Condensing boiler - odd installation

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On Sun, 26 Oct 2003 10:47:21 -0000, "mike.james"

I think that you are missing the point.
It is the case that a lower return temperature improves the efficiency of a condensing boiler, however this is not the complete story.
Any boiler heat exchanger will have a maximum temperature differential that it can support - for a condesning boiler around 20-25 degrees, for a conventional one around 10-12. This is designed around firing and flow rates as well.
The heat transfer through emitters such as the radiators or the pool heat exchanger is proportional to the temperature difference and the flow rate. This is given by the equation energy = mass x specific heat x temperature rise or fall.
If you connect the radiators, as you should, across the boiler flow and return, they will receive the full temperature drop, as would the pool heat exchanger. Typically, you would have a valve arrangement to balance the flow between the two and hence share the heat. If the requirement for the pool heat exchanger is large, one might even have separate pumps for the heating and this; the effect of which would be to increase flow through the boiler when there is more heat demand. In a modulating boiler, this would cause the boiler to modulate upwards while maintaining the same temperature drop. Some condensing boilers with built in single pump even control the pump speed as the power requirements change.
If you connect in series, the radiators and the heat exchanger will get the same flow rate (by definition). The temperature drop will be split between them and so each will only get a proportion and not be able to run at full load. The boiler won't be able to increase its tremperature difference to deal with this, and so the only way to get more heat output would be to increase the flow. Unfortunately, this will not usually be possible because the radiator and pool heat exchanger will probably not be able to take that. Moreover, there would be no way to balance heat between the two.
You have to look at the system as a whole - you can't just focus on one aspect and assume that everything else falls into place.
.andy
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No I'm not if you read my earlier posts you will see that I've agreed that the situation is complicated and I've more or less given up on the idea.
All I'm asking for is clarification of what "engineered to take advantage" means in the previous post.
As you your reply - I follow what you are saying but I don't think the outcome is conclusive because the pool isn't like a room heated by a radiator - it doesn't have to come up to temperature quickly (and it has a much bigger "thermal inertia").
Imagine a standard setup (forget the pool for a moment) and the return temperature isn't low enough to get to the dew point i.e. boiler not working at maximum efficiency.
Now put pool heat exchanger in return flow. Pool water is very cold by comparison with return water flow and should drop its temperature. It might even be enough to make the boiler work at its most efficient. As its in the return it can't alter the efficiency or temperature of the house radiators unless the flow temperature drops.
The heat output from the radiators can't be changed by this because the boiler will just work harder to lift the outflow temperature - this must be possible because its how the boiler is designed to work. I.e. if the house radiators were working as required the return temperature would be exactly what we have with the pool heat exchanger in the circuit.
The only down side is that that pool heat exchanger isn't as efficient because its inflow water is cooler - but who cares? The pools heat cycle is only 4 - 6 hours each day. So just increase it to whatever it takes - 12 hours even.
As to what happens when the house CH isn't needed - just switch it back to a flow and return connection as standard.
For a modulating boiler I can see that there is the additional problem of the boiler always thinking that the house is cold but then there are room stats for that.
mikej
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On Sun, 26 Oct 2003 12:01:28 -0000, "mike.james"

Broadly this means installing radiators that are larger and hence have the required output at a lower flow and return temperature, thus improving the efficiency.
However..... This is a second order effect after having the condensing boiler in the first place because of the modulating effect of the burner. On days when not much heat is required because the outside temperature is high enough, the radiators in a conventional system will have enough output, even at reduced flow and return temperatures. Therefore, averaged over the year, the boiler will be running quite efficiently anyway.
For a condensing boiler, the conventional new system design is to use 70 degree flow and 50 degree return temperatures because most boilers will only modulate down to an output of 10kW or so. At lower implied temperatures the burner would cycle, which reduces efficiency again.
If you were to use underfloor heating, there is good marriage with a condensing boiler because it is normally designed to operate at 50 degree flow, 30 return.
Depending on the boiler, it may be able to modulate even lower. For example, I just looked on mine (a MAN Micromat) and it is running at close to minimum output. The flow is 45 degrees, return 39 degrees, output 4kW and pump speed 35% with the temperature outside being 9 degrees. There is a steady drip of condensate and no visible outside plume.

