Sizing System Boiler to Tank

I am looking to purchase a System Boiler to heat an unvented cylinder 200-300 litres, and heat a 5/6 bed property.

Is there a simple metric to help size the boiler, ie kw/p radiator, and kw p/litre for storage tank.

Presume its always better to go over than under power, a good modulation ratio should handle a slightly over powered boiler.

Any thoughts appreciated.

 

Unless you're in a new build (with low heat loss) you do realise a system boiler is likely to require an additional expansion vessel and quite possibly an additional pump to heat such a property.
A heat only would be more sensible.
Why such a large cylinder, unventeds tend to have high kW calorifiers and the re-heat times are quick.

 

Thanks

Why such a large cylinder
There will be 4 showers in the property.

A heat only would be more sensible.
Confused, could you explain why a heat only would be better than a system boiler for an unvented tank.
My understanding is that a system boiler will have pumps and valves pre-installed in the box, where as a heat only does not.

Is there a simple metric to help size the boiler, ie kw/p radiator, and kw p/litre for storage tank.

 

Correct,

The down side of a system boiler is that the pump and valve gear may not match the heating requirements of the house.

 

Thanks

The down side of a system boiler is that the pump and valve gear may not match the heating requirements of the house.
That needs to be checked before spec'ing.

unventeds tend to have high kW calorifiers and the re-heat times are quick
Looking at a Megaflow Eco Solar PV Indirect (Can't find any figures for Eco Indirect), reheat times 1/2 hour for 200L, 45 mins for 300L @ 45K 15l/m).

Sizing System Boiler to Tank ...
I have read a few threads, with general folk looking for accurate heat loss/gain calculator online tools:
https://www.diynot.com/diy/threads/heat-loss-calculator.380763/

The Steltrad STARS app gets a good review:
https://app.starsapp.co.uk/

This webpage is very helpful, shame the downloadable spreadsheet throw a virus warning:
http://wiki.diyfaq.org.uk/index.php?title=Heat_loss

Can I accurately spec a boiler using these types of tools, all the boiler and tank manufacturers will not provide any assistance, I am told the Gas Safe engineer should spec the boiler - but with the right tools should you not be able to do this yourself, you will be paying the gas bill, so I would have thought be extra diligent

 

Quoting times to reheat is not much value unless they also quote the rise in temperature and the heat output from the boiler ( kW ) as well.

 

Ideally your installer should spec and supply everything, that way if it doesn't perform as expected it's their responsibility, not yours.

Quoted cylinder reheat times are based on the coil rating of the cylinder, which should be available from the manufacturer. Most are around 25kW unless you go for a High Gain version which can get up to 65kW on domestic. They're relevant if you're looking at installing a hot water priority system, which is based on the principle of heating the cylinder as quickly as possible

 

You should be sizing the boiler for the heat loss (and possibly the hot water - see later), not to match the existing radiators. Once that has been done you can compare the required heat for each room with the existing radiator to see if you need to make any changes.
It's about seven years since I suggested the Home Supply calculator; and that's because it was the best of a bad bunch. It's changed a lot since then but I would no longer recommend it as it makes too many assumptions. Stelrad Stars is very good, but you have to sign up to it (earlier versions were downloadable). The Myson Heal Loss Manager is also very good.

Has the incoming cold water flow rate and dynamic pressure been checked? It will need to be very high if you expect to have four showers running at the same time.

Don't forget that the max heating ouput of a boiler will only be required for about 5% of the year. So it is important that the boiler can lower its output (modulate) enough to cover the less cold months.

If the boiler is sized on the assumption that both heating and hot water will be on at the same time, there is a good chance that the boiler will not modulate low enough. The solution is to size the boiler for whichever needs the most heat - CH or HW - and run the system in HW priority mode. this is the way a combi boiler works, but you will not need the high output of a combi as the water is not being heated instantly.

 

Thanks.

We have made separate calculations for the radiators, so work now is to calculate heat loss for the boiler.

We have given up on the Stelrad STARS, after day and half of persisting, it loses windows and doors, is fiddly and finicky, then when you get there the browser tab crashes and you lose all yuor work.

Yes we need to check the incoming water flow rate and pressure, gauge and cup have been ordered., the amount of times 4 showers will all be running, I am hoping will be negligable. if it can power 2/3 I will be happy.

So it is important that the boiler can lower its output (modulate) enough to cover the less cold months.
Yes I can see that sizing the boiler is important for this factor alone.

The solution is to size the boiler for whichever needs the most heat - CH or HW - and run the system in HW priority mode.
Thanks - presume HW priority mode is controlled via some variety of progammable controller and various valves.

 

If you have calculated (how?) the required radiator outputs, then you know what output boiler is required: it's the sum of all the radiator outputs!

The size/type of radiator will depend on the design temperature of the system. Modern systems are designed to run at lower temperatures so they gain the most benefit from the condensing abilities of current boilers.

It's worth checking your results against the Myson calculator; I've never had any problems with it

The you will need at least 25 litres/min at 1 bar pressure

If you need 20kW to give 20°C when it's 0°C outside, you only need 5kW when it's 15°C outside. That's a 4:1 modulation ratio.

Correct. Th easiest way to do this is with a diverter (2-way) valve, which is open to the CH when unpowered but switches over to the HW when powered. If the CH is also zoned using valves, they would come after the diverter valve.

 

Any examples of these, please muggles?

 

What kind of showers?

I have a similar setup to you with a 38KW heat only boiler, separate EVs and pump (pump sizing calculated with the help of D_Hailsham - Thanks!). U/s, d/s and HW zones. I'll share some of my experiences:

1. With multiple zones and a large system, I found a Low Loss Header simplified the setup. It allowed me to have a boiler pump to satisfy my boilers flow requirements and a adequately sized system pump to heat all 26 radiators adequately (20 degree delta).

2. I balanced the system to heat my 300L unvented at the same time as my radiators. The reality is that this is a huge compromise. The HW heating temperature is dictated by the heating temperature of the radiators. This means it takes far, far longer to heat it up, even if you have the KW output to satisfy both. Instead, I have my HW cylinder heat up an hour before my radiators come on. HW priority would be preferred.

3. I find the incoming flow rate to the house fluctuates massively. Off-peak, ive measured it at over 50L/min. During peak times, it can fall dramatically. You may want to consider an accumulator to supplement the incoming flow, depending on the requirements of your showers.

 

https://theintergasshop.co.uk/79-joule-cyclone-high-gain-indirect-un-vented-cylinders

 

22 mins for 500L. Wow. Thanks.

 

Where did you find that info? It's nowhere on Joule's website.

Is it the heat-up from cold time (15°C to 60°C) or the reheat time after 70% draw off? If it's 'from cold time' then you would need a 72Kw boiler to do the job and a coil to match! If it's after 70% draw off , it will need a 50kW boiler and matching coil.

Someone is exaggerating the performance of their product.