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.
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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)
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
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..
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.
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.
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.
The Ideal Mexico is not a modern design of boiler it's conventionally flued - probably the only improvement in the last 25 years is electronic ignition - and maybe not even that. =
I'm not even sure that the Mexico would comply with part L.
25 years? Try 33 years. It came out at the time of the 1970 Mexico world cup, hence the name. The Morris Marina (what a dog) was not even introduced when it came out. How they have been making this clunker for all this time I find amazing.
I think that you are thinking of the older models. These two modulate from 11 to 40 kW and 14 to 55kW respectively. The rig for them has the option of fully proportional controllers with weather compensation and remote access and monitoring.
From the installation guide, they would appear to be larger versions based on the Celsius design - according to the SEDBUK entry , on the market since last year......
The other thing is that according to the SEDBUK database the output is a fixed 41kW, all or nothing. In the spring and autumn the part load efficiency (which is admittedly taken account of in the SEDBUK calc) is likely to be a lot less:
We put twin Keston Celsius boilers in our church a couple of years ago. The cost of the boilers is about the same as one larger boiler but of course there is more work involved in installation (free labour in our case so it didn't matter). In the spring and autumn and when the building is up to temp you can cut one boiler out and run the other more efficiently. The other advantage is that you have usefully degree of redundancy: if one boiler is out for any reason the other keeps going. It does of course also mean that there are two to service, but if the engineer is already there this shouldn't cost too much.
With the return temp of the pool being very low; below condensing temperatures. the burner box will condensate, unless back end protection is fitted. Look inside the existing boiler and I'm sure the burner box will be rusty (assuming the pool is being heated by this boiler).
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I see B&Q still have the Ravenheat 25 kW CSI primary at £399. This is a basic condensing boiler with on-off control, no pump, no pressure vessel and can be open vented or pressurised at over 90% sedbick efficency. Two of these is the same price as the Ideal cast-iron clunker and a boiler for backup too. Well worth considering and implememting as i suggested above in No. 2.
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