I've read most of the condensing boiler discussions in this group and as a result decided to fit a standard boiler - mainly because of not being able to upgrade old radiators to work with the lower return temperature.
I was helping a friend select a new boiler and told them the same story but then realised I might be missing something. They have a heated swimming pool via a heat exchanger plumbed to the CH.
What if they put the heat exchanger on the return flow? That would drop the return temperature even if the old radiators didn't. Am I being too simple minded? mikej
Unfortunately, you are misinformed. Although condensing boilers can take additional advantage of lower return temperatures, they are still more efficient than a traditional boiler at high return temperatures too. There is absolutely no need to replace radiators when upgrading to a condensing boiler. The additional economic benefit is too slight to be worth doing unless the radiators are being replaced anyway. The condensing boiler will usually be capable of maintaining the "standard" 82C flow temperature. (My Bosch Worcester Greenstar II 28HE claims to run up to 88C flow temperatures).
Also, radiators are sized for outside temperatures of -1C. As this is much lower than your average temperature, your radiators are usually oversized anyway and the condensing boiler can turn down the return temperature using its internal logic.
This heat exchanger should be connected between flow and return as normal via a zone valve.
Fair enough, but for most of the year, the reduced output from your rads would be enough to heat your home with the boiler set to say 60 or 70 degree flow (50 deg return) and you would have had the benefit of condensing action. Then, in the coldest part of the year you could jack the flow temp up to 80 degrees (with the boiler stat) and get the same heat from the rads as you would with a conventional boiler. For this period though you would lose the benefit of condensing action, but hopefully only for the coldest month or two of the year.
Sadly, I think there is a flaw here and it revolves around the boiler being optimised (by design) to run at (say) a 20 degree temperature difference, which I don't think is optimal for your system.
I assume you want to keep an 80 degree flow to use existing rads, but then want a 50 degree return to take advantage of condensing boiler action. That's a 30 degree system drop which I don't think the boiler heat exchanger will be capable of overcoming on the heating cycle; as it will be designed for 20. If you slow the flow to promote a 30 degree rise I think you will get a low flow alarm from the boiler.
If you instead run with a 70 degree flow, with 10 degree drop on rads and then a 10 degree drop on the heat exchanger you will have the requisite 20 degree drop, but you will only have a 65 degree mean in the rads. This is better than the full condensing case, but worse than the 75 degree mean of an 80/70 system (13% worse).
So, nice try, but I think if you want to keep your rads, you would be better going with the idea at the top and making do with the non condensing action for part of the year. It's what I've done (on a new system) to keep a reasonable compromise on rad sizes.
But there efficiency drops as the return temperature goes up. And efficiency alters the time to repay - so with an old installation it might make a difference.
Thanks for the reply - but why wouldn't it improve the efficiency to put it in the return. mikej
It is true that condensing boilers perform more efficiently at lower return temperatures and especially below the 54 degree dew point.
However, there are two points here:
- A condensing boiler will generally perform more efficiently than a non condensing one anyway because it is likely to have a larger heat exchanger.
- System design is done around an outside temperature of -1 or -3 degrees. An old radiator system should have been sized assuming that the outside temperature is -1 worst case and that the boiler flow and return are 82 and 70 degrees respectively (a conventional boiler). However, for most of the heating season, the outside temperature is some degrees higher and so the radiator outputs and consequently the boiler output will not need to be so high. A condensing boiler normally modulates and reduces its output accordingly, so naturally will tend towards lower operating temperatures and greater efficiency.
When I refurbished my system, I calculated all the heat requirements for the rooms and discovered that in two rooms that the radiators were undersized even for 82 degree operation. These rooms had always been on the chilly side on very cold days. In other rooms, I found that they were quite oversized. I then recalculated everything and discovered that by buying two new radiators and moving others around, they could all be run at 70 degrees flow, so I did the changes. However, had everything been adequately sized in the first place, I would probably not have bothered.
In other words, it is still worth installing a condensing boiler anyway, even if you don't alter the radiators.
Yes it would, but it's the wrong way to go about things. The heat exchanger in the boiler is able to manage a certain uplift in temperature at a given heat input from the burner. Cooling the return water would have the effect of also reducing the flow temperature quite a bit so you wouldn't get the needed output from the radiators.
The heat exchanger for the pool should go across the flow and return of the boiler in the same way as the radiators and HW cylinder, and the flow through it controlled by a zone valve with some form of thermostat and/or timer.
