Baxi 105e Instant-end of service life advice please.

[FlatMax]

Hi everybody.

 

Our 2002 Baxi 105e instant has been (and still is) a cracking good boiler, but of late it is becoming increasingly unreliable with niggly random issues mainly around control items. I've done numerous jobs on the water set over the years, including swapping out the PCB, thermistors, and other bits which are still readily available just about anywhere.

 

The latest iteration of what can be described as 'random' faulting, is that the boiler locks out when you shower whilst the system is calling for heat; you are then left with the flashing 50C LED and DHW symbol, and the pump running continuously. Powering off for half an hour and reconnecting sees the boiler run its usual ignition check and hey presto, all back to normal! Central heating functions as normal, and DHW as well, just not both together.

 

I'm guessing the overheating is something to do with the diverter valve (the DHW diaphram is new) but my wife has pointed out that the boiler is probably at the end of its service life, is less efficient and more polluting than a replacement, and she would be stuck if I wasn't around when these things happen. All this is very true, so I'm looking for a replacement.

 

The Baxi Duo Tec 28 is the replacement I'd go for, as it sits on the same jig and has the same pipework/flue configuration. I'm not looking for big efficiency savings here, just another boiler that will last 15-20 years and be more reliable going forward. Parts are also readily available for this model and it has a seven year manufacturing warranty. Do members think this is a good combi boiler or are there others out there that are significantly better? 

 

Any advice would be gratefully received. 

[JohnMo]

1 hour ago, FlatMax said:

I'm not looking for big efficiency savings here

Why not when it's easy to do, actual boiler efficiency has little to do with boiler model more to do with how it's plumbed in and controlled. You could probably save a good 20% on heating costs. And get way better hot water performance.

 

So choose your combi of choice, but make sure it's controlled via opentherm, not on off thermostats. But ideally has plenty of turndown so min output is low.

1 hour ago, FlatMax said:

Baxi Duo Tec 28 i

Not the best boiler out there, pretty poor turndown, with a min output of about 10kW, so boiler will cycle a lot at mild temperatures. The best will turn down to under 2kW.

 

Can only run at a set flow temp. So not the most efficient by a long way.

 

Hot water flow rate is 11.5L minute. Just looked at my old boiler and it did 15L minute at the same 35 Deg temp rise. And had a turn down to half the one you are looking at.

 

I would look at other makes and models.

[marshian]

I did the sane thing many years ago - had a glow worm boiler that had given sterling service so chose another glow worm the same KW rating - it was a big mistake!!!!

 

I didn’t take into account insulation improvements to the house, SUDG, CWI improved Loft insulation etc

 

boiler was massively oversized - couldn’t do DHWP, had a terrible modulation level 24 KW max 10 kW min and as a result cycled like a twat in shoulder seasons and wasn’t that great in the winter either

 

find a better boiler as @JohnMo says - preferably with same pipework configuration and flue position and get some real benefits and savings with a better modulation

[FlatMax]

7 hours ago, JohnMo said:

Not the best boiler out there, pretty poor turndown, with a min output of about 10kW, so boiler will cycle a lot at mild temperatures. The best will turn down to under 2kW.

I hadn't even considered this; thanks for the insight. Here in South Devon we don't get much below 10 celsius ambient during the coldest months, so I can keep the house at 20C with a flow temp of 55C no problem. We have twenty rads in total (thirty five panels if you count the double rads) spread over three floors and 2,500 sq ft floor area. Building was new in 2003, so half decent insulation.

 

What would you consider is a good turndown ratio for a modern condensing combi? Our old Baxi has a ratio of 3:1 which oddly, is the same as the latest Duo Tec 28! 

 

Thanks again.

[JohnMo]

A big Viessmann combi

 

A Vitodens 200-W has a 17:1 turndown, so you could install a 30+kW for great DHW performance and still it would tick away down to below 2kW when it's mild. Set the boiler to run WC mode and it would fire up alter flow temp automatically and boiler would run for long periods at best efficiency. Comes with up to 10 years warranty

 

Atag make a good boiler, turndown isn't as good as Viessmann, but a solid machine.

[marshian]

7 minutes ago, JohnMo said:

A big Viessmann combi

 

A Vitodens 200-W has a 17:1 turndown, so you could install a 30+kW for great DHW performance and still it would tick away down to below 2kW when it's mild. Set the boiler to run WC mode and it would fire up alter flow temp automatically and boiler would run for long periods at best efficiency. Comes with up to 10 years warranty

 

Atag make a good boiler, turndown isn't as good as Viessmann, but a solid machine.

