Choosing a boiler again... but can anyone suggest any non-modulating condensing boilers in the 24-28kW range?. The one suggestion I've seen so far is Quantum Heatsave, I'd need too check physical size, cost etc so a choice of more than 1 would be good.
This would be for a Thermal Store system, for which flame modulation is (in theory) an unneccessary waste. On the other hand, I'm thinking I might be better off just 'going with the flow' and getting a standard (modulating) boiler regardless of the redundant fucntion.
Egremont.
PS - there is a background that's been gone into in another thread (which I'm still looking into...)
There's no problem in using a modulating burner for this. Indeed, it can actually be beneficial, as when the store gets nearly full, a non-modulating one can start cycling on the boiler stat when the store stat is not yet satisfied, particularly if the pump speed is fairly slow.
One of the benefits of a modulating one would be that it would modulate to keep the boiler in condensing mode. That would be useful towards the end of a cycle when the temperature of the store is reaching it's demand level and the return temperature would rise, given continued max output. I see this with my own (large) hot water cyl when it starts out on full whack, then backs off a bit towards the end, usually a 20min cycle with the last 5mins backed off to half power.
My Keston Celsius 25 has been reported here once as not liking driving thermal stores, don't know why but I recall it from way back.
You would be better off with a modulating one driving the radiators directly and the cylinder of the thermal store directly, with the thermal store used only for hot water.
In this way, when the heating requirement for the space heating is fairly low, as it is for a large proportion of the year in the UK, the boiler will run at lower temperatures and far more efficiently than it will if you always drive a thermal store with it.
If you drive a thermal store with a modulating boiler, it will wind up to full power for much of the recharging cycle anyway and you will eliminate/reduce cycling.
Lack of cycling is one mechanism for efficiency. Running at lower temperatures where efficiency is better anyway and heat recovery from latent heat can happen is the other.
Condensing boilers of a few years ago had an efficiency advantage over non condensing because normally they had larger and more efficient heat exchangers. More recently, non-condensing caught up.
Operationally, you want the thermal store to recover quickly. You won't be running the boiler at low temperatures during that time anyway, so the only efficiency gain comes from not having cycling. You don't need a non modulating boiler to achieve that.
For space heating, you are better off taking advantage of the fact that by directly driving the radiators, the boiler can modulate down and run cooler while still not cycling. If you put a thermal store in the middle, then you prevent that from happening.
The only exception to this is if you were introducing a "free" source of energy such as solar. In that case a thermal store is useful to combine sources. Other than that, it is a good way to heat the water but not a good way to maximise efficiency from a condensing boiler.
[Had to edit this post, line-breaks may be off...]
OK I've read many of your previous discussions in this area and didn't want to take you back over old ground!. I will mention a few things as relates to my installation:
1) The store has two 'stats to combat boiler cycling (you can blame Dr Evil)
2) My pump is indeed a bit slow (10L/min max), so much so that something will probably have to be done.
3) The water sent to the current boiler (from the bottom of the 1300mm store) seems to be surprisingly cool(ish) in relation to the overall temperature of the store, the space-heating take off is high up which probably helps. This should help condenser efficiency though may not be the whole story!.
As for the boiler... pendulum swinging towards a 'normal' modulating boiler I think. Apart from anything else, Quantum Heatsave seems to be the only known specialist option. BTW can you really still get non-condensers?.
Illegal. Only fitted in exceptional circumstances, which difficult to achieve. Stick to a condensing boiler, as they are cheaper to run.
Look at the Baxi, this can be non-modulating. The two stats work with any boiler and will enhance condensing as the when the stat call for hot water the cylinder will be pretty cool promoting efficiency.
Keston 25 was once on my short-list, partly due to it's shallow depth (a particular requirement) but negative comment from various sources on reliability and support, though not universal, has put me off. At this point, Worcester Greenstar R 24i (I think it was!) is one possible alternative on the fully modulating front, but I think selecting a boiler will be easy(er) if I can reasonably forget about non-modulation.
My choice was limited to only one or two boilers by a requirement for a really long flue and the Keston won on cost of both boiler and the low cost flue (plastic drain pipe) but if that's not important there are many others out there. I've not had any trouble with mine and I was able to talk to directly to one of their technical bods when I needed a hand at commissioning time but as I'm sure you've seen, that's not everyone's experience.
Build quality looks good on a chum's recently fitted non-condensing Worcester Bosch but that's as much as I know of them.
If you are going to hook the boiler up to a thermal store for space heating, it won't make a lot of difference because it will seldom run cool enough to take advantage of low temperature condensing operation. You will get bursts of high power operation as the store is periodically replenished and the return temperature will initially be cool. However, as the store warms, so will the return temperature, and to a point where the boiler is operating in the range of a conventional boiler.
If the condensing boiler drives the radiators directly, the flow temperature can be allowed to fall substantially and the return even more so that the burner will tend to run continuously at low output rather than cycling at a higher temperature range to drive a store.
Use of a store makes good sense for DHW purposes if sufficient mains flow is available. For space heating, unless other energy sources are being added in, it defeats the designed in optimisation of the boiler which will then run less efficiently.
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