under sized boiler ???

[chris watkins]

He taught me everything I know Paul, & then some !!!

Perhaps a few more of you should also pay heed to what he has to say cos from where I am sitting there are some big miss understandings on boiler / system load sizing.

For my part I would just ask you to have a think about what happens to the boiler efficiency during over half of the year when no heating is required i.e. just the hot water, if you are going to add the heating load to the cylinder coil size?
The whole idea of making the cylinder coils so large is to heat it as quickly as possible, with the water coming back to the condensing boiler as cool as possible & without it continually cycling on & off (all in Part L). To aid this there is a requirement that the hot water is independently timed to allow the hot water to be generated first & then once satisfied (& if required), the heating can be switched on.
I see no reason to add any allowance for HW to the boiler size but if so it should be limited to between 2 - 3 kWs as per heating design guide but just think the boiler selected is normally larger than the required heating load anyways.

 

[happyflyer]

But Chris correct me if I am wrong modern boilers will try and match the load connect they do't just bang it at full power, do they, so over size
massively by the size of the will just cost more to buy, or is there something more finite going on. I don't know that anyone here would size boiler like this:


20 kW of radiators
25 kW Mega Flow coil

Boiler size to be fitted 45Kwh, that isn't gong to happen Chris, is it??? Most people will do like you say, but what sort of calculation is that, rule of thumb, I don't think
over sizing can do any harm, big old lofty house, big rads, long heat up time, big boiler rules OK???

 

[SimonG]

Maybe true for modulating gas boilers but it aint for oil. Its all or nothing.

 

[Ermintrude]

If boiler too big for pipework it will cycle all day long

 

[chris watkins]

But Chris correct me if I am wrong modern boilers will try and match the load connect they do't just bang it at full power, do they, so over size
massively by the size of the will just cost more to buy, or is there something more finite going on. I don't know that anyone here would size boiler like this:

Boiler size to be fitted 45Kwh, that isn't gong to happen Chris, is it??? Most people will do like you say, but what sort of calculation is that, rule of thumb, I don't think
over sizing can do any harm, big old lofty house, big rads, long heat up time, big boiler rules OK???

Not so I am afraid happy & it is a common misconception. If you look at modern boilers they only modulate down so far, to around 8kWs depending on their size (I can't remember the exact radio). Added to this the internal controls which have the boiler firing full burn for around five cycles when they are first turned on, presumably as it knows it has to put heat into the system. Only then does it look at the return water temp to decide the level of modulation, if this is <20 degC Δt it may stay off & they have anti-cycling routines built-in which can hold the boiler off for 20 mins once the returning water is up to temperature.
This is causing problems across the board but is most apparent in the larger systems were big boilers need a minimum flow rate to operate correctly hence the increasing need for low loss headers etc.
The big challenge for boiler manufacturers at the moment is to get their boilers to modulate right down to very low levels, I think 8 or even 10 to 1 has been mentioned (new gas valves required). The first of which are just coming on stream now, so there is still hope for all who wish to just guess at the load required & then go up two or three sizes just to be sure!!!
Just make sure the flue doesn't terminate in a windy location as the boiler will be on the lowest burn for so long the flame could get blown out. LOL

 

[Killy Bing]

As simon says its all or nothing for oil, ive seen 40 to 70kw odd oilys on 22, they cycle every few minutes.

As for oversizing gas boilers most fire on high fire, pipework is too small they blast the heat exchanger,, bake it and cycle.

i know atag start on low and ramp up which is far kinder, then again you could have a 51kw boiler which will never give out its full output due to incorrect pipe sizing.

this is where low loss headers work a treat when it comes to zoning.
not to mention eliminate the baking effect which becomes evident on modern boilers on systems which are prone to sludge or havent been cleaned properly.

 

[sheff paul]

He taught me everything I know Paul, & then some !!!

sorry chris for dropping your name,trying to think of a clever chap and your name came up first

 

[Ermintrude]

Why don't oil boys use a buffer or a nice high volume cylinder , perhaps one that's not a coil but more tank in tank? Fort of thing you might choose to use on a soild fuel open vent on ch.

 

[SimonG]

Cost and space. Not necessarily in that order though

 

[Ermintrude]

The usual. Need an oil boiler with a thermal header for external.

 

[chris watkins]

sorry chris for dropping your name,trying to think of a clever chap and your name came up first

OL stop it :blush5:

 

[Nostrum]

Most boilers can turn down on a 4:1 ratio so a 28kw could wind down to a minimum of about 7kw. The ideal vogue has a 7:1 ratio .

This is my pet hate with combis. Whack a 40kw in for hot water then the boiler cycles like mad on heating as the min output is 10kw!

 

[Ermintrude]

Most boilers can turn down on a 4:1 ratio so a 28kw could wind down to a minimum of about 7kw. The ideal vogue has a 7:1 ratio .

This is my pet hate with combis. Whack a 40kw in for hot water then the boiler cycles like mad on heating as the min output is 10kw!

U can set the load on some of the better boilers.

