Boiler Replacement Advice Worcester or Vaillant ?

[ek_1986]

Hi all,

As most of you will know, my boiler is need of replacement and I have been considering alternatives. At present I have an 18 year old combi boiler used to heat both water and heaters and am looking for compatible replacement.

Having spoken with an extremely friendly and helpful chap this morning about boiler installation and recommended choices for boilers, I have managed to narrow down my boiler choice to two -

1)   Worcester Bosch, Worcester Bosch 28i Junior

http://www.heatcodirect.com/index.php?menu=show2&title=%20Worcester%20Combi%20Condensing%20Boilers%20-%20Worcester%20Greenstar%2028i%20Junior%20HE%20Condensing%20Combi%20Gas%20Boilers%20&viewlist=419 @ 793.50

2)    Vaillant EcoTec Pro 28Kw Condensing combination - http://www.heatcodirect.com/index.php?menu=show2&title=%20Vaillant%20Combi%20Condensing%20Boilers%20-%20Vaillant%20EcoTec%20Pro%2028%20HE%20Combi%20Condensing%20Boilers%2028kw%20&viewlist=392 @ £736.00

I was initially looking for a 24kw boiler but have been advised that 28kw would be a better option due to my house being occupied with 4 adults as oppose to children. There doesn’t seem to be a significant price difference to warrant me choosing a 24kw over a 28kw but I’m wondering what other advantages I will get by having the 28kw besides increased output making my heaters warmer 

Also are there any disadvantages over the 28kw to the 24kw?

If anyone has had a better price on either boilers or can suggest a better alternative, I would be very grateful if your input!

Having read exellent reviews on the Worcester Bosch, Worcester Bosch 28i Junior, I am very keen to go with this one.   

Again, Thanks in advance! Without your expertise, advice and tips, I’m sure i’d of been lost

Ek_1986

[Amy]

Just bang the worcs 28i in and forget it, cheap as chips at the moment from my supplier

[plumbskill]

28,s normally give you a better domestic hot water flow rate over the 24,s     i.e. so many litres per minute

[Amy]

24i gives 9.8
28i gives 11.4 litres a min at 35 c

[ek_1986]

28i gives 11.4 litres a min at 35 c

Confirms that I will go for the 28kw  as oppose to the 24.


Its looking good for the Worcester Bosch

Amy which supplier did you use for Worcester? If its cheaper then the site I have posted up, I would be really grateful if you PM me the details..

Is there anything else that I will need to ordering with the boiler besides the flue, thermostat?

I've yet to check the gas pipe to ensure that it is 22mm as oppose to 15mm which some can be.

[Amy]

I always go for the DT20 twin channel digital programmer as it slots in the front and no extra wiring, guaranteed to talk to eachother, easy for idiot installer to wire,  easy for idiot homeowners to use 


2 weeks ago i bought

24i
flue
prog

£775 inc vat
from my local shop, not online. In stirchley birmingham.

Cant see the 28 being more than a few quids more if you get yer shortest skirt on and chat Marcus up 

[KLD]

EK, I haven't looked up your other posts, so do not know whether solar is on your cards as well. The WB web site says the Junior combi boilers are not solar compatible, whatever that means.
Their modulation range is quite limited, so for most of the heating season your new boiler may be grossly oversized (since you didn't say what your CH requirements are, I'm just guessing wildly). The lower output limit is the same for both models, 24i and 28i (the tech specs are very similar, in fact the 28i looks like a 24i with a booster setting for DHW. Amy???).
The standard control option looks very basic, whether or not a weather compensating unit is available I don't know. Well worth investigating, though, since it can reduce your gas consumption and carbon footprint considerably.

I've been through this whole palaver just recently, and am glad I finally came to some conclusion. So many parameters, so many optins, and sooo little time. Good luck to you!

