Replacement Gas Boiler Installation – What Size Boiler?

LucianH

We’re cold and we need a new boiler - old Potterton Suprema 80 is gasping its last breaths .  (And apologies for the long post btw.)

I’ve had three installers out to look at the installation of a new heat-only gas boiler and I’m really confused as to how an installer decides what’s the best replacement.  (And these three installers were picked as a result of looking at many reviews.)

Before I get onto the points that are confusing me, here’s the brief details of the property - it’s a large Victorian property with four large bedrooms and four reception rooms.  The existing Potterton was installed about 20 years ago which replaced an oil-fired boiler.  There are 15 radiators - many of the original radiators have been replaced but there are still about 4 radiators which date back to the original oil-fired installation. A small extension of a single large ground floor room has been added to the property since the boiler was installed.

Let me mention some of my observations following the visits from the three installers:

- I know we’re in the middle of a pandemic but not one of the installers asked to look around the property and view the existing radiator installations or the room sizes.  I can only conclude that the installers base the required boiler power on what is currently fitted (ie, Potterton Suprema 80) as opposed to assessing what the actual required power is from the size of the rooms and radiators.

- One of the installers said that I can’t have the condensate pipe running outside and will need to connect to an internal waste pipe (which would be very difficult in the current installation).  Another one said it was OK to run the pipe outside and the other one didn’t even mention the need for the pipe.

- With 15 radiators (and reasonably sized rooms) I was looking at a 30kw boiler.  I was told by one installer that you do not want to oversize the boiler since it’s better to run a low power boiler at full steam rather than a higher power boiler that will run below its maximum power level (does it not affect reliability?).

- I was also told that the current installation with 22mm inlet and outlet pipes going down to 15mm pipes for the circulating central heating circuit (sorry, don’t know the correct terminology) would not be able to support a 30kw boiler.

So, I have some very basic questions:

- Is it correct you should not oversize a boiler?
- Can a 30kW boiler be fitted with 22mm inlet and outlet pipes going down to 15mm pipes for the circulating central heating circuit?

- Is there a good website where I can get a reasonable estimate of the boiler power requirement which takes into account all room sizes as well as radiator sizes (the Worcester Bosch website seems to require only the number of bedrooms and bathrooms and seems to ignore the number of reception rooms)?

- I would really like to know what the power rating of the current Potterton Suprema 80 boiler is.  I’ve checked the spec but it gives me two figures for two different burner setting pressure:

  Output=20.1kW, Input=25.6kW, Burner Setting Pressure=10.7mbar

  Output=23.4kW, Input=29.3kW, Burner Setting Pressure=13.9mbar 

I’m confused.  How do I tell what power my current boiler is?  I assume the power figure quoted for boilers is the input power so is the Suprema 80 a 25kW or 30kW boiler?

- I’m assuming that it is Ok to run the condensate pipe outside provided it’s well insulated

Any help would be most appreciated

Bendy_House

Hi Lucian.

Yes, it's a bit concerning that none of them looked around! Having said that, they will (hopefully) be experienced, so will have a very good idea of your house's requirements just going by its size and type. Did they know about the extension, tho'?!

Ok, an easy way to get a ballpark would be to know what output your existing boiler is... Along with this Q - does it heat up the house ok?

Yes, there are some good sites that'll help you work out kW requirements for each room, but I don't know what they are.

Yes, of course a condensate pipe can go outside - that's where most of them go. And, yes, it'll need protecting against freezing, and end up either in a suitable drain, or in a mini 'soakaway' with lime chips. If it can be run to an internal waste pipe, then that's better, but it sounds as tho' that could be tricky? The one who didn't mention it at all must have a cunning plan - just ask them.

Ok, the flow and return pipes leave the boiler in 22mm, but then reduce to 15mm for the 'pump'?! That is very bad. What happens then - does it go back up to 22mm?

Most/all modern boilers 'modulate', so can turn down their burners to suit demand, just like you turn up and down a gas hob burner. I think the 'run a small boiler flat out' theory is old school, but hopefully plumbers on here will advise. In any event, if fitting a smaller boiler means it has to be running with a higher output temp just to keep things supplied, then that isn't good for a couple of reasons - one is the obvious extra wear and heat damage, and the other is that a higher return temp will make the boiler less efficient. I would personally opt for a larger boiler running at 3/4rs...

