[Closed] Boilers. To condense or not ?

It will only condense if you run low enough temperatures through it.

Is it a combi boiler e.g. does it do your hot water too?

Or is the hot water heated in a cylinder elsewhere?

I'd suggest in a retrofit/old house you mighten't ever get the boiler water temps low enough to actually achieve condensing performance, hence no efficiency savings and wasted money.

BUT your plumber mate might know better...

Didn't know you could buy non condensing boilers any more!

Just out of curiosity why LPG?

We have just changed our oil tank so looked briefly at LPG. Was more expensive, and in our case harder to install (given regs on the tanks)

It will only condense if you run low enough temperatures through it.
...
I’d suggest in a retrofit/old house you mighten’t ever get the boiler water temps low enough to actually achieve condensing performance, hence no efficiency savings and wasted money.

This seems....confused and frankly just wrong.

https://www.viessmann.co.uk/heating-advice/what-is-a-gas-condensing-boiler

Even in our end of terrace Victorian with solid walls the condensing boiler was doing it's job efficiently - the exhaust gases were cool

Are you sure he said condensing and not combination-easily confused. Combi will give instant hot water without a hot tank in the airing cupboard but are generally more expensive.

This seems….confused and frankly just wrong.

Nothing in your link contradicts what I said 😉

Your Victorian examples are either combi boilers, or have been retrofitted with modern radiators sized to run at very low boiler temps to allow condensing performance.

I'll concede I'm a bit out of touch with domestic boilers, we typically design systems for hospitals/large commercial etc.

No help but Interested in hearing why lpg if you already have a oil tank. Not a criticism, just a question.

No help but Interested in hearing why lpg if you already have a oil tank. Not a criticism, just a question.

Theft perhaps.

Still on an antique (25years maybe) combi, non-condensing. It's incredibly simple, does the job and low maintenance. From what I've heard from friends/family with condensing, they seem to have more issues and are costly to fix, or replace them after 10 years (though that may be the boiler/service industry pushing them into it).

Purely anecdotal and me being sceptical 😄

However, at this stage, is it worth looking at non gas alternatives? Given I think it's not long now, if not already started, when new houses are not going to have gas supplies.

Been looking briefly at stuff like heat pumps and not convinced they'll give enough heat (and is there really enough heat you can draw from the air or ground during a British winter?). It sounds like going back to the horrors of night store.

Been looking briefly at stuff like heat pumps and not convinced they’ll give enough heat

Not really comparable, you need a well insulated house. In which case yes.

Having had lpg in my last rental

I'd rather be cold than have lpg again

On oil again and thankful.... LPG should have been cheaper but in the real world it was not.

But then someone had to pay for the free tanks they give.

Been looking briefly at stuff like heat pumps and not convinced they’ll give enough heat

You'll be fine. I moved to the US and have an air source heat pump for both heating and cooling and it gets both hotter and colder than the UK here in Virginia. Since you'll likely have one more for heating than cooling, the tech there gets better every year. They are less efficient at extremes but very good in general.

it gets both hotter and colder than the UK here in Virginia

Google historic data suggests otherwise unless your version of the UK stops at the Watford gap.

Fwiw I don't know anyone that's been lumbered with one in a new built that is happy with their performance at 57° north.

Oil boilers are in the region of £3,000 plus fitting. I can buy a non condensing combi Baxi for about £750.
Due to regs,if I put a new oil boiler in then the tank must be moved,around 30 metres in my case.Also due to regs, the new oil boiler cannot go in the same place as the original.Putting another oil boiler in is just full of difficulties.There is no room for a gas tank so it will be the 47 kg bottles.
The house,I think,is pretty well insulated and my yearly oil consumption has only been around 500 litres for heating and hot water.

That’s fair enough Op. we had a major headache getting a new oil tank to meet regs.

Condensing is more efficient. Non condensing is old technology. Personally I wouldn’t be going for the cheapest available option.

Just a comment regarding condensing/non condensing.

When the part L(I think) building regs changed in the late naughties (I think), we then could not fit new boilers with less than 86%(or about there I think) efficiency, apart from some exceptions.
At the time non condensing boilers were generally 75% efficient so it meant that plumbers told clients condensers only.
However the tech has evolved and now non condensing boilers are meeting the requirements and can be installed.

Condensing is more efficient.

Sometimes

Non condensing is old technology.

No it's not, we specify and install some very advanced, modern non-condensing boilers. Condensing is simply not appropriate for many applications and requires more delicate, less durable components which are pointless if your system isn't designed to condense.

No point going for a more expensive option if you then operate it in a manner that prevents it condensing, it just becomes an expensive and delicate non-condenser!

OP mentioned it being a combi-boiler though, in which case it will operate in condensing whenever it is heating the incoming cold water.

