What size boiler?

[davebew1]

Firstly may I thank those of you who replied to my first question and for taking the time to do so. Your comments were very useful.

I wonder if I could ask your advice on the size of the boiler I need as I am getting conflicting quotes.

I have a 4 bed detached house which has twelve radiators and a hot water storage system presently heated by the outgoing boiler. The recommended replacements are the Baxi 18 and a similar sized Worcester by one firm but another chap has recommended that we go for a bigger capacity Baxi (I think he said Baxi 50) because of the size of the house.

I would be grateful of your views as I would hate to chose a boiler that may be overworked.

 

[swidders]

How long is a piece of string? Boiler should neither be overworked nor underworked for efficiency.

If you want to do your own estimate, use a btu calculator (free online downloads for this) to work out required output for each room and find total? Then add a bit - am sure installers will post a percentage soon enough. Then look up specs from any manufacturers website or online supplier.

not sure about Baxis- the pro's on here seem to favour other makes which browsing through historic posts will indicate - but avoid any recommendations from someone who calls himself BigBurner.

 

[Dan Robinson]

Your second chap is a fool. 18 - 24 kW is fine -Worcester also OK at a push (see me historical posts). Avoid Baxi's like a Rag-man's trumpet.

 

[gas4you]

Are you sure the second chap wasn't talking in Btu's rather than kilowatts?

In that case he would have been recommending a 15Kw.

 

[Dan Robinson]

Fair point Dave - but he would still be a fool

 

[namsag]

So what problems have you had with baxi solo/megaflo`s then Dan

 

[D_Hailsham]

I have a 4 bed detached house which has twelve radiators and a hot water storage system presently heated by the outgoing boiler. The recommended replacements are the Baxi 18 and a similar sized Worcester by one firm but another chap has recommended that we go for a bigger capacity Baxi (I think he said Baxi 50) because of the size of the house.

Use the Stelrad Elite Catalogue to work out the total wattage of your existing radiators. Then add on 2kW for the hot water

Use the Sedbuk Boiler Calculator to work out what size boiler you really need.

If the Stelrad total is greater than Sedbuk, get a boiler which is midway between the two calculated values. If the Sedbuk total is greater than Stelrad, get one the same size as the Sedbuk total.

The reason for this logic is that radiators tend to be oversized so there is no need to buy such a big boiler, the oversized rads will also help keep the boiler running in condensing mode. If on the other hand the rads are undersized, its better to put in a slightly larger boiler so you have the spare capacity when you install larger radiators.

 

[Dan Robinson]

So what problems have you had with baxi solo/megaflo`s then Dan

Thankfully few 'cos we don't install them, I certainly don't like the build quality on the ones I have come across during repairs.

Megaflo's are OK I suppose, but they're just a tank, so as long as the fecker doesn't leak then fine... OSO are better though!


Baxi must be getting desperate though, a few months back their regional sales manager phoned me asking to fit Baxi's again. The only way he could have got my details would have involved quite a bit of digging through their records.

I kinda felt sorry for him really - especially after my little speach about reliability, customer service, trusting the installer etc.,

 

[Deleted member 105431]

Baxi are in dire financial trouble, they are making redundancies and cutting costs left right and center. They are trying to boost sales any which way they can.

Serves em right for all the fooking carp boilers they made!

 

[namsag]

Now there you go Dan limited product knowledge the Baxi Megaflo is a sealed system boiler not a tank .
I would imagine most reps are trawling records just now trying to drum up business every merchant i have used in the past is phoning me every couple of weeks asking me to buy off them again .
It is starting to bite out there and they are all starting to panic

 

[Dan Robinson]

Not at all... Just referring to different things...Providing a wider scope to your post.

They're just a re-branded Potterton something or other anyway...aren't they?

I must confess to not being very interested in the various pueces of excretia that emanate from Baxi toers though.

 

[bengasman]

The layman's guide to system-boiler sizing.

Measure and add up the lengths of all the radiators in metres, doubling the length for double radiators.

This method is based on rads of 600mm high, so make a rough correction of height times 1.2 for 500mm high and divided by 1.2 for 700 mm high.

You know have the exact size of your requirement in kilo watts.

As a new boiler should be using an up to date programmer giving you the option of selecting completely seperate times for ch and dhs, there is no need anymore to add capacity for the cylinder like it used to be done in the olden days.

 

[D_Hailsham]

Measure and add up the lengths of all the radiators in metres, doubling the length for double radiators.

I can buy that one. A 600mm radiator gives approx 1kW per metre length.

This method is based on rads of 600mm high, so make a rough correction of height times 1.2 for 500mm high and divided by 1.2 for 700 mm high.

But this is where I get a bit lost as I don't understand what you are trying to say. A 500mm radiator will give out less heat than a 600mm of the same length, but you correction seems to suggest that it gives out more; and vice versa for a 700mm radiator.

Surely, if you have 500mm high radiators, you should divide the length in metres by 1.2 to give the output in kW; and, if you have 700mm rads, multiply the length by 1.17 to give output.

As a new boiler should be using an up to date programmer giving you the option of selecting completely separate times for ch and dhs, there is no need any more to add capacity for the cylinder like it used to be done in the olden days.

That assumes the hot water cylinder is never heated at the same time as the radiators, either because they are timed differently or because a diverter valve is used, not a mid-position. But there will still be many people with mid-position valves or S Plan systems who want to be able to top the hot water up in the middle of the day when the central heating is on.

 

[bengasman]

you are right, was indeed thinking back to front.

as for the diverter, bearing in mind that a proper install most likely includes a (power)flush, new valves and new controls, changing the valve if that is what it takes, should not be too big a job.
the difference in heating up between completely separate times for ch and dhw and both at the same time is so significant that it is well worth the minor extra expense.

 

[D_Hailsham]

the difference in heating up between completely separate times for ch and dhw and both at the same time is so significant that it is well worth the minor extra expense.

Are you saying that, if you have a diverter valve set to give HW priority and you want to 'top-up' the cylinder in the middle of the day, when the heating is going full blast to keep the house as 21C and it's -10C outside, the HW will heat up so quickly that you will not even notice the temperature drop in your living room due to the heating being temporarily off?

I agree that heating a cylinder full of cold water at 5C up to 60C will use a significant amount of heat and will take a much shorter time if the boiler is just heating the cylinder. But when the cylinder temperature has only dropped 10C (the differential on a typical stat) before it will be reheated, the time difference will probably be negligible - and I would rather stay warm than reduce the reheat time by a few minutes.