Condensing boiler - odd installation

[IMM]

6%?? look at More like 12% according to sedbuk. If the system is engineered to take advantage of a condensing boiler, then more like 25%.

[mike.james]

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

[mike.james]

Are you saying pumbworld or similar (checked other online prices) are ripping us off? (See other post for quote) mikej

[mike.james]

Now this brings me back to my original point. What does "engineered to take advantage" mean - lower return temperatures? Hence my "silly" idea of putting the pool's heat exchanger in the return flow. mikej

[Andy Hall]

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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[Andy Hall]

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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[mike.james]

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

[mike.james]

temperatures?

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

[Andy Hall]

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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[IMM]

Ideal ICOS "system" boiler £740.25 Including VAT

Ideal ICOS "heating" boiler £675.63 Including VAT

Two ICOS condensing boilers at approx 24kW each is about the same price. So may as well get two of them and couple them up.

System boilers come with pressure vessel and pump.

[mike.james]

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

[IMM]

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.

[mike.james]

Two ICOS boilers cost about the same as one Keston when you take into account the extra installation costs not to mention the increased space, flue etc and not being as sophisticated.. mikej

[IMM]

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.

[Andy Hall]

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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[IMM]

Fitting two side by side is not that much in time. The flues come with the boilers. These boilers are "small". He is running a hotel. Having two boilers means backup if one drops out. Bad for business if no hot water or heating.

[IMM]

see other post for quote.

[Andy Hall]

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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[mike.james]

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

[Andy Hall]

In fact Keston and others make ready to go rigs with two boilers and all the controls and plumbing to do just that..... There is one for the C40 and C55.

-

.andy

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