That's true, and thinking about it further, it probably is not a good idea to try to marry up the pool heat exchanger and the house heating anyway.
The typical way of working with a condensing boiler when running with radiators and a hot water cylinder is to have either a divertor valve or two zone valves so that the heat is diverted to one or the other. There are good reasons for this - the first being that you normally want to replenish the hot water quickly, and the water has become cold in the cylinder. Therefore, the boiler is switched to it and run at full power, (often at a level above that for the heating) for a relatively short period. Then it is switched back. It also saves having to balance the needs of very different heat loads.
Considering the comparison of a pool heat exchanger and a radiator system, these are also very different in thermal inertia, as you say. The pool is going to require a lot of heat for quite a long time, whereas the radiators quite a lot less. Balancing could probably be done, but a more sensible solution is likely to be to have a separate boiler for the pool.

There is a common confusion here, also perpetrated by suppliers. The dew point is not a holy grail or thermodynamic orgasm to be achieved at all costs. It simply represents the temperature at which there is a phase change from steam to water vapour or water and latent heat is released. In terms of efficiency when comparing against temperature, there isn't a sudden step, but an increase in the *rate* of efficiency with falling temperature.

The heat output from the radiators will certainly be changed by this because the boiler can only provide a maximum temperature *difference* if cool water is pumped in. For example, if you just had the radiators connected and the boiler operated at full blast it might have a return temperature of 50 and flow of 70 if you have fitted larger radiators. If you now drop the return temperatrure to 30, the flow will not remain at 70, it will drop to 50 and the radiator outputs will be less. Remember that the radiator outputs to the room are proportional to the difference in temperature between the mean value of the radiator temperature ( given by (flow-return)/2 ) and the room air.
The point is that the boiler heat exchanger is rated to give 20 degrees temperature lift at full burner output - that's it. At lower outputs it may operate with lower temperature differentials.

This is why connecting in series would create control problems.......

.andy
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wrote:

Right - now I've got it. Many thanks.
I'm focusing on the return temperature thinking that the boiler can take care of the flow temperature and bring it up to the higher level needed by the smaller rads. You're saying that I have to have a higher return temp to ensure that the smaller rads are kept as hot as they need to me. So if I do drop the return temp I'm effectively opting for the lower flow temp.
I need to think about this. (and read the technical stuff on the Keston)
mikej
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On Sun, 26 Oct 2003 13:49:38 -0000, "mike.james"

Exactly. It is a kind of circular argument, I'm afraid.
It's easy to just focus on one aspect, when in reality it's all inter-related.
A useful, although not totally applicable analogy is an electric circuit.
Think of it like a circuit with a 6v battery and two 6 volt bulbs. You can't increase the voltage of the battery.
If you connect both bulbs in parallel and measure the current, if the battery were perfect, the current from it would double when compared with just having one bulb across the battery. This is analogous to doubling the flow rate and you would double the power output from the battery in effect.
However, if you connected the bulbs in series, you would get half of the voltage across each and reduced brightness. You could compensate this by going for 3v bulbs - analogous to upping the radiator size.
This is a gross simplification, of course - I am not sure what the direct electrical equivalent of a condensing boiler would be :-); biut it illustrates the point..
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Yes. Larger rads.