BUFF

I am not an expert but imo:
It's a 23.4kW max (output) boiler that you have. (How do you currently heat your DHW, are you happy with that or could this also be an opportunity to upgrade that too?)

I have  1/2 as many radiators as you do & a 29kW max output combi boiler (Worcester Highflow 440CDi) on 22mm pipes with some even dropping down to microbore - it's been fine for 13 years (but, if you can, read the installation manual for the boiler that you are thinking of).
The thing that I would be concerned about would be the gas supply - is it adequate for a proposed more powerful boiler (mine needed upgraded)?

You might want to read https://idealheating.com/boiler-size & anything similar - bearing in mind that they will of course recommend their own models ...

A modern condensing boiler with load & weather compensating controls should allow boiler modulation to better match output with demand (assuming that demand is not higher than max output) as well as save fuel compared to your existing set up. Hopefully you have TRVs on your rads too (except where your room stat is).
When looking at competing boilers it might be worth comparing just how low they can modulate down to for heating.
Also, in order for the boiler to condense efficiently the flow & return temperatures will almost certainly need to be appreciably lower than those that your existing system runs which may negatively affect how quickly your house heats up from cold.

It is normal/preferred practice these days to run the condensate to an internal waste as it saves drilling a hole in the wall & lots of callouts for boilers not working due to frozen condensate discharge. Afaik it is still allowable though to run externally as long as suitably insulated etc.

nofoollikeold

1. It is correct that you should not oversize a boiler, or at least not by much.  Modern boilers adjust the heat they produce (and thus the amount of gas being used) to reach a target value.  To do so they "modulate" the heat produced, and a good quality boiler should be able to produce from around 15% of maximum rating up to 100%.  If you grossly oversize the boiler, it will modulate as far as it can, but then will turn off and wait until more heat is needed when it will fire again.  This is called cycling and is harder on the boiler than running constantly, albeit at a lower level.  A very crude calculation is 1.5 to 2kW per radiator.  Given the number of radiators and likely construction of the property, 30 kW sounds about right.  The correct way to size it is to calculate the heat loss for each room and sum them, allow 10% for initial warm up, and correct for any other factors (principally the difference between the average of flow and return temperatures and the normal temperature expected in the room) The sum of these figures for all rooms, plus an allowance for water heating (2 to 5 kW depending on cylinder recovery time) gives the required heat output of the boiler. 

2. It is normal to have 22 mm pipes providing a "spine" running to each floor, and around that floor, with short lengths of 15 mm pipe coming off the flow and return spines to feed each radiator.  If your system is nearly all 15 mm pipe with only short lengths of 22 mm going into the boiler it would not support a 30 kW boiler and 15 radiators, at least not with hot radiators.  I ver much doubt if this is what you have.

3. The boiler you have is 80,000 BTUs - equivalent to 23.4 kW.  This is the heat output of the boiler, but it needs 29.3  kW of power to get there, which means it is about 80% efficient.  To achieve 23.4 kW of output heat it requires a certain amount of gas to be burned, and this needs to be supplied at a pressure of 13.9 mbar.  A lower pressure will supply less gas in the same time, resulting in a lower power output.  

4. There are various web sites which purport to give you heating loads.  In my opinion they aren't very accurate, and the further you get from a cavity wall, insulated loft "standard" property, the less accurate they become.  You really need to use an accepted method to calculate the heat loss for each room so that you can sum them.  You can get worksheets off the internet to help the calculations.  

5. You can run the condensate pipe outside, although it really is much better if you can avoid it.  If outside, it should convert to 32mm (1 1/4") plastic pipe inside the building, then run with a decent fall as short a distance as possible to the nearest foul water gully.  It is mildly acidic, so must be run in plastic pipe.  It can be run to a soak away if absolutely necessary, but must go through a neutraliser (usually marble chips) which must be renewed from time to time.  