Klaus

[Amy]

I havent got any specs on weather comp for the valiant but all german boiler manufacturers will have something because they have been streets ahead for years, except it seems, the WB

heating modulation range is 7.2 - 24 kw on the WB for both 24i and 28i

[chickensoup]

 Hi Klaus,
             You mentioned that the heating modulation is limited, i think that 7.2kw min ( like Ames mentioned) is superb and just about the norm across all range of boilers. All condensing boilers are modulating so will adjust their output according to the load, as for the difference between models 24+28 is just the domestic plate heat exchanger size....that's it...no boost button or turbo mode! again the boiler adjusts its output according to the dhw load.
                  As for compensator's, unless your property is well insulated the savings in energy would be minimal  offsetted against the cost of the unit as you would only expect to save a few % more with a unit.

                  chicken

[KLD]

Well, we discussed the modulation range of gas boilers here before. Just because the manufactures call it "fully modulating" doesn't make it any better then it is. A factor 3 between max and min output seems pretty standard in the WB / Vaillant etc class. What I haven't seen anywhere (haven't looked very thoroughly  ) is some statistics about real domestic heat loads. So, I can only go back to N=1, our own house. Part of it is 150-years-old 2ft solid stone wall, part of it is 70ies brick and block and part brand new extension. Pretty standard I would recon. A classic pattern (for us) is that only a few rooms are heated, then you get some passive solar gain, the outside temp is say 10°C, so overall you still want some heating. The heat load at that point drops to maybe 1 or 2kW. With a 7kW min power, the boiler starts cycling. Looking at our daily data (oh no, did I mention the d-word?  ) this situation is not a rare occasion. I.e. ideally you either find a boiler that is truly fully modulating, or you install a big buffer tank.

The weather comp control helps a lot to bring the system temperature down, thereby increasing the chance for condensing operation.

Just my two pence
Klaus

[Amy]

So look at the Vailant with the heat store piggy backed on to it.
I installed one 6 weeks ago and it will either buffer 20 ish lts or work in combi mode at the flick of a switch.

Best of both but i dont know what the lower modulation output is

[chickensoup]

 Klaus,
          You have some fair points there, but if a boiler were to modulate down to 1kw the temp of the water would be very low...maybe 25-35 deg....guesstimating! probably arrive at the rads lower , deltaT @ the rad maybe 20 degrees(great if you've oversized rads)....would the average room gain any heat from that? well it probably could if it were passive haus standards, but as most European homes are underinsulated and very draughty the design engineers probably didn't feel the need to have such a low min output.
                           You say you only use a few rooms, have you considered zoning them, do you have trv's?

[lightfoot]

Klaus,
          You have some fair points there, but if a boiler were to modulate down to 1kw the temp of the water would be very low...maybe 25-35 deg....guesstimating! probably arrive at the rads lower , deltaT @ the rad maybe 20 degrees(great if you've oversized rads)....would the average room gain any heat from that?

Why ?? - unless otherwise controlled, the boiler flow/system mean water temperature is only likely to drop, if the output from the boiler was less than the that being consumed by the system, ie the boiler couldn't keep up with demand - energy in, needs to equal energy out....

The ability of a boiler to modulate it's output - to match the required load - would be based on the return temperature.  So during part load conditions, ie a mild day, there will be little temp drop across the system - thereby requiring only a small input from the boiler to raise the flow to the design/required temperature.  If the output of the boiler exceeds that required by the system, then the boiler will come up to temp and be switched off by it's stat, until the heat is dissipated - so the boiler may cycle.

Many houses in the UK, will require the design peak heating load for around only 5% of the heating season - for 50% of the heating season, it will only require around 25% of the design peak load - and at times less than this.  When utilising a combi boiler, you also have to consider the DHW demand - which often requires a boiler output far in excess of the max/min space heating requirements - this is where good modulation (high turn-down ratio) comes into play, to suit the needs of the household and be as efficient as possible/practical....

