I am about to replace our 1960's Thorn M gas boiler with a Condensing boiler..
Evidently condensing boilers run more efficiently than non-condensing boilers. But my understanding is that they rely on a return temperature of 55/60C ish (I'm not sure what a typical flow temp for a condensing boiler is - same as non condensing boiler ?)
I believe that my system runs with a flow 82C of and a return of about 71C. Not checked but that would be typical.
So what sort of efficiency am I going to get out of my brad new Condensing boiler? Surely not much better than my old Thorn M?
Is there anything I can do to my system to help the situation. I had a plumber come in recently who told me that I should be fitting thermostatic controls to all radiators. Is that a regs requirement? Isn't that going to make the situation worse?
Nope. Whilst condensing boilers do like a low return temp, they do not require one.
Although larger radiators will enable you to maintain output at lower temperatures, the electronics within the boiler are capable of reducing the temperature when it is not very cold, anyway, so your small rads can still run cooler. Even at exactly the same temperature, the condensing boiler will eat your old Thorn for breakfast in the efficiency stakes.
Loads better.
It is one way of meeting the regs requirement. The cheapest way, in fact.
It will depend on the behaviour of the boiler controls. One thing to remember is that condensing boilers usually modulate, meaning that the burn rate is reduced when the heat requirement is less in warmer weather.
One thing to realise is that the 54 degree dew point is not like finding the Holy Grail. On a condensing boiler, the efficiency improves with reducing return temperature. Above the dew point this happens at one rate with respect to temperature. It happens at a greater rate below the dew point. There is a graph on the data sheet of the Celsius boiler on Keston's web site which shows this.
The second point is that the existing radiator sizings will have been designed on the basis of 82 flow, 70 return and a worst case outside temperature of -1 or -3 degrees. However, these days are relatively rare, and most of the time, the output needed to keep the place warm is a lot less. So for at least 9 months plus of a typical year, the boiler will modulate down as will temperatures to more efficient ranges. When needed, the boiler will be able to deliver 82/70. Even though it won't be condensing, because it will have a better heat exchanger anyway, it will be more efficient boiler than the old one.
The old boiler is likely to have a seasonal efficiency in the 50-60% range. Seasonal efficiency is described on the SEDBUK web site, but is basically the behaviour characterised for the typical UK climate over the whole year rather than simple lab optimised figures.
Prior to the change in rules mandating condensing boilers in most cases, non-condensing boilers were reaching 78-80% on this scale, partly because of better heat exchangers, partly the use of fanned flues and partly absence of pilots (possibly others). Decent condensing boilers all achieve 90-91%.
So, if you do nothing with radiators you could expect to achieve better than 80% and not 90%.
You could replace radiators with larger ones or ones having more panels, or do what I did and move some round and replace some.
This is worth doing anyway and is likely to provide a better ROI than radiator changing, so if you do one thing, that is it.
No because the boiler will modulate output down when the heat isn't required. You leave one radiator where the room thermostat is located without a TRV
"Rely" is too strong a word in this circumstance. A modern boiler will be significantly more efficient than an older counterpart by virtue of more sophisticated control systems, and better design anyway. A condensing boiler even moreso as a result of its larger heat exchanger and its ability to recover what would otherwise be lost heat.
Probably about right...
I would expect it to be significantly better - since the old boiler will be fairly poor by modern standards even if comparing against a non condenser.
The boiler will do much of the work for you, since it will use its ability to modulate to actually match the heat it provides to the load. It can also use this capability to maximise the amount of time it spends condensing. In many cases it will run the system cooler, but for longer (which is preferable to bursts of full output, which result in overshooting the desired temperature, followed by gaps of zero output)
Yes, the controls on the whole system will need to be up to spec. So TRVs on all reads except the one in the room with the room stat (which provides the interlock to turn the whole lot off when the demand is reacted (can also do it with all TRVs and a flow sensor)). Fast recovery cylinder, fully pumped, and zoned if it is a big house.