There is no room for a gas tank so it will be the 47 kg bottles.

The OP in a months time!...

[img] [/img]

...47kg cylinders are IME the worst way to go. We had them many years ago and got rid as soon as the smaller bulk tanks were available. These could be sited in places the big bulk tanks couldn't go.

This was 20+ years ago though so regs on siting may have been tightened.

Indeed where's ourmaninthenorth with his to takes of 200 quid a month gas bills with 47kg cans

Nothing in your link contradicts what I said 😉

Your Victorian examples are either combi boilers, or have been retrofitted with modern radiators sized to run at very low boiler temps to allow condensing performance.

I stand corrected - https://www.theheatinghub.co.uk/why-our-condensing-boilers-do-not-condense

Basically if you see a plume coming out of your boiler vent it's not working efficiently. Current house should be the exception as we're running very low temperature underfloor heating (flow temperature is below 30C even when below freezing) but even that might not help as it's all mixed down. What I do notice is a plume when it reheats hot water...

I had LPG in my previous house. Be cheaper to burn £50 notes! Utter nightmare, also difficult to know exactly where you are with remaining supplies. The bottles can be nicked too.

My current 30 year old oil boiler seems fine. The heating engineer said to me last service that he shouldn't say so, but advised not to replace it as modern boilers are fragile and offer a very low improvement in efficiency. False economy.

I have just spoken to a couple who live a few doors away that are running an lpg combi from the 47kg bottles. The 2 bottles fuelling now have been running from February.Same size house as ours.
He thought the gauge on the switching system was faulty because of the low gas use,but after inspection it's not.The bottles are just over £60 each so he's more than happy with the running costs.

The 2 bottles fuelling now have been running from February.Same size house as ours.

Do report back , I find that incredibly hard to believe unless they have an alternative primary heat source.

I used to use 47kg bottles for heating my very small reasonably insulated house. Very expensive. At one point we were on a bottle a week although that was on the really bad
Winters back in 2010/11 (?).

I’d find 2 x 47kg bottles since last Feb almost
impossible to believe to be honest unless there’s another source for heating and/or hot water.

Maths behind it -been doing this recently for my gas heater my my garage office

24kw boiler consumes 24kw energy per hour.

1kg of propane has 14kw energy in .

47kg has 14*47 = 658kw of energy

658/24 = 27 hours of energy

So either my maths wrong or you have say 60hours worth of energy in the two tanks.

If your neighbour hasn't changed tanks since February he's has roughly 6hours of gas usage a month.

I think his wife's been changing them on the sly.

Pretty sure my combi is on for more than 6hrs a month just keeping the tank warm in winter.

By way of comparison that 500l of oil you use currently to heat your home has 5000kw energy 210 hrs In same boiler ) in so a factor of 5 more than your pair of lpg bottles

So that is 500l of oil currently costing around 200 quid on a bad day . For the same number KW of lpg you'll be 600ish I reckon going on 60 quid a bottle

Now I'll happily be corrected on any of that because it's fag packet maths but it stands to reason based on my (and others) experiences with lpg

I am pretty short of options because of siting issues both of the boiler and any tanks.
On radio 4 a few weeks ago was an article about heat pumps. The " heating expert " said that radiators do not need to be red hot to the touch and if they were then it's wasting fuel.
I read an article that about 1.5 kw is needed per radiator. With 8 radiators that would be 12kw. Most boilers seem to be around 24 so that appears way over what's required.
So,would turning down the temperature of the water on the heating side actually give greater efficiency ?
My wife's eyes glaze over whenever I mention the subject of lpg Vs oil.

Just an update on my neighbour with the lpg.
I think he has been a little economical with the truth as I've just seen him carrying a bucket of coal into the house !

Be careful with lowering water temps, the radiator is sized to give a certain output (e.g. 1.5kW) at a certain mean water temp (e.g. if your boiler produces 80C water, and the water leaves the radiator at 60C, then the mean water temp is 70C).

If you lower the mean water temp then you need a bigger radiator to achieve the same output!

That's why heat pumps (only really capable of 55/60c water temps) really only work in very well insulated new-builds, or require massive radiators (or you use your existing radiators but they basically stay on 24/7 trying to keep up...). They are also pretty poor at low temps and are barely any more efficient than old school electric heaters, less of a problem as the grid moves towards renewables but still expensive.

I’ve just seen him carrying a bucket of coal into the house

Lol! Probably has electric showers as well 😁

24kw boiler consumes 24kw energy per hour.

Only if it's running flat out.

I’ve stayed off this thread for multiple reasons but what the hell, here goes.

The size of a radiator is based on the space/room heat loss and is the sum of the air load and the fabric load.

