Advice on choosing the right boiler system after house extension

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I'm hoping a learned contributor to this forum may be able to offer me some insights into choosing the correct boiler setup for my house that is currently being extended. I thank you in advance for your input.

My property will end up as with the following; 8 bedrooms, 4 bathroom, 3 living rooms, 1 kitchen diner space. All bedrooms and living rooms will have 1 radiator each. The kitchen/diner space is to be installed with 25m2 of underfloor heating.

I need to decide on the heating and hot water solution to implement sufficient for the house. My assumption up till now was to get a Megaflo or equivalent system installed. My contractor is telling me i should have 2 systems; leave the existing combi for one part of the house and add a Megaflo for the rest.

What is the best solution for the heating and water requirement?

Can a Megaflo meet the requirements for all rads and bathrooms? If so, what size/model would be appropriate?

Is having 2 systems a feasible idea or even advantageous in anyway?

Any insights will be greatly appreciated.
 
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An unvented cylinder setup would certainly be most appropriate for that size of house, but you need to know your incoming water mains flow rate & pressure before getting too far into it as there's little point having four bathrooms if your mains can only cope with running one at a time.

Two boilers might be better than one, but we'd need to know the calculated heat loss of your completed property before even beginning to advise on that
 
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When you say 2 systems, do you mean 2 boilers? Your post suggests that the UV is the second system and not an additional boiler!

From what I can understand from your post, your contractor is suggesting that your current combi remains supplying DHW to certain parts of the house and the UV will be used for the new bathrooms.
A UV (megaflo etc) has no relation to providing central heating, actually, as far as the boiler is concerned it is just another “radiator” (heat load on the CH) and is controlled by valves and switching.

So, the size (and age) of your boiler is now in question... is it able to cope with the additional load that will be placed upon it? Extra radiators, UFH and say a 300L UV?

Also bear in mind that if continued to be used as a combi for DHW to other parts of the house, then it will prioritise that demand during all other heating phases and will interrupt them.

You might also want to consider a separate “towel warmer” zone off of the primaries and you may also need a header to cope with the size of the extended system and demands.

I would be insisting on meeting and talking with your contractor’s plumber/heating engineer.

As you already have a combi, it would seem that your flow rate is ok, but get it checked as per @muggles

Speak to the plumber - not just the builder he works for!
 
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there's little point having four bathrooms if your mains can only cope with running one at a time.

If the requirement is 4 bathrooms and the incoming water supply is inadequate then the use of cold water storage in the loft should be considered amd maybe more than one vented hot water cylinder to reduce pipe distances between storage and taps.
 
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If the requirement is 4 bathrooms and the incoming water supply is inadequate then the use of cold water storage in the loft should be considered amd maybe more than one vented hot water cylinder to reduce pipe distances between storage and taps.

OP: Yes, worth “considering”.
If you have/will have the space for kit and decent static head to provide you with a satisfactory flow for showers and baths, else you’ll need pump(s).
 
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Thank you all for your input.

To understand how to proceed further, I will consider some of your suggestions and also present the following comments/queries;

I understand that an UV cylinder is usually installed with a system boiler, so my guess is that the builder's suggestion is to add the UV and system boiler in conjunction with the existing combi. Would this be an efficient and acceptable solution?

Would a large enough combi be sufficient for all rads in 12 rooms and a 300L UV? If so, what model of combi would be up to the job?

How would i calculate the incoming flow rate and pressure? How would i calculate the heat loss of the property?
 
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Thank you all for your input.

To understand how to proceed further, I will consider some of your suggestions and also present the following comments/queries;

I understand that an UV cylinder is usually installed with a system boiler, so my guess is that the builder's suggestion is to add the UV and system boiler in conjunction with the existing combi. Would this be an efficient and acceptable solution?

Not really. You don't want two boilers if you only need one. Either keep the existing combi and run an extra circuit to heat the cylinder, or do away with it and install a new system or heat only boiler instead.

Would a large enough combi be sufficient for all rads in 12 rooms and a 300L UV? If so, what model of combi would be up to the job?

The model that will be up to the job will be the one that you've calculated to be up to the job in the property heat loss calculations. You don't want a new combi though, if you're going for a new boiler then a system or heat only is needed

How would i calculate the incoming flow rate and pressure?