 

^ WHS beat me to it

 

Valiant are catching up with their recent boiler so also worth consideration - their controls are also highly regarded

 

Intergas are also in with a shout 

 

Whatever you go for find an installer that does specialises in the brand - boxt and other "box" movers are unlikely to get the best out of the install

[FlatMax]

Thanks to everyone who has been kind enough to post their replies.

 

I think I now have all the info required to make an informed choice. What I really need to do is find an installer with whom a reliable relationship can be established; we don't have that at the moment.

 

 

[JohnMo]

Have a look for a Heat Geek installer in your area. They have been trained properly on low temperature heating systems etc. if you go on to the heat geek website there's a page for finding local installers

[FlatMax]

**UPDATE**

 

Following all the great advice kindly offered on here, I managed to establish a working relationship with a WB approved installer just a couple of miles away. The ageing Baxi 105e has been replaced with a WB Greenstar 4000 rated at 30kw for DHW/24kw for CH. I calculated the heat loss for the building at 12.5-15kw (it's a 2002 build of 340m2 set over three floors, with total volume of 1,120m3) at ambient of 5C and inside at 16C. We don't like the house too warm, so these metrics are fine for us.

 

The boiler has a turndown ratio of 1-8, so 3kw up to 24kw is the operational output range. Before looking at weather compensation, I've been experimenting with simple inputs like range reduction, flow temperatures, and proper use of our existing TRVs; the system remains under the control of a Hive Mini on/off arrangement. The original twenty radiators were substantially over-sized when the house was built in 2002, so this will work in our favour for a 'long and low' regime. I've range rated the boiler down to 50% (12.5kw) and reduced the CH flow temp to 50C; I have also dropped the DHW to the same value and might go lower than this eventually. All the rads were balanced with the TRVs wide open and heat loss across the panel is around 10C. All the rads across the three floors heat up fairly evenly from cold, and after two hours the house is really very comfortable. The boiler's return temperature after two hours of heating is 44C and it's condensing really well; the amount of water it discharges is extraordinary.

 

Now the fun bit for the techies on here. Having range rated the maximum CH output to 12.5kw, how come my Octopus smart meter says I'm using 20kw per hour of gas when the boiler is firing from cold? The half hourly plot does show the boiler modulating down a bit as the system heats up, but it's still using around 18kw each hour. The input to output ratio for the boiler is almost the same, so I am completely mystified. Am I wrong in believing that 12.5kw is the most the boiler can consume for CH?

 

[JohnMo]

25 minutes ago, FlatMax said:

ambient of 5C and inside at 16C

Pretty flawed heat loss calculation you should have done it at -3 outside and 20 inside, but your boiler is still ok for that. Your just fooling yourself with overly small heat losses and too smaller range rating figure.

 

Range rating to your calculations for heat losses at +5, what happens when you get a zero degree day, and your house drops to 11 degs? Because your boiler output is too low. Think I would rerun the heat loss calculation for realistic figures and range rated accordingly.

 

44 minutes ago, FlatMax said:

from cold, and after two hours the house is really very comfortable

First alarm bell is this. If you letting the house get cold (16 is cold in my book and that's your target temperature), the boiler needs to run balls out (especially when range rated right down), so may actually be struggling. Taking a long time to find a comfortable running point. To get a house to 16 with radiators in 2 hours is a long time.

 

For me 

You have range rated too low

You allow too much set back when heating is off - at a target of 16 why would you set back at all.

 

Just let the heating run 24-7. Let everything stabilise. Your boiler will start to run more efficiently and modulate down on its own.

 

Range rating is good for stop start heating as it allows a level of heating output more suited to the house at startup, but you have to careful, analogy 

If you are trying to fill a bucket with a hole in it (your heat loss), the hole allows 10L per minute to leak out. You open the tap (your boiler) and it flows at 10L per minute you will never fill the bucket. Open to 11L per minute it going to take a long time to fill. That is where you are now. Boiler running hard for too long.

[marshian]

Where do you sit on this table?

 

[SteamyTea]

5 hours ago, FlatMax said:

boiler is firing from cold

The useful efficiency will be pretty poor initially as little condensing may be happening.

The 12.5 kW you have limited to may only be a rough setting and is the thermal output. The efficiency may be down to 65% at start up, which is getting close to 20 kW.

Or 

Was it heating water then?