 

[happyflyer]

All I was trying to say was it's no big deal if your boiler is oversize, it's not a hanging offence, just take this situation, custard say I need a new boiler now mines leaking and I am putting and extension on the house next year, can you fit me one that big enough, no sorry love more than mi jobs worth, you will have to have a 20 kw now and next year throw it down tip and get a 35 Kw when you have had the extension built, come on never in the manner of man...

 

[Nostrum]

U can set the load on some of the better boilers.

Not strictly true as the gas valve has limitations. You can restrict the maximum output, but as far as I am aware there is no domestic boiler on the market that can adjust its minimum rate lower than the factory pre set.

Apparently the sedbuk bands are changing soon, so maybe this is why the vogue has a 7:1 gas valve and I am sure the others will follow suit. The Baxi G.A also has a 7:1 gas valve but only 20% of the time, the other 80% of the time it is waiting for heat team to attend.

 

[Ermintrude]

I was going to mention the GA digital gas valve

 

[happyflyer]

The problem is simple, you can ramp the gas valve up and down and the fan speed, but getting the combustion air volume to match the gas is beyond the electronics in a domestic boiler, I think they still do a good job of getting it near to how you would do it commercially or industrially.

 

[IDCHAPPY]

The new Vokeras have a 1:10 modulation

 

[Nostrum]

From Vokera

The Unica i range boasts an enviable modulation ratio of 10: 1, this high ratio enables the boiler output to be reduced to as little as 10%. For example the 28kW Unica i can reduce its output to just 2.8kW, so when the home only requires 3kW, the boiler will modulate itself down to 3kW. Boilers with an inferior ratio, for example 4:1 would only reduce to 7kW, resulting in wasteful ON/OFF cycling and reduced efficiency. A high modulation ratio can significantly reduce wasteful on/off cycles, which in turn, will increase component longevity and helps to ensure stable domestic hot water temperatures.
In simple terms; 10:1 modulation = better fuel efficiency, increased component longevity, optimal comfort.


So there you go, this is why boilers shouldn't be oversized.


Oh, but I still wouldn't fit a Vokera..

 

[happyflyer]

I worked for Hamworthy 40 years back on the Dual fuel Rotary Cup burners up to 20,000,000 Bthus in old money, flame length up to 15ft long, turn-down about 30 to 1, in the days before GSR and Corgi when men were men, the noise from the burner caused most of my tinnitus 99dB+ , we used 50 gallon oil barrels as the outer core for the silencers on the main air intake, a client was posh if they had silencers fitted, now you would get drummed off the plant, at hamworthy we perfected an excess oxygen cell, it controlled excess oxygen in the flue gases perfectly, the burner air damper had a small slave damper on it this was operated by the cell, you guys with your 25 kws gas boiler meeeow.

 

[Ermintrude]

The new Vokeras have a 1:10 modulation

Like that's going to work..... Vokera? What they mean is boiler will fire 1 in 10 attempts after first year! Well efficient. Auto anti cycling and it never overshoots temp curve!

 

[happyflyer]

From Vokera

The Unica i range boasts an enviable modulation ratio of 10: 1, this high ratio enables the boiler output to be reduced to as little as 10%. For example the 28kW Unica i can reduce its output to just 2.8kW, so when the home only requires 3kW, the boiler will modulate itself down to 3kW. Boilers with an inferior ratio, for example 4:1 would only reduce to 7kW, resulting in wasteful ON/OFF cycling and reduced efficiency. A high modulation ratio can significantly reduce wasteful on/off cycles, which in turn, will increase component longevity and helps to ensure stable domestic hot water temperatures.
In simple terms; 10:1 modulation = better fuel efficiency, increased component longevity, optimal comfort.


So there you go, this is why boilers shouldn't be oversized.


Oh, but I still wouldn't fit a Vokera..

That's the way, where are they made not in the UK!!!

 

[Dotty]

I worked for Hamworthy 40 years back on the Dual fuel Rotary Cup burners up to 20,000,000 Bthus in old money, flame length up to 15ft long, turn-down about 30 to 1, in the days before GSR and Corgi when men were men, the noise from the burner caused most of my tinnitus 99dB+ , we used 50 gallon oil barrels as the outer core for the silencers on the main air intake, a client was posh if they had silencers fitted, now you would get drummed off the plant, at hamworthy we perfected an excess oxygen cell, it controlled excess oxygen in the flue gases perfectly, the burner air damper had a small slave damper on it this was operated by the cell, you guys with your 25 kws gas boiler meeeow.

Now you're talking my language.

Thought I loathed and despised rotary cup burners with a deep and abiding passion, was glad to see my last one go a few years back.

 

[happyflyer]

Now you're talking my language.

Thought I loathed and despised rotary cup burners with a deep and abiding passion, was glad to see my last one go a few years back.

Haven't seen one for years but I would bet a lot of money there a load still going in India at lot when out there in the 60s, a real engineering job. I worked as Area Manager for Selectos for 3 year went over to Dublin to service potato crisp oil heaters they were the size of a London bus, we would fire it from one end and the flame would be about 2ft in dia and 12-15ft long oil only in Dublin there was no gas then.... lads don't know there born, used a fork lift truck to take the burner off it was that heavy.

 

[Nostrum]

Alright you condescending old git!

Go and learn how the remote control for your video recorder works!

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