Your idea will only work when heating is on, as the heat exchanger is on the return pipe, which is "silly".
As Andy and myself have highlighted, a boiler should work "inside" a flow/return temp difference. Outside this for sustained periods can cause big problems. When control is on the boiler you are fooling it and it may not react the way you expect, or want.
Look at a swimming pool. What is the ideal temp? 23C? Most of the time the return temp from the pool will be between 15-22C, cold enough for excellent efficiencies in a condensing boiler. Assuming the pool is indoors. If outdoors then the return temps may be very much lower. The boiler will be operating all night to raise it a degree or two. Most swimming pool have the pool heated all day and the temp setback at night. It is knowing when time the night setback has to return to normal day temp, as it will take hours to raise the temp.
I would go for basic condensing boilers, and have stand-alone control for the pool heater, DHW and CH. This is a commercial setup, so control it as such. The likes of the ICOS and Keston Celsius are for domestic properties.
B&Q have for sale the Ravenheat CSI for 400. This is basic. Two of these will do with separate stand alone controls.
On the pool heat exchanger have a blending valve set to the minimum the boiler heat exchanger can have. Say 80C flow and the boiler heat exchanger temp diff is 20C, then set the return to 60C. There is a danger with a pool of having a very large temp diff, so the blending valve puts the heat exchanger into the correct temp diff range. You could have the blending valve set to 40C and the boiler flow temp to 60C.
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Can't fit larger rads hence my "silly" idea.

No that isn't the reason its silly. Its easy to arrange for the exchanger to be connected to the flow when the house heating is off (really off as in summer).

Why do you think my arrangement would put the boiler outside its temp difference. If you read the previous post you'd see that it brings it into its desired delta T.

Currently the pool gets 6 hours of heating in winter and the house 12 hours plus. I can't see there is a problem in extending the pool to 12 hours - or as long as it takes using a thermo controller. At this point you seem to be telling me that it might work.
My main point is that in this situation there is a huge heat sink that could be used to cool the return flow and hence increase efficiency - its a fairly unique situation.

mikej
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"mike.james" wrote:

This is silly. There is no way that condensing boiler are double the price. Typical increase for a condensing boiler is around a couple of hundred more. [Vaillant have not caught up with this yet and are still trying to flog condensing boiler against a 400 quid margin 8-(]
Whilst the 1600 would seem resonable for somthing like a Keston C40 or C55. I have my doubts that you can get 40kW of conventional boiler for 800. Perhaps they are thinking of "bolting" [1] a couple of really cheap conventional boilers together - but that's not comparing like for like. Actually 1600 would be fair for a couple of C25s bolted together and that would give more control and reliability options. [1] Not literally 8-).
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"mike.james" wrote:

Its an Ideal Mexico Super CF4140 - 140,000 BTU - 41kw 872 v Keston C55 (55Kw) 1,548.89 or the C40 (40Kw) at 1,360.90
(Prices I've got from pumbworld which don't correspond to the prices quoted in the installation which are up by about 10-20% which makes the gap bigger) mikej
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On Sun, 26 Oct 2003 10:44:40 -0000, "mike.james"

Poor choice. The SEDBUK seasonal efficiency figure for this one is 78.6%. In other words, it only *just* scrapes into the minimum legal requirements for efficiency according to the building regulations. Fairly shortly, the minimum will be increased so this type of product will be off the market and is probably why the price per kW is relatively low. The manufacturers, knowing that the lifetime is limited, are getting what they can for the remainder of its (short) market lifetime. As far as I can see, it isn't on their web site any more, so that should provide a clue.
Also, note that you are not comparing apples with apples. These Ideal products are basic, non-system boilers and do not have a pump. This particular model is also conventional flue. For a new installation, this does not really make sense because of the need to provide appropriate room ventilation (= cold air from outside) and the likelhood to have to buy a flue system to go with it, pushing up the price.

Both of these products come in at at around 90% seasonal efficiency, include a pump and can work with cheap plastic waste pipe as a flue.
All of this is before the difference in energy saving is taken into account.

.andy
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Very good point. One I can use in getting them to requote with a better conventional boiler.

Ah but the flue is there - it isn't a new installation.

Well the flue is there - and they are charging to take it down if the Keston goes in! (So is the pump.) I personally like the spec of the Keston - I'll see if they can quote for a different conventional boiler.
mikej
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On Sun, 26 Oct 2003 11:40:14 -0000, "mike.james"

Which would put the difference between this and a much more efficient condensing model even less.
Don't forget also, that the manufacturers run loyalty schemes for the trade. Go into any heating merchants and pick up one of the trade magazines and you will find the manufacturers offering tools, cash backs, designer clothing and trips to the sun. Then ask yourself whether the installer is acting in your interest or his.
You will also find that installers tend to stick with what they know and what they have always done. This could be another reason why you are being offered ancient technology.