Calculated properly you might find that 30 kW is a little low for the property, and opt for a slightly larger boiler.  This would also provide some buffer should you decide in the future to extend.  

knightstyle

Surely the calculation for the size of boiler needs some more basic information such as isthere adequate insulation in the loft, are the walls cavity and do they have any insulation, good double glazing?   ETC.
Remember that the boiler only needs to replace the heat lost from the building so installers should take the above into account.

Bendy_House

Fab stuff from NFLO as always.

Lucian, you can do all the calcs and arrive at '30kW', or you can hold your index finger high above your head, waggle it a bit and then say '30kW is fine'.

It's a roughish science, but bear in mind that not all the rads will be pumpin' at the same time. Bedroom rads will be a morn and late eve thing, or will likely be turned quite far down. 30kW is a lot, and that should do most stuff.

And obviously whatever is going on with the pump needs a solution.

As asked before - does your current boiler heat your house fine? If so, 30kW sounds a tad high to me. (No calcs done, of course...)

tacpot12

It is correct that you should not oversize a boiler. A boiler that is sized correctly should be able to heat your home on the coldest day that we might have over the next 20 years. Most days, the heat load will be much less, so even correctly sized, it is already too big most of the time. 

As the existing boiler can only have an output of 24kW as a maximum (the name Supreme 80 comes from the output being 80,000 BTU, which is about 24kW), you should only need a 24kW boiler as a replacement, so the issue of the 15mm distrubtion pipes  goes away. If the Potterton wasn't heating the house adequately, you might need a larger boiler and the distribution pipes might need to be upgraded to move the extra heat to the radiators. 

BUFF is completely correct when they said to get the efficiency benefits of the condensing capability, the flow & return temperatures will almost certainly need to be appreciably lower than those that your existing system. But you will have trouble acheiving a sufficiently low return temperature as you need to heat hot water in the cylinder and you need this to be at least 60 C to avoid legionella, and you need the water to be hot enough to heat the house on cold days. 

One solution to consider is a hot-water priority arrangement where the flow temperature is set to at least 60 C when the cylinder needs heating and the flow is directed to heat the cylinder only (it doesn't go to the radiators). Once the cylinder is full of hot water, the flow temperature is set based on the outdoor temperature and is allowed to circulate through the radiators. This means that most of the time the flow temperature will be low enough that, even with your existing radiators, the return will allow condensing, and on the coldest of days, the house will be warm (but you won't acheive condensing performance).

If you need a new cylinder (or if you already have one with a fast recovery coil) I would definitely consider the hot-water priority with weather-compensation during heating and a high set point for hot water production. If you need a new cylinder and can get one with a fast recovery coil, you will reduce the time it takes to reheat the cylinder - while the cylinder is reheating, you can't get any heat to the house with the priority arrangement.

So you can have a modern condensing 24kW boiler toheat your house. 

ComicGeek

Heat loss calculations overestimate the heating demand for at least 99% of the year as they don't include for internal heat gains from people, equipment, cooking etc, as well as solar heat gains. They're looking at the very worst case scenario which might be a few hours each year. They're fine for new builds and whole heating system replacements, but the vast majority of existing houses would fall well below the calculated demand. Also no point putting in a larger boiler if the installed rads and pipework can't deliver the heat. 

If the existing boiler output is fine and gives you the heat you need, then just install a new boiler of the same output. No need to overcomplicate things - unless the client is complaining that the house is cold, or is planning a big extension, I wouldn't be putting in a bigger boiler.

Most modern boilers have an anti cycling mode built in, so the issue of slightly oversizing is not really an issue. The issue with poorly insulated (or none!) houses is that the temperature can drop noticeable while the anti cycling mode is still running, and you're waiting for the boiler to kick back in - you really need to make sure that the thermostat is in the coldest part of the house, and TRVs fitted elsewhere. 

In your situation a crude calculation is probably worse than no calculation. A detailed calculation will only tell you that the whole heating system is undersized, so where would you go from there?? So the simple answer is to replace like for like unless there is a reason not to.

Another_Level

The output is most probably not been altered since installation and still on maximum output 24kw (80k btu) Usually the installer or anyone altering the output would move the supplied self adhesive arrow so it points to the correct value on the data badge behind the right hand side panel.

