Megaflo Hot Water allowance in boiler sizing

[nbullus]

4 bed detached house with 2 bathrooms.

We have just ripped out the old 1960’s one pipe system with gravity HW and pumped CH. The layout is being upgraded to a sealed un-vented 2 pipe system with a system boiler and 210L Megaflo indirect cylinder. Both the boiler and cylinder are being housed together in the garage and CH and HW will be independently controlled with an ‘S’ plan system using 2 x 2-port motorised valves.

I have calculated the CH requirements, which comes to 12 kw (including 5% pipe loss and 10% boiler margin). I am struggling to understand the allowance to add for HW so that the CH does not suffer when both are calling for heat at the same time. Some sites say 2kw, some say 3kw. Personally I think this should be determined by the size of the cylinder and how quick you want the tank heated – am I right?

What I need to understand is how you work out how much heat is going to each of the HW and CH circuits for a given boiler size when they are both calling for heat at the same time? Is it 50/50 split or based, or as I suspect, something more complicated than this as, e.g. length of pipe work on each circuit.

For example, if just added the 3kw allowance for HW and therefore fitted a 15kw boiler, what’s stopping the Megaflo taking more than its 3kw allowance and therefore reducing the heat going to the CH circuit? Just because I added 3kw for HW does not stop the Megaflo, with a massive 24kw rated coil inside, zapping as much heat from the boiler as it can take?

Obviously to ensure both circuits are fully satisfied with no impact on either circuit I would need in excess of 36kw which is rather high just to keep CH happy and get the quickest HW heat up times. However, I’ve worked out that it will take around 4.5kw to heat the full 210L in 3 hours, or around 6.8kw to heat it in 2 hours which is quite acceptable (assuming rise in water temperature from 4 deg to 60 deg). So if I put in an 18kw boiler, how will my CH suffer when both are calling? What determines how much of the 18kw the Megaflo will take and how much the CH will take in each hour?

Any help would be appreciated, but please only comment if you actually have some expertise in this area.

 

[gas4you]

I would allow at least 3Kw for the size of Megaflo you indicate, possibly 4Kw.

You are correct in youe assumptions about the coil being rated at 24Kw, but in reality you do not need to worry about this.

Assuming you have a new set of full controls with this new set up, I would recommend setting the HW to come on about 30 mins before the ch in the morning, so the cylinder will get up to temp quickly with the full power of the boiler dedicated to this.

When the ch comes on the boiler will be working just for this. At other times of the day, eg evenings, the boiler will be ticking over, probably with several trv's shut so if you need to re-heat the cylinder it will have spare capacity.

Usually first thing in the morning is the only time the boiler would be asked to work at maximum.

 

[nbullus]

Thanks Dave for your prompt reply.

Yes we have a new two channel programmer being added as well so we can kick in the HW earlier, and most radiators will have TRVs fitted.

I had a feeling this might be the right way to go. I think the 4kw HW allowance is what I will use which puts the total requirement to 16kw and just over the 15kw limit of the boiler I had in mind. I think I will therefore go for the 18kw model (regulates down anyway) as this gives me 6kw spare capacity for HW (on top of any spare capacity from the CH in the evening) to get some very good recovery times when the CH might be on.

Putting the HW on first with the full 18kw going in should heat the whole tank (assuming it's empty which some days it won't be) in 25-30 minutes before the CH kicks in, so great advice.

Just out of my own personnel interest, how do you work out the output split to each circuit if both were calling together first thing in the morning - any idea?

Thanks again,

Nigel.

 

[Daniel198960]

Hi Nigel

I know this is a very old post but I am having the same issues in calculating the requirements for the water heating when the central heating needs to also be on.

Can you tell me if the 18kW system boiler was fit for purpose in the end?

I have spoken to various manufacturers to gain clarity from them and whilst the 3kW to 4kW has been mentioned a few times, one did tell me that 10kW is allowed for on new build houses.

I have underfloor heating and radiator rating requirements of 14kW so this takes me into the 26kW range of system boilers eg. Ideal Vogue Max, etc.

I still haven't received an answer from anyone as to how the boiler splits cylinder and central heating requirements. I assume the boiler will work to its maximum and evenly distribute the power across the whole circuit given that all water is being heated at one time in one heat exchanger.

I hope to hear from you!

Regards

Daniel

 

[Madrab]

Ideally you would start your own post and then refer the original poster to draw their attention, especially given the last time the poster logged on was 2009.

The coil inside the cylinder has a specific output to the water, given in kw/h. In other words, it can transfer a specific amount of heat into the water per hour, that is usually calculated from cold. It is usual to then add 10% for fluctuations and heat loss through pipework etc. I wouldn't ever quote 10Kw just because it is a new build, the draw is dictated by the size of the coil, not whether its a new build or not.

That requirement is then added to the central heating requirement to give a total system draw if everything was calling for heat at the same time. The other consideration is that it is all calculated at maximum possible output but CH/HW systems are rarely used at that level. How it splits the requirement is down to the system design.

