measure the room, add a fiddle factor and multiply by aunty's telephone number.
There are a number of web sites around that purport to do the sums - these seem highly variable.
The only two methods I would trust are
- Radiator manufacturer calculator programs (e.g. Myson, Barlo - I have the Myson one if you want it) but check the U values they use with those in the Building Regulations Approved Documents and other sources.
- Calculator or spreadsheey and do the sums by hand.
28 mins reheat is not super-fast and you could get better than that if you wanted it.Remember that if the boiler is reheating the cylinder then the house will begin to cool - more rapidly if not well insulated. If you have any rooms where there is a particularly large heat loss because of exterior walls and large window areas then you will notice more quickly.
It doesn't matter for the cylinder. It would be OK for the house if it really is 14kW.
Both boilers will modulate, although curiously, the 635E goes from
10.5 to 35kW, whereas the 622E is from 13.4 to 22.4kW at 80/60 operation.
Avoid an unvenetd cylinder.
The boiler is good. Try the Glow Worm condensing range. Vaillants inside (same company) but cheaper....and very quiet.
The weather compensator is worth it. Also zone off the CH upstairs and have a stat/clock on downstairs and upstairs. That will save a hell of a lot of gas not heating parts of the house unnecessarily.
The Glow Worm HXi 30 (Made by Valliant) goes from ~5kW to 30kW. Mate this to a heat bank and Voila!!! Mate it to an integrated heat bank and have the CH off the store in two zones (a flow and return on the store for each zone) and an optimised stat/programmer on each zone and you will not be disappointed. May as well do it properly.
In message , Andy Hall writes
I did wonder..........
I think I do have one of these somewhere; pretty tedious as I recall. Perhaps I will impose on you if I can't find my copy ?
I could do them as well :) Again it gets pretty tedious :)
Hmmm.I shall look into that...
More sums then :)
Either way, you do need to make sure that you have correct U values for the different materials used in the construction of the house - especially external walls and roof - these make the largest difference to the outcome usually. The other is to make reasonable allowances for air changes. There are standard figures for different room types. If you have an older property or a draughty one, then allow a bit more there.
If you want the Myson program, drop me a mail. (andyh at hall dot gl)
Albion is one vendor, but most have one.
There are boilers on the market which will go to a lower output. 7kW is fairly common now and you can get down to 3-4kW but these tend to be more expensive.
The advantage is that they will cycle less frequently and that does save some energy.
If you are doing this renewal exercise, you could also consider running the system at 70degrees/50 degrees rather than the conventional 82/70. This can be done anyway in spring and autumn by turning down the boiler thermostat. You will enable the boiler to operate at a better place on the efficiency curve. Basically efficiency improves with reducing return temperature and below the dew point (about 54 degrees) condensing takes place and the rate of improvement of efficiency with reducing return temperature increases. There isn't a magic step change when condensing happens as some people think.
If you want to improve efficiency in general, you could look at designing radiator outputs to match the heat losses at 70/50 rather than 82/70. Radiator manufacturers publish a table of correction factors which are applied to get true output for a given mean water temperature (i.e. (flow + return) /2) and air temperatures. The nominal figures are based on an EN standard testing method using 90 degrees. The practical effect is that for a room at 20 degrees and
82/70 operation, the factor to be applied is 0.9. For 70/50 it's about 0.6. In other words, if you drop the temperature, you need the radiators to have about a third more output.I did this exercise when I upgraded my system. I checked each room and its radiator and found that several were oversized and could be left alone, some were moved to different places, and three were replaced with double panel where they hadn't been before. It was a reasonably painless exercise when they were all removed for flushing anyway.
The effect is that in spring and autumn, my system will drop to around
40 degrees flow and in winter, seldom exceeds 60 degrees.Obviously YMMV depending on whether the radiators were oversized in the first place.
Matt, the Glow Worm goes down to 5kW and it is far from expensive.
Andy, any chance you could send me the Myson one? My e-mail address is real.
Cheers, Richard
On its way
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