Could take an average of the two stated outputs = 22kw 

The radiator heating load can be roughly calculated if the boiler does your hotwater as well an additional kw load for the unknown sized hot water cylinder. Any extra radiators going to be fitted in the future ?

Do all of your radiator heated rooms, get hot enough? 

Higher output boilers may require 28mm primary flow & return pipes, 28mm heating zone valve, 28mm pump valves. The boiler manufacture may require 28mm pipes or it's installer good practice. Although a site visit walk around may suggest otherwise.
The suprima has a pump over run, many new gas boilers dont have a pump over run which can create problems,  site visit 

Is the existing system a Sealed system or open vented ?

Many gas boilers are range rateable where the installer accurately sets the kw output to match the kw demand. Intergas boilers are very adaptable.
Their hre24ov a heat only 23kw output boiler can be range rated down to 19kw 16kw 13kw Just need an intergas specialist.

Boilers 

LucianH

Crikey, what an amazing response.  Thanks so much guys, that’s given me a lot to think about.

There were a number of queries that were raised by some of you:

1) It’s not a sealed system – it’s open vented standard installation with feeder/expansion tank

2) In relation to the pipe diameters I think NFLO hit the nail on the head.  I’ve just checked again and the pump is seated on 22mm pipe and it looks like there’s 22mm pipe disappearing under the floorboard so I’m hoping that there is a 22mm “spine”.  (Without taking up carpets and floorboards it’s difficult to confirm though.)

3) Never heard of “hot water priority” and “fast recovery coils” before.  We do need a new cylinder so I’ll look into that and see what the installers say. 

4) And, in relation to the main question as to whether the existing boiler is adequate - the answer is “just”.  We do have to supplement the heating with a very non-environmentally friendly (but oh so cosy) open fire in the living room during cold snaps.  Given that the current boiler is 24kW I would like to have added a good 10% to 20% headroom on top of that for the new boiler.  Looking at the Worcester Bosch website they have a 27kW (Greenstar Ri) which is about 15% higher power than my existing one.  I was tempted by the 30kW Greenstar 8000 Life since it has a 12yr warranty but that’ll be nearly a 30% increase and, from the comments so far, it may be too much.

5) Finally, having looked at the datasheet for the Greenstar Ri, there is one spec that I now find confusing (and, please remember, I don’t know what I’m talking about here!).  For the “Useful Efficiency” it is quoted as 88.2% “At rated heat output and high temperature regime”.  It then implies that the efficiency increases to 97.3% “At 30 % of rated heat output and low temperature regime”.   There’s no mention of regimes in the user manual so I’m assuming the boiler automatically switches between the low and high temperature regimes such that it is efficient for most the year but only kicks into “high temperature regime” when the likes of the Beast from the East pays a visit.

Bendy_House

From what I can see, the 27kW Greenstar Ri can have as low as a 9kW output when required. It's actually quite hard to 'oversize' a boiler these days, due to the fact they do 'modulate' as mentioned before (eg, you'd usually choose a combi boiler for it's DHW performance, and not on the number of rads you have, so most could well appear 'too big'.)

Interesting point raised before too - since it's a heat-only boiler, it also has to heat up your DHW cylinder. For that, the temp would need to be 60oC plus. I'm guessing, therefore, you'd have to have the boiler's output going at over 70oC to enable the cylinder to heat up quickly enough. Does this mean that, when outputting to CH, it'll also be at 70+ degrees?

The answer is - I don't know. But I think it's worth an email to WB to ask.

Certainly, as the 'demand' is near to being met (ie, rads nicely warm/DHW cylinder nicely topped-up), then the boiler will modulate down to suit, as it won't be able to shift its hot flow fast enough. But, it does still suggest that for a lot of the time, it'll have to output at 70+ degrees, which isn't the most efficient.

Another thing to consider in the whole caboodle - I'd personally take the opportunity to lose the F&E tank in the loft, and make it a sealed system. And if your system currently is Y-plan (ie a three-port diverter valve splits the flow betwixt CH and DHW), then I'd change that to 'S' (which has two, 2-port valves). These are small changes to make, but will give you a better system.