Even with the normal return temperature you'll probably find that most condensers have an efficiency advantage. If you look at page 3 of the Keston Celsius leaflet
formatting link
you'll see this.
Remember that the return temp will be lower whenever the system starts from cold and during the spring and autumn when the boiler only cuts in periodically.
Yes, but it will still be more efficient than an old boiler.
If you compare new conventional with new condensing, the new condensing will be more efficient, averaged over the year than the new conventional because the condensing model has the opportunity to operate at lower temperatures in (say) the autumn and spring. The conventional boiler will always operate at the higher fixed temperature.
Yes, but you will still get better efficiency from a new condensing boiler than a new conventional one and both will be better than an old boiler. How long the payback is is difficult to say.
If you are making the comparison by looking at the cost of changing all the radiators to get as much efficiency as you can out of a new condensing boiler, the payback is likely to be extremely long and not worth the effort. For me, it was worth doing because I could achieve the desired result without having to change *all* the radiators.
In a way, but the flow temperature would be substantially reduced and probably not give the required outputs from radiators
All boilers drop in thermodynamic efficiency as the return temperature goes up. The differences between a condensing one and a traditional one are:
The condensing type is ALWAYS much more efficient than the traditional type at ANY flow or return temperature.
The condensing type has even greater efficiency still at lower return temperatures.
Don't get bogged down thinking about item 2, when item 1 alone is enough to justify a condensing boiler. Remember also, that condensing boilers are intelligent and will drop the return temperature if they detect they can get away with it.
It would increase the instantaneous thermodynamic efficiency. However, it will cost much more to run, as the controls would not actually be keeping everything at the correct temperature. There is more money to be saved ensuring everything is only heated as much as is required, than by trying to get an extra 5% on the thermodynamic efficiency.
Your scheme is likely to end up heating the swimming pool to higher temperatures than you wish, which will cost you a fortune, even if that heating was done efficiently.
Modern condensing boilers have load compensation control that takes care of rads sized to 80C. The big market is in replacement boilers and the makers designed to suit. The only case where you don't fit a condensing boiler is when the plume is a nuisance or there is no drain connection for the condensate. Otherwise "always" fit a condesning or "no-flame" boiler.
Not all. some are quite dumb and have on-off control. My condensing boiler does not modulate with a one fixed burner rate. I control it by using an outside weather compensator, which reduces the return temp to suit . The compensator has boiler anti-cycle control. Those with modulating load compensation control are ones to fit on an existing system.
From what I understand he wants a plate heat exchanger on the boilers return to lower the return temp further, that would only work when the CH is on. On a fixed rate boiler this will increase the flow/return delta T alarmingly. On a load compensation boiler the flow temp may be reduced too much as the boiler aims to keep the delta T and may think the house is up to temp.
Surly this would only be so on the coldest of days - when the system was working at less than full capacity the benefits of a condensing boiler would apply?
Depends on how the software is programmed. Not all do it the same. Some attempt to maintain the delta T and the lowest return temp as possible. Not difficult.
Thanks for all the comments - as always the subject of condensing boilers seems to stir a reply :-)
I appreciate the "condensing boiler is always more efficient" argument but my friend is running a small hotel and been quoted (rough figures) £800 for a conventional boiler to do the job and £1600 for a condensing boiler. So if the efficiency advantage isn't fairly large the payback time is going to be long. It seems that there really is a choice to be made here.
As to my "silly" idea of putting their pool heat exchanger in the return loop I'm still not sure how silly it really is.
The point made about control rather than efficiency is a good one but control could be added with couple of valves and a thermostat. I can also see that there is a problem with a modulating boiler thinking that the house was still cold because the pool exchanger was still lowering the temperature and I'm still thinking about this one. Given the house has termo valves on all the radiators and so does the pool - how does this change things?
But after saying this I now think that the general vote of "it isn't worth the gain in efficiency for the complexity increase" - but I thought it was a nice idea.
I have the same problem justifying a condensing boiler. Conventional combis are cheap and easy to fit. The best case difference in efficiency afaik is no more than 6%. This would save me perhaps £25 a year. So as far as I'm concerned the condenser is not viable esp as I don't intend to stay in this house forever and I don't think having a condenser vs. normal combi would help to sell the house at all..
If you were to do this it would be equivalent to putting radiators in series which is just not done.
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.
What general vote? There is little complexity increase. £800 to £1600. he is being ripped off. Condensing boilers cost little over regular boilers these days. Get another quote.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.