Don't assume that it would not need to be replaced even for a new CF boiler - it may not be up to spec.

.... and it's reasonably new?

I think that you have to make your own decision, but I don't think that your logic is quite correct.
There doesn't seem any point to me in replacing ancient technology with new ancient technology when you can have much improved energy usage with a new condensing boiler.
If you take a look at the Sedbuk site, they indicate that you can easily save 150 per annum on energy in a large house installation when changing from an older boiler of about 65% efficiency. They are about right - I've done it - and that is with with a 25kW boiler. If you factor up to a 40kW boiler, the saving could be 250, although this is for going from a 65% efficient product to a 90% efficient one. For a comparison between a 78% - 80% efficient conventional boiler you would approximately split the difference, so say 100-150 pa.....
At current gas prices, I would be surprised if cost recovery were not achieved in 5 years, and there is no evidence to suggest that gas prices will drop......
.andy
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But it would be nice if the gap was narrowed by a price reduction not a price increase.

The flue is fine and the Mexico is replacing a similar boiler so installation costs are less.

I agree but my friend is more interested in money than technology.

In my friends world this gives about 5 to 8 years to pay back the invesment. As he is thinking of selling up in at most 5 years you can see that I can't swing the argument on cost - which is all he seems to be interested in.

This is a better argument, if rising prices could be better :-) mikej
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wrote in message > >Very good point.

Andy is on about efficiency, which cascades to money.

Running a 140,000 btu/h boiler to heat a pool will pay for itself in around 2.5 -3 years. Do some looking and cheaper condensers are had. It appears he wants to pay domestic, and fit domestic, for a commercial setup. Typical small hotel mentality. Just have a shower in those places, and that is why foreigners laugh at our plumbing.

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Precisely but you don't seem very consistent when you convert to costs. On the one hand the estimate is 5 to 8 years and you've just quoted 2.5 years without explaining how or why.
The reality is that my friend is being offered two boilers one at 800 with an 80% efficiency and 1600 with a 98% efficiency. The first has lower installation costs because it replaces the original boiler so the difference is in fact greater than basic price.
In addition the 98% assumes "engineered to take advantage" which as you say means larger rads. This isn't the case so its unlikely the difference will be 20%. Even so the payback time isn't 2.5 years based on his current gas bills it will save 200 (assuming 20% really is achieved) and that means 4 years to recover the extra just in the boiler cost. If the efficiency is less then it will take longer to pay back. mikej
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Conjures up thoughts of........
Quote - Sybil: 'Do you really imagine, even in your wildest dreams, that a girl like this could possibly be interested in an ageing brilliantined stick insect like you?'
Quote - Basil: 'Well... may I ask what you were expecting to see out of a Torquay hotel bedroom window? Sydney Opera House, perhaps? The Hanging Gardens of Babylon? Herds of wildebeeste sweeping majestically...'
Quote - Basil: 'So that's two eggs mayonnaise, a prawn Goebbels, a Hermann Goering and four Colditz salads... no, wait a moment, I got a bit confused there, sorry...I got a bit confused because everyone keeps mentioning the War, so could you...' German: 'Will you stop talking about the war!' Basil: 'Me? You started it!' German: 'We did not start it.' Basil: 'Yes you did, you invaded Poland...'
Quote - Basil: (on phone) 'If you're not over here in twenty minutes with my door, I shall come over and insert a large garden gnome in you. Good day.'
.andy
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On Sun, 26 Oct 2003 13:00:20 -0000, "mike.james"

