Calculator recommendations????

[Harvest Fields]

Sorry, I read 8kW for up to 200l. What would you allow for a standard 117l cylinder then? <8kW, presumably?

Off the top of my head about 5 or 6

Edit due to spelling error

 

[chris watkins]

I've been told all sorts of things, including that if you have a 22mm pipe from the hot outlet of a combi, then you'll have reduced flow to the bathroom because 'the water slows down when it hits the 22mm pipe'.

Hence why I need to understand everything and why I question things.

In a 2 bedroom house with 5.5kW heat losses at design temperature, you'd specify a system boiler capable of providing 13.5kW at whatever flow and return temperatures you have decided on? Is that right?

If you did that you would have 8kw more capacity for the HW generation but why? especially if you think about what is going to happen to the boiler during the summer (& for that matter the spring & autumn) when there is no (or reduced heat load) the boiler is then going to be oversized. I know in this case the cylinder coil could probably take the 13.5 but buying a larger boiler than the max heat load makes no sense.
I also know they can modulate but they are designed to try to match there stated outputs with the system, if they can't they will cycle which is wasteful.
We need a matched boiler to the system demands where it can adjust it's output to stay in condensing mode for as long as possible.

CE54 recommends 2kW (or 3 if very large HW demand)
Domestic Heating Design Guide suggests 2kW for 2bed, 1bath (2.5kW right up to 4bed, 1bath 1shower) only then 3.

So.. where does the 8 or 5 or 4kW come from (not having a go, just asking the question)?
I have even had people suggest that the cylinder coil rating should be added to the heating load so it can do both at the same time. Heat load at -3C might be 15kW + 18.3kW (125L Megaflo) would = 35kW boiler selection, then they wonder why it don't work!!

 

[Ric2013]

Unvented cylinder?

No, vented.

I quote my own system because it's the only one I've had the leisure to observe in more detail than is probably healthy. My boiler can't modulate. Previous house I put a new cylinder in to run off the existing 13kW output zombie boiler. The cylinder manufacturer told me I didn't need fast recovery on a boiler that weak... and it also cycled like a mad thing once the cylinder had lost its initial chill.

I must say I'm thinking more like Chris Watkins in that I'd rather have the boiler run flat out without excessive cycling than have an oversized boiler.

 

[SimonG]

No, vented.

I quote my own system because it's the only one I've had the leisure to observe in more detail than is probably healthy. My boiler can't modulate. Previous house I put a new cylinder in to run off the existing 13kW output zombie boiler. The cylinder manufacturer told me I didn't need fast recovery on a boiler that weak... and it also cycled like a mad thing once the cylinder had lost its initial chill.

I must say I'm thinking more like Chris Watkins in that I'd rather have the boiler run flat out without excessive cycling than have an oversized boiler.

Auto correct of the week award goes to Ric. Zombie boiler!

Got me frightened already.

 

[ShaunCorbs]

That’s why

Unvented required more heat / they have a bigger coil cap

If you look at gledhill 300l you will see the coil is rated at 25kw max for a re heat time for 45mins I think from cold

 

[Ric2013]

No, I meant 'zombie'. The walking dead. Decades old, inefficient by modern standards, and running off grandfathers' rights, but still in service. Saw the word in "Installer" magazine and thought it was a common term, but I think it is ONLY "Installer" that uses the term.

 

[Ric2013]

That’s why

Unvented required more heat / they have a bigger coil cap

If you look at gledhill 300l you will see the coil is rated at 25kw max for a re heat time for 45mins I think from cold

Thanks Shaun. Someone said 'any cylinder' so I assumed this included vented.

I wish I'd fitted a fast recovery in my last house: every time the boiler took an hour to reheat the water I sighed and wished I'd stuck to my guns and bought a fast recovery instead of listening to the idiot who told me my boiler wouldn't cycle. Another thing I've been told grr .

 

[chris watkins]

That’s why
Unvented required more heat / they have a bigger coil cap
If you look at gledhill 300l you will see the coil is rated at 25kw max for a re heat time for 45mins I think from cold

Vented or unvented an indirect hot water cylinder installed into a dwelling (heated by gas in this case) must be Part L compliment, this covers the re-heat times which must be 25 mins given the required boiler power is available.
Re-heat means 70% of its volume through 35 - 40 degC

 

[Ric2013]

Vented or unvented an indirect hot water cylinder installed into a dwelling (heated by gas in this case) must be Part L compliant, this covers the re-heat times which must be 25 mins given the required boiler power is available.
Re-heat means 70% of its volume through 35 - 40 degC

The cylinders I'm referring to are part L compliant, according to the manufacturers.
Thanks for your post - it explains a lot if re-heat times are quoted at 70% of the volume @35-40K in 25 minutes. Since we put the thermostat a third of the way up the cylinder, it's probably just below that 70%, after which the coil is half covered in heated water and heat transfer drops considerably.

What would be good would be a standard that requires a cylinder filled with 20°C water to satisfy a stat set at 60°C and located a third of the way up the cylinder, in 25 minutes, with a flow temperature to the cylinder coil of 80°C and sufficient flow to ensure the return is no lower than 70°C. You'd need to define the standard to allow for 75/55° operation too.

At least we know it would then work well, rather than just hit an arbitrary target (how do you measure the temperature of 70% of the water anyway?). But as someone who worked at the BRE when they were writing the Code for Sustainable Homes, commented 'we knew when we were writing it that it was ultimately a load of rubbish'.

 

[king of pipes]

I did a large town house with 20 rads they came out at 24kw on their own so chances are you are correct 30kw would be more like it . Kop