Gas combi's seem to be getting better (I'm not particularly up to speed with gas boilers, as I don't have much to do with them these days), but a 3 or 4:1 turn-down ratio is nothing to write home about these days - 10:1 would be more like it, if you require a good DHW output (2 or more bathrooms etc), aswell as a efficient CH system - but often comes at a price (investment).  As Klaus has said, you can improve the performance of a over-sized, non/low modulating boiler by utilising an appropriately sized thermal store as a buffer - which is the approach of some boiler manufactures - especially non-modulating, condensing oil combi's.

As well as TRV's, zoning and variable speed pumps etc - where appropriate, the addition of a intelligent weather comp system - either directly controlling the boiler and/or a 3/4-port mixing valve, may well vary/reduce the flow temp to suit the predicted heating requirements - which will reduce the output from the rads/ufh (but that's the idea - rads/ufh are sized for peak load) and as Klaus has said, this can further help a condensing boiler operate efficiently.  However, if a boiler can't modulate it's output to suit, or if the heat can't be buffered by some kind of thermal store etc, then the boiler may well cycle inefficiently.


Lightfoot.

[KLD]

Thank you, Lightfoot, as usual there is little one could add to what you said. As an example for the investment possible, have a look at the Evinox range of boilers. I've never seen one in vivo, but the specs look fantastic. I'm sure the price is at the "reassuringly expensive" end 

Chickensoup, our own system is fully weather compensated, divided into two principal zones (rads, UFH), and single room controlled. The boiler is a Viessmann Vitodens 200W, modulating between 4 and 19kW. The 4kW is still too much for a lot of the time. The additional investment necessary to match something like the above mentioned Evinox gadget, though, I rather have directed to better insulation. At the time of designing the system I didn't dare putting in a 500 or 800l TS to act as a buffer for the CH, since I feared the solar DHW heating wouldn't work as well (re: all that discussion about stratification during the last year, or as Martin put it: the arms race ). So here we go, the usual world of compromises.

All I tried to do on this thread is to say: hey, did you know about these options? I strongly believe that our decisions are more balanced when we have a wider knowledge of the possibilities. And this forum is great in helping to achieve that!

Cheers
Klaus

[lightfoot]

For the record - a primary thermal store doesn't necessarily need to be 500 or 800+ litres.  Should you wish to utilise a thermal store as a buffer between a gas/oil boiler and the UFH/rads - then it only needs to be sized to accommodate the excess heat produced by the boiler, during part load conditions. 

For example, if you had a 24kW boiler that could modulate down to a minimum of say 7kW - but your CH only had a part load requirement of say 2kW.  Then, depending on the system design, you may only need to allow 100 litres or so for a buffer - to help reduce the number of start/stop cycles per hour and help keep the boiler in condensing mode. 

You may wish to use the TS to supply or pre-heat the DHW and could also choose to heat this thermal store from other inputs - such as solar and/or a WBS/boiler etc - but their storage/buffer requirements may differ from a gas/oil boiler and would need to be considered at the design stage. 

Obviously the advantage of using a combi boiler is that it heats the DHW on demand - therefore potentially negating the need for DHW storage - which maybe a issue if space is at a premium and will help reduce the standing losses.  However, you will need some kind of storage if you wish to incorporate solar into the system - so to my mind, it makes sense to make the most of this storage capacity and extend the working range of the boiler, by utilising a appropriately sized TS to buffer the CH as required and potentially improve the DHW flow rate that you would get from a combi boiler. 

Again, if the boiler and CH are connected directly the TS (no coils) - then you will be heating the store from the top down and can heat as much or as little of the TS, as and when required.  If for any reason, the system requires a large TS - that is disproportionate to the potential solar input - then you always have the option of using multiple level solar coils, side-arm heat exchanger or some other means of stratifying the TS, to concentrate the solar input. 

The challenge is to design a system that makes the most of the resources available - a efficient system needn't be too complicated, nor break the bank, just maybe a little different from the UK standard - it's good to look/think outside the box.