If those are his decision making criteria then fine - I wouldn't waste my time with any more discussion.
Another factor that one could consider, since this becomes more of a commercial environment is that in five years, this older type of boiler will be off the market. I would have thought that being able to offer a potential buyer lower running costs at that stage would be an interesting selling point. If I were a buyer, and noticed that an inefficient system had (realtively recently) been installed, I would start to look quite closely at what other capital economies had been made.
Presumably a property requiring this size of installation is going to be on the market for a bob or two, so nickel and diming over a couple of hundred quid in capital outlay seems somewhat strange,......
I'd let him get on with it,.......
.andy
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To a certain extent that's my feeling but I just couldn't resist thinking about how the pool could be used to make the whole thing work better and perhaps get to the 20% extra promised by the boiler. I still sort of think that there should be some way of making use of a low temperature sink but I'm beginning to see that its more difficult than I first thought. mikej
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On Sun, 26 Oct 2003 13:52:23 -0000, "mike.james"

Even if you connect the heat exchanger in parallel to the radiators (as they should be) there will be a downward effect on the return temperature to the boiler.
Think about the flows of water and the heat loads. Let's say for simplicity that you arranged equal flows through the radiators and through the pool heat exchanger, but that the pool heat exchanger represents twice the load of the radiators. I am assuming that the pool heat exchanger is not limited by its dT - which actually with good sized stainless steel plate jobs will be the case - they can be capable of transferring 200kW in a very small size. Since everything else is equal, you will get a twice larger temperature drop on the return from the heat exchanger than from the radiators. When the water is mixed to return to the boiler, it will be lower than with either load connected alone.
I have an arrangement like this for providing heat to my garage workshop. It runs with a separate circuit and via a heat exchanger, for a number of reasons. When there is a heat requirement to the garage (which is generally equivalent to a couple of large radiators), the boiler return temperature does drop and the burner is wound up a bit as is the flow.
The one questionmark that I raised with this is that with the pool, the heat load is going to be vastly greater than that for the radiators. Therefore to get a reasonable distribution of heat, some balancing would be needed to effectively throttle back the heat supply to the pool as IMM suggested.
They do have rather different characteristics as well, which is why there is generally a switchover arrangement for heating cylinders. This is why having two separate boilers may have proven to be an effective solution, simply because the load characteristics are so different.
.andy
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1. Assume a Keston Celsius with load compensation control. When the flow and return temps become closer together the system assumes the house is nearing, or up to, temp. It then modulates the burner down. When the return and flow temps become wiser the assumption is that the house is becoming colder and winds the flow temp up.
CH is only on the house warms up and the boiler drops the flow temp. Fine. The pool heater is switched in. The return temp from the pool is very low (17C) and this blends with the return of the house heating (say 30c) to say give a combined return temp of 21C. Great for boiler efficiencies. Less gas is being used. The control system thinks the house is cooling and ramps up the flow temp to max. Fine, as the pool heater needs this heat. This means the flow to the house is too high. This is fine as TRV rad valves will compensate for any heat gain; assuming they are fitted in the rooms.
2. The Keston boilers quoted are small commercial jobs with no modulation. With these boilers it is best to fit a Danfoss Randall BES 5000 outside weather compensator (166 from discountheating) . This will drop the return temp of the boiler to the outside weather promoting efficiency. The room TRVs will compensate for any highs or lows in the room trimming off locally. The pool heater kicks in. No matter what the combined heating and pool return temp is, there is enough heat flowing to the pool to heat it. The burner will stay full on as the return will have a good flow and a constant low return temp. The boiler will take a long time to make an impact on that volume of cold water. The pool temp is only around 21-23C. That will work. This goes for a couple of basic Ravenheat condensers too.
With either of the above have the DHW heating on a priority, cutting out the pool heater to heat the cylinder.
Better still have one boiler do the house and DHW and one do the pool (assuming a 25kW boiler will heat the house). With a few valves to alternate boilers if say the house boilers drops out, always having a backup for essential services. Or if there is a constant large demand for DHW, it may be best to have the pool and DHW on the one boiler with a priority system for DHW.
Now it depends on what deals you can get with boilers. The two boiler option in a commercial concern is by far the best.
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