Nope they will help. As the TRVs close down, the rate of flow through the rads falls, giving bigger temperature differentials across the rads and lower return temps - this improves boiler efficiency still further. As the system as a whole comes up to temp, there will be a time when the boiler sees a rise in the overall return temp (since the heat being dumped to the house is falling) this will allow the boiler to modulate down further.
| The second point is that the existing radiator sizings will have been | designed on the basis of 82 flow, 70 return and a worst case outside | temperature of -1 or -3 degrees. However, these days are relatively | rare, and most of the time, the output needed to keep the place warm | is a lot less. So for at least 9 months plus of a typical year, the | boiler will modulate down as will temperatures to more efficient | ranges. When needed, the boiler will be able to deliver 82/70. Even | though it won't be condensing, because it will have a better heat | exchanger anyway, it will be more efficient boiler than the old one.
My house is so well insulated that it stays nice and warm on radiators rated 3kw downstairs to which I switch another 1.5 kw upstairs in very cold weather. These are similar to Stelrad Elite P1
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from which I cribbed the ratings. I did the heating calcs for these 25 years ago, since which I have improved the insulation, so I believe that I could now use smaller radiators. I have uses a 10 kw conventional boiler, for Central heating and domestic hot water, without problems for 25 years.
My problem is that the smallest combi boiler I can find is the eco Hometec EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw. Clearly much too big for my house.
Surely through most of the year, on minimum modulation this will cycle on and off and loose most of its condensing advantages. DHW is OK
Not really. If you replace it and don't go the combi route, you will need to replace the hot water storage cylinder as well with a fast recoivery one to comply with the building regs. That will swallow the
I wasn't aware that the cylinder needed to be changed, only that if it was changed, it would have to be Part L1 compliant. Any existing cylinder kept would need to be fully pumped, with a thermostat added, though. Obviously, an ancient scaled up cylinder with only a crappy add on jacket is probably worth replacing anyway.
Do you have a link to this requirement please John? I have seen people advise the use of fast recovery cylinders, but never stipulated as a requirement for Building Regs!
L1 compliant cylinders must have a certain amount of insulation and a certain size of heat exchanger (so the boiler can heat it quickly and turn off, reducing primary circuit losses). A true rapid recovery cylinder will actually have a much larger heat exchanger than that mandated for L1, although the L1 is much better than the standard cylinders installed 10-20 years ago.
I believe you only need to fit one when installing a new one, or replacing an old one.
| Dave Fawthrop wrote: | | > My problem is that the smallest combi boiler I can find is the eco Hometec | > EC16S which is rated by SEBUK at 22.0kw regulating down to 3kw. The | > companies literature rates it at 16kw ish modulating down to 2.5 or 3.1kw. | > Clearly much too big for my house. | | Not really. If you replace it and don't go the combi route, you will | need to replace the hot water storage cylinder as well with a fast | recoivery one to comply with the building regs. That will swallow the | 20kW output nicely and give fast recovery times.
I do not need hot water storage, the EC16S will give more DHW flow than I have at the moment.
This is actually the MAN Heiztechnik Micromat. I have one of the larger versions, but of course not a combi.
The control system is more sophisticated than most boilers in that there is also weather compensation and slope control.
This means that it will modulate down such that the flow temperature is at around 40 degrees. The pump also reduces in power to as low as
20% of full run rate.
Once you fall below the minimum 3kW modulation level, the boiler will cycle, but at this low temperature on a very long time constant.
Since your existing boiler is a 10kW one, it will be cycling on the thermostat at full power and 82 degree operation. I would suspect that there is quite an overshoot on flow temperature as well.
Running the burner at 3kW and 40 degrees means that the boiler is operating well into the high efficiency slope anyway and spare heat is distributed within the building. Much of the efficiency loss through cycling with conventional boilers cycling is flue loss when the boiler no longer should be providing heat.