Using a flow meter and a pressure gauge, surprisingly enough. You should attach your pressure gauge to an outside tap, turn it on, and run enough mains taps to reduce the pressure reading on the gauge to 1.5 bar. Then use the flow gauge to measure how much flow you have. You may need to use more than one tap and add the flow rates together

How would i calculate the heat loss of the property?

Using a heat loss calculation sheet
 
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How would i calculate the incoming flow rate and pressure? How would i calculate the heat loss of the property?

You shouldn’t have to!
That’s the job of the Plumbing and heating contractor!
There are, however, numerous resources online that will explain how to do the above.

Look, this is how it normally works...
Customer approaches builder(s) for prices on a construction project, hopefully against drawings. Builder then calls whichever of his plumbing contacts is available, or he doesn’t owe money to, usually late at night (yes; been there), sends them a crappy photo of the drawings via SMS and asks how much to do the heating and plumbing? “Rough price, won’t hold you to it”. Plumber asks some important questions, builder bluffs his way through them and usually declares “it’s all straight forward, just like the last one you done for us”.

Builder then comes to you with an all-in price and you accept; you’re then stuck with what the builder has decided you’ll get for the money and will resist any variations as these will undoubtedly hold up his “marching on” schedule for building walls into boxes and making them pink, so he can invoice you and get onto the next job!

What you should do is ask for a meeting with the plumbing and heating engineer, on site, and discuss with him what facilities and performance you are looking for at the end of the job... your “comfort” expectations, how you’ll live in and use the system and fittings... this needs to cover all second fix items that you’re considering as they will dictate structural supports, demand, supplies, drainage, etc. Once briefed, he can then build a proposal and associated cost, which you can cut back on if beyond your budget, but it’s important for you to have the option rather than get what you’re given.

It’s also worth calling in an “independent” contractor of your own, as he will have no ties to your builder... but this has to be handled diplomatically if you intend to proceed this way, so as not to ruffle feathers.

That’s my advice mate... and it comes from experience of getting on site to commence works for a builder and then, having met and chatted with the customer, quickly discovering that the lovingly chosen fixtures and fittings they have on order do not tally and will not work with the construction and supplies specification and my name will be on the end performance... situation quickly deteriorates from that point and makes for awkward conversations!

Hope that helps you.
 
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Dilalio's advice is correct. especially this

but it’s important for you to have the option rather than get what you’re given.

It is you and your family who will be living with the system. Most builders / plumbers want to install that which will give them the most profit. They will not have to live with it.
 
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Dilalio's advice is correct. especially this



It is you and your family who will be living with the system. Most builders / plumbers want to install that which will give them the most profit. They will not have to live with it.

Most?
 
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of builders / plumbers I do ( sadly ) feel that most is the appropriate word. If considering heating technicians / engineers then most may not be the appropriate word

Then again Bernard... there are also “many” customers out there who are rude, arrogant, ignorant and couldn’t give a toss as long as it’s cheap... and when it fails... it’s the plumber’s fault!
I am currently lucky enough to give those sorts a wide berth and, believe me, it feels absolutely wonderful! :LOL:
 
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Thank you for all the advice provided in response to my query.

The conclusion i have drawn is that there seems little benefit in keeping 2 boilers. The most logical approach would be to install the appropriate system boiler and cylinder based on an assessment of the heat loss attributes of the house, which should be done with an appropriately qualified and objective technician/engineer.
 
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Thank you for all the advice provided in response to my query.

The conclusion i have drawn is that there seems little benefit in keeping 2 boilers. The most logical approach would be to install the appropriate system boiler and cylinder based on an assessment of the heat loss attributes of the house, which should be done with an appropriately qualified and objective technician/engineer.

Worth keeping in mind that if you only heat part of the house, then having multiple smaller boilers will work better and more efficiently. At the expense of having to maintain multiple boilers, though.

If you have zoned the property (which you should do in such a large house), then two boilers can also aid in balancing the system. From experience (and help on this forum), I learned that a large system can benefit from a Low Loss Header, which depending on the boiler and the central heating circuits, may also negate the need for more expensive semi-commercial/commercial pumps. If you search the forum, you'll find info on LLH's and Plate Heat Exchangers.

You also need to decide how many of your bathrooms you want to be able to use simultaneously. This will determine how large your HW cylinder will need to be. If your mains flow rate is not up to the job, you can have accumulators installed to supplement the water requirement for short periods of time (enough time to have multiple showers simultaneously).

Make sure whoever installs the system has experience with large systems. Maybe even use a commercial heating engineer. Pipe sizing is crucial.
 

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