Under steady state (for ease of calculation) Each surface will have a heat loss (or gain) determined by the thermal properties of the fabric and the respective temperatures either side (eg, 22 deg C inside, -5 deg C outside). This applies to all surfaces of the space. Some losses will be beneficial gains if internal the the building (ie heat loss from bedroom into a stair/landing is not lost but if it’s to the external it is. Walls, floors, roofs. Not all surfaces are homogeneous (walls can have windows and doors) so have different thermal properties (usually described as a U value). Each space should be calculated individually.

The air load is ventilation and infiltration. Air is at external ambient (say -1) that coming is is noticeable and needs heated! This is normally denoted by air changes. You need ventilation for a variety of purposes, removal of pollutants, moisture, smells, etc.

So your room rad size is the sum (Qt) of all the fabric losses (Qf) plus the air loss (Qa). It’s usually to add a margin to that (between 10 and 20%) for ‘quick heat up’. It’s a safety margin.

The size of a boiler is the sum of all the radiators (plus your distribution losses, ie your pipes in the floor or wall do not provide much meaningful heat gain inside the thermal envelope, but if you’ve a 20% margin you can ignore this...).

That is the heating load, what about hot water? A HW storage cylinders usually 150-280L dependent on property size. You need to allow for heating this from cold (10 deg C in winter) to usually 60 deg C (for an hour anti-legionella precaution). How long you got to allow it warm up, an hour?, two?, overnight? That will determine the water load.

The two together you got a boiler size.

24kW will be (usually) well over capacity for this, it this is a system boiler.

A combi boiler heats water instantaneously (ie from 10 deg C to at least 40+). 24kW’s won’t give you that much (a shower, but don’t go trying to wash dishes at the same time). 28 or even 30+ are common. On district heating schemes, the heat exchanger for these are normal 50kWs...).

Pros and cons of combi vs system:
Combis take up less space (no HW cylinder) and have no HW standing losses. You only heat what you use.
Restricted HW flow rate. You only got 24kW m=Q/Cp’delta’T. You want to use more than one water outlet? You’ll probably need a bigger boiler then..
When the combi fails (note when, not if) you lose both your heating and hot water. And in my experience (ie twice) it’s been the depths of winter...
The boiler capacity for combi will be approx 2x a system, bigger incoming fuel pipe, etc.

I’m not going to start on Trail Rats calcs but despite them not being correct, the trend is. Buying LPG in cylinders is about the most expensive way you can buy fuel.

Metalheart - straight out of CIBSE 😉

I see others have commented on the flow temperature thing.

The amount of heat transferred from the heating surface (ie radiator) to the space is determined by size and temperature difference. A high temp rad has a higher gradient so is smaller. The lower the temperature difference between mean surface temp and room, the more heating surface you require (ie bigger rads). 13thmonk has already dealt with this.

Heat pumps lose efficiency (CoP) at higher flow temps. This is why the regs have diffident minimum CoPs for heating vs HW HPs.

ASHP’s are least efficient just when you need them the most. As already mentioned they ain’t really must better than immersion heaters (most ASHP will have exactly that for the worst case, a direct heating element). This is because coils frost up. It’s 4 degrees outside, you pass that over the coil and it takes, what, 6 degrees out of the air so it’s now -2.... where does the cold air go?

Only if it’s running flat out.

Are gas boilers different to oil ? Mines either on .....or off.

straight out of CIBSE 😉

Straight out my head (but, yes, CIBSE, or it’s predecessor originally 🤪).

Basically if you see a plume coming out of your boiler vent it’s not working efficiently.

My boiler is currently flowing at 55 degrees yet there is still a small plume. Outside air temp is -2.

Are gas boilers different to oil ? Mines either on …..or off.

Cheap/shite ones are on/off. But even yours won’t be on 24/7/365... 😜

Decent ones modulate. Oil is more expensive to get good modulation, gas is easier. Gas will easily go down to 30% (and condensing boilers are more efficient at low fire). Oil tends to be hi/lo fire...

My boiler is currently flowing at 55 degrees yet there is still a small plume. Outside air temp is -2.

I had lazily and incorrectly associated plumes with condensing but of course it is the opposite way round, the plume represents water vapour condensing and giving up heat to the cold external air, which means it HASN'T condensed/given up heat inside the boiler = less efficient.

I can't be bothered looking up how low your water temps need to be to condense 100% but I guess a plume could also represent e.g. a fouled heat exchanger preventing 100% condensing?

Never suggested it was on 24/7

But continuous work hours from a tank was where I was trying to get to.

I knew they wouldn't be highly accurate but -how far out are they because it was within the hour for the usage of my 13kg butane canister in my 4kw gas fire. -which truely is on or off -the joys of 1983

As of next week it's being retired to the tip to be replaced by a van diesel heater running off kerosene. Was supposed to be a temporary thing till Jan but they just extended working from home till April. And gas cans will bankrupt me.