I have seen people advise the use of fast recovery cylinders, but never
Yup sorry, I might have overstated the requirements slightly, reading the detail :
"1.43 For systems incorporating integral or separate hot water storage vessels, ways of meeting the requirement include: a) arranging for hot water storage systems to meet the insulation requirements of BS 1566, BS 699, BS 3198, or BS 7206 (as appropriate); or b) in ordinary cases, insulating vessels with a
35mm thick, factory-applied coating of PU-foam having a minimum density of 30kg/m3. (For unvented hot water systems additional insulation should be provided to control the heat losses through the safety fittings and pipework but without impeding safe operation and visibility of warning discharges. (See Approved Document G.)"
And:
"1.44 Provisions should enable efficient operation without excessive boiler firing and primary circuit losses. A way of demonstrating compliance for indirectly heated hot water storage systems would be for the size of the heat exchanger to be at least that recommended in BS 1566, BS 3198, or BS 7206 (as appropriate) and for them to be served by a pumped primary system."
I have not looked at BS1566, or 3198, but it looks like there is a rquirement to meet L1, but that does not need to be as fast as a "fast recovery" cylinder. Just well insulated and fast ish!
| TheScullster wrote: | | > Do you have a link to this requirement please John? | |
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| | > I have seen people advise the use of fast recovery cylinders, but never | > stipulated as a requirement for Building Regs! | | Yup sorry, I might have overstated the requirements slightly, reading | the detail : | | "1.43 For systems incorporating integral or | separate hot water storage vessels, ways of | meeting the requirement include:
| overstated the requirements slightly
is the understatement of the year. You posted
| Not really. If you replace it and don't go the combi route, you will | need to replace the hot water storage cylinder as well with a fast | recoivery one to comply with the building regs.
Which is outright wrong.
As I am putting a combi system in specifically to get rid of the hot water cylinder, and have worked out that the smallest combi will provide more liters per min of hot water than the present system, and more importantly will provide one bath after another, which the present system struggled with. I will stay with the combi.
Most DIY domestic combi installations will have the same requirements as myself.
Most domestic installations clearly require more CH boiler output than you which is why most boilers available are in the 20-30kW range. Didn't it occur to you that the reason that there were so few available at the bottom end of the range was because your requirement is not typical?
You mentioned the MAN (Eco- Hometec) EC16 boiler. Although this make is an excellent boiler, the hot water output rate from the combi section is disappointing because the input power level is so low at
22kW for HW. This provides a flow rate of 10.5 litres/min at a temperature rise of 30 degrees. The industry standard is to use 35 degree measurement and so the flow rate is actually 9 litres/min.
This means that during the winter, it will take 15 minutes at least to fill a bath.
If you are happy with that level of performance, then fine but it is more than a stretch to say that most installations will have these requirements.
It should read, "you will need to replace the hot water storage cylinder with a part L1 compliant one (or better) to comply with the building regs"
(This does make the (fairly safe) assumption that a 1960's installed system will not already have a compliant cylinder)
My error was in describing this as "fast recovery" which in fact is a bit faster still than the "ordinary" part L1 compliant ones.
On a more practical level, cylinders don't have an unlimited lifespan, so if you are replacing an ancient boiler and the cylinder is also as old, now would be a good time to replace it before it leaks.
If you are using a condensing boiler, then there is no reason to not use a fast recovery cylinder.
(there used to be the issue that the load they placed on the system could drop the return temperature too far, and cause condensing on non condensing boilers - this would either damage the boiler, or require more a complex install to add blending valves on the return to keep the return water temp above the dew point)
The smallest combis (typically 24kW) will deliver water at 9 lpm (at 45 degrees) in warm weather (less in winter). So there is something seriously wrong with your storage system if it can not match that. A badly designed an implemented one should do 20 lpm (at 65 degrees), and a good one 40+ lpm (to which you can also in addition add cold water at a significant rate).
As for "one bath after another", yes it will, so long as you don't mind it taking 15 mins to fill each one.
Fine by me, by we wern't talking about you ;-)
You are IMM and I claim my five pounds!
That seems like a rather presumptuous statement...
