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.

Is this a new build or retrofit? What's below the UFH pipes?

So your boiler has a minimum output of a out 10kW, which is pretty huge. So you really need most of your radiators on, while you try to run the UFH. Bringing down your flow temp will help, as it will slow down the system and reduce radiator output, so thermostats cycle less. Tell us more about what is controlling your radiators?

If you count is 1 full cycle in under 10 mins, you have issue to fix. Your boiler should start, modulate down and run for a prolonged period.

100L cylinder with a 32 kW boiler takes under 4 minutes to go from 45 to 60, as it's only 1.75kWh heating capacity in the cylinder. So to get a useful run time you would need several hundred litres. Hence my recommendations

Remove actuator from valve and truly to rotate the valve with your finger. It should be easy to move. Manual valve is an isolation valve, so likely to have a handle on it. May look similar to this Do you have any drain valves upstream of the UFH but after the motorised valve. Will look like this?

1. I wouldn't bother 2.Not sure what you will gain from this 3. That just makes stuff way more complex, and sure it will gain you much. Read a few Baxi datasheets and manuals and they don't seem the most sophisticated of boilers and not the best output turndown. With this in mind, you need places to dump heat, as your summer house will struggle to a absorb the heat on offer and I would suspect your boiler doesn't run for long and has plenty of cycling, which will ultimately reduce life, but use way more gas than needed. Turning down flow temp could lead to issues with cylinder heating, so do it small steps, you need to ensure the cylinder heats in one long boiler run, not lots of short cycling, which will be the result if you trim heat too much. The ideal way to run UFH is low and slow, so I suspect the UFH mixer could be turned down. To help the boiler out, need other parts of your heating system open. This is basically a recipe for boiler short cycling, higher gas consumption than you could have. So timings I would set all the same, run whole house a little warmer. Next steps and questions Does boiler accept opentherm control? You really need to run the heating system differently, so priority domestic hot water, opentherm is an easy way to implement this. So basically you run one flow temp for cylinder heating and another for central heating.

Few things, motorised valve not opening valve. A manual valve closed in that circuit. UFH pump not starting UFH mixing valve stuck and not sucking any new hot fluids in.

There climate is a lot different from ours, they are hotter in summer and way colder in winter, which is way better than the just above freezing, high humidity winter climate we have, for ASHPs. So defrosts are way less of an issue. Then it's just a matter of sizing correctly.

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. 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.

Some simplification sounds the order of the day. A couple of radiators kicking a huge boiler into life for 5 mins isn't an efficient way to run a heating system. Will wait for the additional information requested above.

But would life be worth living afterwards, "I would of or I wouldn't of" being mentioned many times, comes to mind

Solution is simple - get rid of the blockage. Either remove down pipe and unblock or leave in place and blow out debris with a hose pipe from the top. Maybe while your up there clean the gutters if that's what feeds the down pipe. Most window cleaners now do a gutter cleaning service, get them cleared every year, or if plenty of trees around more often. Fix the problem, don't cure the symptoms.

What in the rest of the heating system, as I assume you are not using 32kW just to heat a garden room?

Nearly twice the cost of the UK! That's expensive heating, a time of use tariff and direct electric heating would cost about the same when you add in boiler efficiency.