No. When the store calls for heat the bottom will be very cool. This is the return temp, which is makes the boiler very effcient. Whhe the store warm sup this will gradually rise. So, for most of the re-heat time the boiler is operating effciently.
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Not in the least. Many modulate on the flow temp (maintain a constant flow temp). Those that modulate on load compensation will modulate down when the return and flow temps narrow down.
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I can think of other reasons to operate a heatbank - mains pressure hot water without using a combi for one.
Perhaps a good reason not to bother with a condensing boiler then (yes, I know about the supposed law mandating them, more b*ll*cks interference in my home by the government blah blah...)
Tim
Not so.
Not so. Any boiler will operate efficiently on a thermal store. It must be able to maintain a constant flow temperature.
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Non modulating condensing boilers are available. Boilers which modulat to maintain a flow temp, are perfectly suitable: W-B Greestar, Glow Worm. Condensing boilers are more efficient than non-condesning, even at hig temperatures
-- Doctor Evil
You've missed the point. While the boiler drives the radiators directly, the return can always be cool except in the depths of winter.
Your approach cycles the boiler in the same way as if the boiler is running a DHW cycle
This is not the most efficient operating mode for a modulating and condensing boiler.
Continuous, lower output will be more efficient than cycling into a store or cylinder even if the return temperature is low.
Yes of course and I didn't suggest otherwise. My point was specifically in regard to modulating, condensing boilers. If you were to use a conventional one then a long run is sensible.
I think that this is one case where it is worth doing something despite silly government meddling I've certainly saved considerably on gas consumption using a condensing boiler.
My point was very specific and is correct.
The point was clear and correct for a modulating condensing boiler.
I didn't say that it wouldn't work with a store and indeed with respect to hot water production makes good sense.
My point was purely about putting a store between boiler and heating load when the boiler is a modulating and condensing type. In that scenario, the boiler will not be operating in its most efficient way when just driving the radiators. THe effect of the store would be to cause cycling, and even if the return is cool will still be less efficient than running the boiler continuously at lower output.
There are two reasons for this:
- THe boiler won't have the possibility of operating at the lowest end of its temperature range
- It will be cycling (albeit occasionally) which is not as efficient as a continuous low power burn.
It will still not have the opportunity to operate at the low temperatures possible with a direct connection to the radiator load and it will also cycle.
I've made the point, it is correct for the scenario described (modulating, condensing boiler) and I am not going to repeat it.
A tall cylinder can have wide stratification. The bottom of the cylinder can be very cool indeed, especially when the store calls for the boiler. There is no inefficient boiler cycling either.
You can also have a section at the bottom of the thermal store that is kept to the temperature dictated by an outside weather compensator. Once again no inefficient boiler cycling and the boiler operating at very low efficient return temperatures.
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That's good, it is wrong.
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Not so.
A store of water between (a buffer) is the best thing as it eliminates boiler cycling.
Not so, cycling eliminated
It will if an outside weather compensator dictates the temperature of the bottom of the store.
Not so. Boiler cycling eliminated as the thermal store (buffer) prevents boiler cycling. Boiler directly connected to the rads cycle once the heat demand is below the lowest boiler output.
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Ah, my apologies. "Modulating" condensing boilers is a new one to me.
I'll probably get a significant gain just by fixing the roof insulation(!). Which is slightly non-trivial as a dormer conversion is involved and I don;t want to do anything to bung up the already limited airflow between roof and upper ceiling. Still thinking about that.
Tim
Most of the newer ones do do this, and the general mode of working is that when they do a hot water cycle (i.e. reheat the cylinder or store) that they go to full power - the philosophy being to get the maximum amount of heat as quickly as possible into the cylinder.
However, condensing boilers operate more efficiently the lower the return temperature. When below the dew point (at which condensation actually begins to take place) they become even more efficient (although realise that there is not a sudden Nirvana). In effect what happens is that the rate of change of efficiency increases below the dew point. Therefore the lower the return temperature the better. One aspect of this of course is that the lower the operating temperature, the lower the flue gas temperatures and hence less heat simply going out with the flue gases. The other aspect is that there is always an inefficiency involved with starting the burner whatever the type of boiler. Therefore the less times it goes on and off the better.
All of this takes you to the point that it is most efficient to run this type of boiler continuously at lower temperature than to force it into a situation of reheating a cylinder in bursts. For the hot water, you have to do this, but for the space heating you don't.
If this type of boiler is connected directly to the radiators and you have TRVs, as the room temperature reaches the set point, you have effectively balanced the building heat loss with the radiator outputs. THe TRVs will begin to close. If the boiler is directly connected to them, it is able to sense the reduced heat demand and modulate down accordingly. Thus, in the typical UK autumn/spring situation, which is a lot of days of the year, and you need *some* heat, then you can have the boiler operating in this equilibrium mode at low temperature.
If you put a store in the way, you will be emptying it at a low continuous rate of (e.g.) 8kW, but then when most of the energy has been used, replenishing it in bursts of perhaps 25kW - all controlled by a thermostat on the cylinder. Therefore you prevent the boiler behaving as it was intended to do - you force it to run at the high temperature end and even though it will probably be one continuous burn to replenish, there will still be more of those than if the boiler had been left running continuously at lower output.
