UKC

Sorry for further diluting the limited amount of (mainly) gritcentric climbing discussion on here but I know there are some well informed DIY types on here and I can't be arsed joining the weirdos on DIY specific forums.

Basically I may have to scrap a very expensive 35kW Veissman boiler due to what I think is probably sludge blocking internals (still debating whether to pay for a strip and flush).

Anyway I need to size a modulating systems boiler (probably Ideal) but I can't find any info online about the recommended sizing for a known kW of radiators. Essentially I have 21 rads totalling about 32 kW of output but because I have a Evohome system each rad (and therefore room) is self-zoned. As such I am rarely if ever going to need to fire all rads at the same time in any situation (i.e. upstairs and downstairs (would never normally be working at the same time). I can't imagine requiring more than 20 kW at any one time. Furthermore, because the Evohome system calls for heat on individual rads there would be benefit in lowering the minimum output as it will often only be heating 2/3 rads when topping up individual rooms especially in Autumn/Spring. The lower the boiler output the lower the minimum output which should reduce the risk of cycling in that situation.

I am wondering if I should go for 26 kW - any thoughts/experiences?

Hmm, how old is the boiler?

Stick some chemicals in, clean the system, put in a strainer and it should be fine?

If the boiler is old then maybe now is the time to replace it. 

Only 5/6 years old. It's cycling all the time due to internal overheating, providing about 5 kW output and there seems to be no head on the internal pump which suggests a blockage to me. I'm getting an estimate for a strip/flush but a new heat exchanger will be about £800 which is well into new boiler territory...

That is why (gas prices may change this!) I stick with my ancient, never ever goes wrong boiler. Manufacturers expecting such a short working life of their products should be banned from selling their products, and I thought Veissman were a 'good' brand. 

Does anyone make boilers with a 25 year service life any more? 

I would be powerflushing the whole system anyhow as if that one sludged up so will the next one.

Agree with your logic that 32kW of rads does not need 32kW of boiler especially if you keep the place warm - you only need that kind of power for a heat from cold scenario which ideally shouldn't ever happen. Plumbers tend to over-spec though because no-one has ever called them back to complain that the system gets hot too quickly.

Ideally get a modulating boiler that controls its own heat output, but they generally only work in conjunction with their own controls. I have a worcestor bosch with their own "wave" controller which I find very good at keeping the boiler running on almost no output, so warm rads and not blazing hot which minimises temperature swings and means you can run it overall colder and not notice. 

When researching before installing our central heating system, about 10 years ago, my understanding was that big radiators were good (although they aren't now big enough for a heat pump) but you were better off having as small a boiler as you can get away with. The boiler benefits from the water circulating at a lower temperature but  works better when it is working hard. 

To be fair the boiler was fitted b (by a cowboy) to a very old one-pipe system (cast iron rads c. 1940s). We had the whole house re-plumped with modern two-pipe and new radiators. I think it might be too late for the boiler though.

This does sound like a circulation problem and not boiler specific issue which is unlikely to be blocked. I’d suggest investigating the pipework directly before the pump where the header and feed pipe join the system. 
it’s at this “neutral” point of the system that problems such as this can occur.

No, too much "value engineering" using plastic parts which degrade. Same problem with cars now. 

Not sure if it's the plastic parts that are the issue, the boiler problems I've had have all been electrical and this has been the case with many friends who've had issues.

I have 8 radiators in my house, but you have 21 ???????  Surely you have a maintenance team to deal with these issues

In my lesser experience, the high kW is useful when having a shower, not necessarily for the rads. I'd size it on the DHW requirements, but then again... 21 rads????

Get an independent qualified heating engineer to give you a qualified opinion on your existing Boiler. If it is sludge a powerflush should shift most of it. BUT -- if you change the boiler and it was a sludge problem you'll just have sludge in a new boiler

Don't forget to add inhibitor before you fill it up again, you can squirt it in a rad by unscrewing a bleed valve.

It's mostly all new pipework as stated above. When the secondary pump (7 m max head) is running on full (there's a low loss header) you can feel the water flowing through the furthest radiator. That's why I think it is an issue in the boiler as it has definitely been exposed to substantial sludge. I've had a guy have a quick look and says the internal pump is fine.

I've also personally re-piped the first 2 m from the boiler as part of my investigations.

It's a systems boiler with a hot water tank. I think recovery for a 120 litre tank is still only about 15 minutes with 26 kW is the coil is rated that high.

Our boiler is 29kW (Worcester Bosch Highflow 440) but very slightly undersized for our rads (there are 18 of them but including a couple of very large ones.) It has never caused a problem. We could also do with a system like Evohome, I might start a separate thread for recommendations. How do you find it? Can't help with the rest of your post I'm afraid.

Evohome works great in my experience - Honeywell have solid pedigree in this area and the whole system was a breeze to install. The automatic TRVs aren't that accurate so you need to calibrate a bit for each room , which you can do on the units themselves. Once locked in they keep the selected temperature in each room very steady.

I think for a poorly insulated large Victorian property, the ability to zone by room (or groups of rooms) is the best way to reduce energy usage. People need to be disciplined with door closing mind to maximise the benefit, though in my house with 4 young kids is fecking challenging!

I've had a Worcester Bosch Greenstar 27CDi boiler in my small 7-room terraced cottage, running 7 radiators, for 14 years. Excellent in every way. I suppose as gas gets much more expensive I may have to have a total re-think, but would need expert advice on this.

Interesting, thanks. I currently have a boiler controlled by a Hive smart meter so thought their system might be easiest to install, but it gets poor reviews. Can you do the Evohome as a DIY job or does it need a pro?

Re: your second paragraph, I had come to the same conclusion (along with trying to fill in some difficult insulation gaps and draft holes), but not managed to do it yet.

Likewise I have a 35kW Viessmann...had never heard of them until I bought the house. It powers 20 rads (just counted them!) and various bits of underfloor heating. It does a pretty good job in a c. 360m2 Victorian house, but not sure I'd want anything smaller? The previous owner installed a Tado system, which I'm still getting to grips with, but seems to work well enough in giving the room by room flexibility.

The only electrical wiring you will need to so is swap the wireless boiler relay over (from the Hive one). I was happy to do mine but you need to take the front of the boiler off (I would not consider this needing 'gas safe' qualifications). The HR92 motorised valves are very easy to fit BUT you must make sure you TRV heads are compatible, there are a range of adaptors supplied with the TRV but not surprisingly Honeywell recommend their TRV. I specced Honeywell TRV heads on my new rads so that was easy enough for me. I think what you get with Honeywell over a company like Hive is a lot of established engineering from a firm that is very large and has been around a long time so less buggy products and fewer concerns about future proofing. The evohome system has been around a number of years and is on at least a second iteration of controllers and actuators.

The HR92 seems to last about a year on AA batteries for our most used rooms and the system tells you when they need swapping out.