No it shouldn't. There is no requirement to replace the hot water cylinder with a compliant one when changing the boiler.
Read the following. Paraphrased,
When replacing the boiler, you must fit an efficient one, and update any controls and provide fully pumped circulation and boiler interlock. There is no need to change the hot water cylinder.
When replacing a cylinder, you must fit an efficient one and update any controls and provide fully pumped circulation and boiler interlock. There is no need to change the boiler.
Note that the actual boiler efficiencies suggested below have been superceded with a new table since publishing of the original Part L1.
Christian.
2.3 Reasonable provision where undertaking replacement work on controlled services or fittings (whether replacing with new but identical equipment or with different equipment and whether the work is solely in connection with controlled services or includes work on them) depends on the circumstances in the particular case and would also need to take account of historic value (see paragraph 2.9 et seq). Possible ways of satisfying the requirements include the following:-
b) Heating boilers. Where heating boilers are to be replaced in dwellings having a floor area greater than 50m2, providing a new boiler as if for a new dwelling i.e:-
(1) In the case of ordinary oil or gas boilers, providing a boiler with a SEDBUK not less than the appropriate entry in Table 236, together with appropriate controls following the guidance starting at paragraph 1.36. (In the case of replacement boilers installed in the period up to 31 August
2002, it would be reasonable to provide a less efficient boiler provided the heating controls comply with specification HR2 or HC2 given in GIL 5937;
(2) In the case of back boilers, providing a boiler having a SEDBUK of not less than three percentage points lower than the appropriate entry in Table
2;
(3) In the case of solid fuel boilers, providing a boiler having an efficiency not less than that recommended for its type in the HETAS certification scheme.
c) Hot water vessels. When replacing hot water vessels, reasonable provision would be to provide new equipment as if for a new dwelling following the guidance beginning at paragraph 1.43.
d) Boiler and hot water storage controls. So that replacement boilers (other than solid fuel boilers) and hot water vessels can achieve reasonable seasonal efficiency, the work may also need to include replacement of the time switch or programmer, room thermostat, and hot water vessel thermostat, and provision of a boiler interlock and fully pumped circulation. Section 4 of GPG 30238 gives more advice on how this can be done.
| On Fri, 09 Sep 2005 08:42:59 +0100, Dave Fawthrop | wrote: | | | >
| >As I am putting a combi system in specifically to get rid of the hot water | >cylinder, and have worked out that the smallest combi will provide more | >liters per min of hot water than the present system, and more importantly | >will provide one bath after another, which the present system struggled | >with. I will stay with the combi. | >
| >Most DIY domestic combi installations will have the same requirements as | >myself. | | | This is complete nonsense. | | Most domestic installations clearly require more CH boiler output than | you which is why most boilers available are in the 20-30kW range. | Didn't it occur to you that the reason that there were so few | available at the bottom end of the range was because your requirement | is not typical?
I took into account my atypical requirements and note the abundance of high output combi boilers available for those with larger properties and less insulation. The SEDBUK list gives 7 boilers at over 40kw, which should cover any domestic requirement.
| You mentioned the MAN (Eco- Hometec) EC16 boiler. Although this make | is an excellent boiler, the hot water output rate from the combi | section is disappointing because the input power level is so low at | 22kW for HW. This provides a flow rate of 10.5 litres/min at a | temperature rise of 30 degrees. The industry standard is to use 35 | degree measurement and so the flow rate is actually 9 litres/min.
The EC16 is now no longer produced, so I may mow have to go for the larger EC23S, | | | This means that during the winter, it will take 15 minutes at least to | fill a bath.
My bath fills fine with 8 l/m, with 22mm pipe and soft water,from the present system.
The higher output combi boilers referred to above have much higher DHW flow rates. Using the Eco Hometec EC 38 as an example, merely because I have the data to hand, gives 22 L/min DHW which should be enough for anybody, without storage. | If you are happy with that level of performance, then fine but it is | more than a stretch to say that most installations will have these | requirements.
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