If you lower the mean water temp then you need a bigger radiator to achieve the same output!

Yes, but radiators are usually oversized (quick heat up margin I mentioned earlier 😜). And the design conditions are usually only met for, oh, 5% of the year. That means 95% of the time your radiators are bigger than they need to be anyways...

I can’t be bothered looking up how low your water temps need to be to condense 100% but I guess a plume could also represent e.g. a fouled heat exchanger preventing 100% condensing?

Do you ever get 100% condensation inside the boiler though? I would've thought there would always be a small loss to the exhaust.

It's true that there's a much bigger plume if the hot water is running (higher flow temp) but I question the idea that you won't see anything at all if your boiler is at an efficient flow temp.

I can’t be bothered looking up how low your water temps need to be to condense 100%

Deep condensing is dependent on the return temp.

There is a reason the En ISO thing for rads is based on 60/40. Isn’t 55 the maximum for condensing to commence?

And I’m with ransos, you never going to get 100%.

Due to regs,if I put a new oil boiler in then the tank must be moved,around 30 metres in my case.

I could be wrong as I've not done any Oftec training, but I thought there was no legal requirement to upgrade an existing tank as long as it is safe and not leaking. I've certainly installed oil boilers* and used the existing tank even though it didn't meet current regs. Last one was maybe 12-14 months ago so things may have changed since then. If it's the cost associated with moving the tank - probably getting a new one - that's putting you off an oil boiler then I'd double check the rules.
* with a tame and somewhat housetrained Oftec guy, not on my own

Also, from my experience of talking to customers with LPG boilers they are exceedingly expensive to run in the cold months. That's in Scotland though, YMMV.

We're on LPG. I don't find it too spendy.

Cylinder refills do look like double the cost of a tank, though.

If I was replacing a boiler now, I'd be taking a pretty serious look at heat pumps.

Something else to consider, my first combi boiler that went fut, I asked the (commercial) rep what the anticipated lifespan for the particular model was.

Eight years...

The complexity required to achieve the required efficiencies for domestic boilers reduces their economic life expectancy. From memory (commercial) condensing boilers are predicted as 15 years (domestic probably less). With PCB and fan failures it’s not the same as atmospherics with bimetallic strips....

And I’m with ransos, you never going to get 100%.

Correct - even with commercial boilers in new systems (ie ones that are designed for condensing operation) there is always a lot of condense draining out of the flue drains.

Inlaws have lpg bottled gas in a 1930s poorly insulated house occupied by two 80yo, dread to think how much it costs them.

Quite a timely thread as we used our first lpg tank during the night and I had the other bottle turned off so I could see how long it actually lasted. We have been living in the static for exactly week today but had the heating on a week earlier to dry out paint etc. The controls are no way optimised (going to get a Nest thermostat) and the insulation is shocking but I can now see that those saying 47kg lpg tanks are expensive to be right! I have just bought 2 new rads and a towel rail rad (dual fuel) to put in but now wondering if I should just install some electric heaters instead. We have a 50A supply so have replaced the cooker with a 12A oven and induction hob.

for the record the new build is going to be heat pump with underfloor heating, triple glazing etc. We have 3 acres of south facing land so plenty of room for the outside piping.

We have 3 acres of south facing land so plenty of room for the outside piping.

Lucky you, hope you are going to keep us updated with your build so we can make constructive comments as you go 😉

Something else to consider, my first combi boiler that went fut, I asked the (commercial) rep what the anticipated lifespan for the particular model was.

Eight years…

Exactly. The financial and environmental cost of this is crazy. Don't forget you also have to service the thing annually as well to get that 8 years and any kind of warranty.

When I self build I'm not having gas - the economics and environmental cost just doesn't add up in my head when you can indulate to great standards.

When I self build I’m not having gas – the economics and environmental cost just doesn’t add up in my head when you can indulate to great standards.

This is one reason I'm having a solar array put on the roof in January and my name's on the list for a powerwall.

My next boiler won't be oil. And ironically one of the main high pressure pipeline supplying you lot with gas is the reason I can't have gas as they won't cross over it with gas lines for such few houses.

Re existing tanks, I recently had a replacement oil boiler installed, the plumbers comment from his Oftec inspector was 90% of installations don't meet current regs, if its basically sound and no work is done on the tank installation, then leave that part of the certification form blank and move on.
Tank in question is in a far corner of the garden up against wood boundary fence (surrounded by other gardens). New tanks are expensive, so that was a relief really.

There was a big push to get the new "blue flame" low NOx boilers, but I decided not worth as a lot of extra money, even if that gets a 10 year gaurantee. (Grant Vortex Pro)