Look at the cylinder coil specifications.

 

[Daniel198960]

Hi Rob

Thanks very much.

Being new to the forum I didn't even think to post a new topic but will do so now.

By account of your calcs, that would mean 14kW (for UFH, rads) + 24.5kW (coil of MegaFlow 300i) + 2.5kW (10%) = 41kW!

I will struggle to find a system boiler at that size so goodness knows how a homeowner or heating engineer would specify a property bigger than out 4 bedrooms and 3 bathrooms.

The 10kW used to specify a new build is done so, according to Worcester, as they are chiefly responsible for the design of the system and the kW output required of the boiler.

We have assumed rads are on at 100% with no TRV control as well as the water being used in large quantities.

The chances of this might only be a concern on Christmas day.

Regards

Daniel

 

[Madrab]

Diversity can be applied to the required calculations. It's rare that everything will call 100% all at the same time for extended periods of time, even then if everything did, there would be reduced input at the end points, not no input.

If the concern is that the HW will run out then a HW priority system would be designed.

You really need a well qualified resource that has experience with advanced system experience onsite to get onsite and do a proper survey and design. You might find that you will need a large boiler, if you don't want to have any dip in delivery to both the CH and HW, especially if it will be considered a non domestic setup (if it is to be used as a guest house).

 

[Daniel198960]

Thanks again Rob! I have had an engineer specify it but as I expected, he took estimations of kW outputs whereas I added up the kW of each radiator he specified and added this to the kW output given to me by the underfloor heating company that supplied me the system! His own calculations were a fair bit higher and led him to advise a 35kW boiler as he added in 7kW for the cylinder heating.

What he didn't appear to appreciate is that both requirements should not be added together and only the cyl. coil rating should be accounted for to give the speedy reheat times that the cyl. technology gives.

If the 35kW boiler modulates, why is a larger boiler an issue? Will it still be cycling on and off more than the correct sized unit even though it's modulating?

 

[Madrab]

Modulating means when there is a call for heat, then the boiler can reduce it's output down rather than cycling on and off. A larger boiler may struggle to modulate right down to the minimum requirements but again that would need to be calculated and what that lower limit may be.

The boiler would be rated by the usage requirements, adding all the components together gives a maximum starting point, that can then be balanced against how the system will be used, what may be needed when taking into account levels of insulation, correct heat loss calculations, diversity, etc.

i.e. the floor mass wouldn't cool down that much in the 30 odd minutes that a cold cylinder would take to re-heat, neither would the overall house temp unless poor insulation, draughts, space design (open plan against rooms) etc.

There are a large number of factors to take into consideration but it all needs to be done onsite by someone that is qualified and experienced in end to end advanced system design.

 

[Daniel198960]

Rob, thanks very much for your great help! I have arrived at the conclusion that a 26kW would be ample in allowing the boiler (Ideal Vogue MAX system) to modulate down to 5.2kW but also give me the benefit of the cylinder technology in that it will fully utilise the 24.5kW coil inside the 300L cyl. and will give me the fastest possible re-heat times in instances where I have run out of hot water.

I only hope that the 5.2kW is low enough and that there are not times when I, for example, only need 3.5kW of power and the boiler cycles on and off as it can only provide 5.2kW of power. This would make the Worcester Greenstar 8000 a better choice as it can modulate far lower albeit with the disadvantage of an aluminium heat exchanger!

 

[Madrab]

Don't forget, it is really difficult to design a system that won't cycle every now and then, the trick is to keep the number of times it does to a minimum. That comes from a proper setup, the users interacting with the system, a little monitoring and fine tuning. As usage changes then so should the setup. A lot of the newer SMART controls can help here too. A lot of them have a level of self learning and will automatic build in efficiencies. Does need to be setup by an experienced hand and used properly though.

Too many people just leave a system at default and then pay for it in the long run.

Must be honest though, I wouldn't pick either of those boilers but I am a bit biased towards Intergas, as an installer. Ideal have never been a boiler of choice and I am keeping an eye on the 4/8000 series to see how they get on but I won't hold my breath as all they have done really is just jazzed up the box, internals are very similar. IG's parts, long term, can be a bit more expensive and their UK position isn't maybe as strong as some others but with only 10 odd parts inside of which only 4 are moving, then they are a very good bit of kit IMO and I've never had an issue with support and I look after quite a few.

 

[Daniel198960]

Hi Rob, thanks again for great input. Do you agree in principle with the direction I am taking in opting for a kW output just big enough to put the full amount of heat through the cyl. coil?

The Vogue Max has good reviews and in a recent survey of which boilers gas engineers would install in their own own, came top with 22 votes of a possible 100.

If installed by a Max approved installer, I also get 12 years warranty so I can't help but think it is worth a chance. In addition, they use stainless steel over the aluminium equivalent heat exchangers found in Worcester's range.