Calculator recommendations????

[SimonG]

Heat water outside peak heating times.

Back to Rileys original question. Would you not be better just doing a quick full house heat loss calc?

 

[Welder]

100% a proper full calc to demonstrate your professional skills.
100% no reason whatsoever NOT to have an uvc come on before heating and to top up when not on. Only a fool wouldn't.

 

[chris watkins]

Back to Rileys original question. Would you not be better just doing a quick full house heat loss calc?

http://www.domestic-gas.co.uk/documents/CE54.pdf
This was designed to do just what you want. i.e. it provides a total heating load for sizing boilers without the need to do room by room full calculations as you would for heat emitters.

PS. you can take a view on whether you allow 2 or 3kw for DHW, personal I don't.

 

[Last Plumber]

I would normally calculate the heat loss from the house and also use the chart I linked to, to work out what they have now and what they need. That way you show them the full picture.

 

[Ric2013]

8kW for any cylinder up to 200l Sounds excessive IMO.

My own boiler is only 8kW and will heat the cylinder from cold for around 20 minutes and then starts cycling. In practice, we tend to reheat cylinders before they are stone cold, so I can't see a cylinder accepting 8kW very often.

 

[Harvest Fields]

8kW for any cylinder up to 200l Sounds excessive IMO.

My own boiler is only 8kW and will heat the cylinder from cold for around 20 minutes and then starts cycling. In practice, we tend to reheat cylinders before they are stone cold, so I can't see a cylinder accepting 8kW very often.

Just what I was always told and what I now work to mate.

 

[Ric2013]

Just what I was always told and what I now work to mate.

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?

 

[Harvest Fields]

No. Because clearly a 2 bedroom house wouldn’t have. A 200 litre cylinder.

 

[Ric2013]

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

 

[ShaunCorbs]

8kW for any cylinder up to 200l Sounds excessive IMO.

My own boiler is only 8kW and will heat the cylinder from cold for around 20 minutes and then starts cycling. In practice, we tend to reheat cylinders before they are stone cold, so I can't see a cylinder accepting 8kW very often.

Unvented cylinder?

 

[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