THe two exceptions that I made were a) if it's a conventional or non-modulating boiler - there you would make an improvement by having it recharge the cylinder once as opposed to driving the radiators directly and cycling to match effective power output to load; or b) if you want to introduce other sources of energy such as solar. Since these change the economics, it may be that the overall equation works better by having a store in the middle.
The roof is generally the biggest loser of heat in a house and usually the easiest and cheapest to fix. However, it's worth doing the heat loss calculations. For example, if you already have 100-150mm of insulation in a roof space and nothing in the walls, you will probably get a better result from doing something about the walls than adding another 100mm in the roof. As you say, definitely avoid doing anything that reduces ventilation. This is asking for trouble.
If you are tight for space, you could consider sliding sheets of Celotex into the awkward spaces. It comes in various thicknesses and has about four times the insulating property for a given thickness than glass fibre mats.
Which is exactly what it will do when heat demand is les than the minimum output of the boiler. Most of them it is around 8 kW, some go down to 5 kW. You will get cycling.
But not enough and inefficient boiler cycling occurs.
Putting a thermal store between the boiler and the rads with the temperature controlled by an outside weather compensator will eliminate boiler cycling and operate at very low return temperatures promoting efficiency.
When house only requires 1kW of heat, it can just take this from the thermal store at the exact temperature dictated by an outside weather compensator. The boiler stays off until the store water drops below what the outside weather compensator dictates. Then it comes in to reheat to the required temperature in one long low temperature efficient burn.
Most modulating boiler modulate to maintain a flow temperature. These are perfectly suitable for a thermal store. Even those modulating on load compensation operate quite well too. Best to buy:
a) A condensing non-modulating boiler b) A condensing modulating to maintain a flow temperature.
These generally are cheaper and less sophisticated and less to go wrong. Keep it simple.
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Hi Andy
Yes - thanks for the excellent explanation.
It makes sense - though I have an overriding reason to use a heat bank in my setup. Not a problem as we already have a not-very-old back boiler which should be good for 20 years.
Do they make condensing back boilers?
I suppose that it would be possible to have an intelligent controller between the heatbank and boiler, that would "learn" the characteristics of the bank and based on various measurables (rate of temp drop at various vertical points for one) and could make a decision to run the boiler at partial output. Be quite a nice little project with a PC, then turn it into a micro-controller package.
If modulating condensors are indeed becoming the norm, I'm sure that one of the more advanced controller manufacturers will think of doing this.
Tim
It is not.
It doesn't, as it is incorrect. See my post on this.
Yep. Try the Gledhill range. They have "smart" thermal stores with a large pcb controlling all. It learn the characteristics of your boiloer and knows whe to bring it in. I have one. Try a BoilerMate on your back boiler - have the CH run off the store. Leave the back boiler temp full on an the store does everything else. No clicking on and off noises in your living room. It will reduce fuel bills.
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(Changes topic)
I was looking at Celotex/Kingspan with some interest. I know that "reject" panels can be obtained at lower cost.
Now my quandry: Bungalow with pitched roof + later (20 years ago) added dormer conversion. 4" roof timbers, space between roof and plasterboard assumed to be stuffed with glass wool. Dormer roofs themselves are approx
6" timbers(!) with a miserable 4" of glass fibre (noted when facias replaced). Remaining roof voids outside of the upstairs floor area have a mixture of knackered wool on the underside of the roof and bits of crap on the floor.I don't really want to have all the plasterboard off, so I'm proposing to redo the voids first. Getting rid of the crap on the floor would be good as it's nasty to route cables through - I would like the voids to be reasonably clean for storage purposes.
I was considering either:
a) Put 50mm celotex on battens to leave 2" gap between tiles and celotex but this would add complications with adding soffit vents everywhere, and I'm not sure if doing so would impede the already dubious circulation of air to the bits of the roof which are packed with glass wool and then have plasterboard underneath for the dormer ceilings.
b) Replace 4" glass wool under the tiles and use breathable membrane to contain dust. Ditto: not sure if this would affect airflow, seems that it would.
I don;t believe there are much in the way of ridge vents, but the timbers (that I can see) seem fine and when the dormer felt was replaced last year, the builers found no evidence of rot.
The aim here is not to spend a huge amount of time and money to do it "right", but to merely improve on the useless crap that's there now in teh most practical way. Surprisingly it's actually quite warm up there already and the gas bills are sane.
Next time the roof needs re-tiling, then I'll consider adding insulation between the tiles and top of rafters.
Any ideas gratefully received.
Tim
There are a number of models now going down to around 4kW.
One also has to look at the amount of heat required, the number of days in the year and times of day as well. For the large proportion of days and the UK climate the heat load is likely to be at or above the boiler minimum and continuous operation will result.
With your approach the boiler will *always* be forced to do high power burns into the store so there will never be an opportunity for it to operate in its most efficient range.
This of course is untrue.
This is nonsense under the conditions described which are more typical of the real world.
This is an attempt at obfuscation.
This is an attempt to alter the agenda.
I haven't seen any.
Your scenario is a good reason to put in some thermal buffering.
That could be worth doing, although I fear that you might have relatively little control over the boiler behaviour. Really it has to be hot or off. Nevertheless I think that you could probably do some predictive stuff to make it more responsive in anticipation of demand and to turn it off in good time rather than overheating the store. I presume that you are going to fully pump it. That will also give you some good opportunities for control by timing the pump on off ahead and after the burner.
Given what you have, I think it's definitely worth a go.
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