Found this post by Water Systems (about half way down this http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3494&page=3 page.

Cranfield University did studies and concluded that 15% energy savings can be had by using thermal storage. A Google brings it up.

The lower the return temperature of a condensing boiler the greater the efficiency. If you look at a gas condensing boiler graph, the boiler starts to condense at around 53C return water temperature. That is the key point. Just above and below doesn't make that much difference in efficiency. One the return temperature drops to 30C the efficiency rises sharply.

At 65C the efficiency is 87%
At 53C (dew-point) the efficiency is 89%
At 30C the efficiency is 97%
At 25C the efficiency is 98-99%
At 20C the efficiency is near 100%

So 10% more efficient using lower temperatures, which a heat bank gives. A cylinder heated by a coil will not get near the condensing efficiency re-heat figures of heating a cylinder directly.

Look at this:
http://www.keston.co.uk/downloads/pdf/cel25-b.pdf
There is a graph here (the boiler is now discontinued)

A heat bank using a plate heat X will give temperatures at the bottom of the store at 5C above the incoming mains temperature (see Any's post to confirm this). That could be 10-15C in the bottom of the cylinder, while 65-70C at the top. When the boiler kicks in the boiler efficiency is approaching near 100% on most of the cylinder re-heat. Have the heat bank set to 65C and the efficiency is beyond question.

HTH,
Bri.

Have I got this right? :- Non- condensing boilers had high exhaust temperatures in order to avoid condensation at ANY time, which would cause corrosion of the heat exchanger. This meant they wasted heat via the high exhaust temps, AND by failing to extract any of the latent heat available by condensation.

Condensing boilers are resistant to corrosion, and so for a given return temperature can operate with a lower exhaust temperature, even though this may mean condensation happens only some of the time.

So - condensing boilers are more efficient, even when they are not condensing.

Also, am I right in thinking that some of the condensing takes place in a heat exchange between exhaust and incoming air? If so that would allow a contribution to efficiency from condensation but with a higher return temp than you might otherwise expect.

The figures Vaillant quote for my boiler are about in line with brig001's figures above.

The thermal store argument goes like this: With a conventional boiler coil you are limited in how much heat transfer occurs in the coil. So if you put in water at 70C it might come back at 50C, thus limiting condensing efficiency gains. With a directly-heated thermal store you put hot water in the top of the store whilst taking cold (20c?) water out of the bottom. Exactly how well this works depends on how much your boiler will raise the temp in one pass (and is often complicated by having a TMV to mix return water in order to able to set the ouput temp to top of tank to (e.g. 70C)). But the basic point is that with direct heating the return water will usually be much colder than it is when heating via a coil. This is actually apoint about direct/indirect heating, not cyclinder/thermal stores per se, it's just that a store is often heated direct.

I think you are right that summer operation is probably least efficient. Which helps make solar a good idea. Of course winter operating being efficient does require a low-enough return temp from the CH. Is it, in practice?

Hi,

Its worse with oil as the dew point is 47 deg and realistically the max temp rise you can achieve throught eh boiler jacket is 20 deg. so thats an outlet temp of 67 deg. Problem is if you check the brochure specs you'll find many that quote 50 deg as the min return temp.
many europeon oil boiler configuration use the thermal store and the return water mixer. In the uk we tend to operte boilers in a very uncontrolled way. If you can balance to achive a 20deg rise throught the boiler, you wont get a 20 deg drop throught eh rads so the mean temp climbs and climbs until it cuts on high temp then you are back into the cycling mode. Probably 99% of instalations have the boiler - radiator load un-balanced.

Cheers
Mike up North

Posted By: CWattersIf you have a thermal store that can provide power without firing the boiler. Only when the thermal store cools does it fire the boiler up. If set up right with the right speed pump the boiler can run flat out until the store is upto temperature with no cycling.

Gas boilers generally can modulate their burners so this is less of an issue for them but you still want to be sure that the boiler isn't oversized.

Good point. Thermal stores and fixed rate burners, gas or oil, go together well. Have two cylinder latched in stats for anti-cycling.

Oil boiler have a larger heat exchanger water jacket than modern gas boilers. Some are so large they are like small thermal stores, so much so a plate heat exchanger can be slapped on the side and the thing is an instant combi, with the reserve of heat in the heat exchanger acting as a nice heat buffer giving initial high

A good thing about a thermal store is that does not care about the size of the boiler. I assisted a friend with an oil installation. He had a small commercial oil boiler offered to him near dirt cheap. I said take it. It was way oversized for his house. It was I think around 110kW to 150kW - we sued 35mm flow and return pipes from the boiler to the store. He had the room for it in an outhouse. I suggested a thermal store with two anti-cycle cylinder stats. The boiler would kick in and re-heat in no time at all. Then it would stay off for along time while the CH drip fed heat into the house for the store. He never ran out of DHW even with a few body jet showers and two teenagers using them at the same time. When the cylinder was down on heat and the cylinder stats called in the boiler, the power of the boiler was pumping heat into the store faster than the heat being extracted.

The great thing about oil boilers is that you are not constrained by a gas meter and mains gas pipe size. Being such a big boiler, it still only needed the same sized oil tank as a boiler perfectly matched to the house. It never used any more oil, probably less.

Interesting -I didn't know about the 15% savings with a TS and condensing but would concur on the principle. Have just fitted a new Qudos 28 (Keston) with 2 x 500 litre TSs (cellar access was small) plus 2 hot water tanks and 5 solar panels. Each room is independently zoned (large rooms) with oversized rads so don;t need more than 50 C. I am hoping for nearer 50% rather than 15% - but the 15% you quote will help achieve that!

From 30-40 store temps, there is a lot of condensate dribbling - lessens markedly at 45 and by 50 the boiler, which is 60 + by now, is modulating down and vitually dry.

I have chosen to fit separate DHW cylinders. That's another debate in itself

Water systems/Wookey,Mike, Cwatters,
Do you have any suggestions for my particular situation:
on average per boiler (and they are not easily linked):
10 bathrooms
300m2 mostly Victorian property
poor water pressure thus loft tanks
existing system is regular boiler heating cylinders and heating separately

You are right hotels have their own needs as the customers are from all walks of life. However in many ways the problems faced are much the same as for a regular house - same/similar occupancy patterns and needs. The main difference is simply the neccessity to cater to more tastes.

I am still interested in peoples suggestions for the hot water set-up - just think of it as a massive house with lots of bathrooms that may get used at the same time.

The article you listed above raised the point of "super-micro double glazing " - whats that? slim-lite?
Is that any good?

I have just been reviewing my windows and I am leaning towards secondary glazing at the moment as the U-values achieved are pretty good - supposedly 1.7 ish with low e glass, which isnt bad. Refitting the windows seems to be highly problematic, and in fact highly expensive.