Rad sizing

[mutley racers]

9k. as long as i come in cheaper than that i should be laughing. i have done around 4 whole new installs!!! geesh, 5000 i can only hope i get to that many!!! I better start delivering those leaflets!! It will take me half a day to insulate all the pipes let alone wire the system too.

Am not about speed, as long as its nice and neat and i am happy with what I have done I will be chuffed and be able to sleep well that night.

I have been checking out prices on websites down this way and to convert from an open vented to a combi is 3k. that is expensive. That's including all the scrap. so around £3200

 

[mutley racers]

being a plumber I was half expecting a Rolex!!!!!

 

[tamz]

As Fuzzy said don't worry too much about being fast. Better to do it right first time.
Speed comes when you don't need to think about what you are doing because you thought about it half an hour before while you were doing something else.

You can but a fake rolex for a tenner and wear it to your work. Makes them think what you are getting for their money :lol:

 

[kay-jay]

rad schedules allow for correct running temps so no need to oversize anything

I was taught to oversize on a combi and rads swap if the original boiler was not condensing. Because to set up the new boiler correctly and to maximize condensing capability the return temperature must be as you quite rightly say 55 degrees (dew point) this means that same size rads won't be jumping hot like with the previous boiler so to get the same btu output (not surface temperature) from the new rads the replacements
Must be oversized.
Sent from my GT-I9100 using Tapatalk 2

 

[AWheating]

i find the city plumbing web site rad calculator very good if your after a quick rad cal. Long hand is better but takes time which unless you need to provide heat loss cals to a builder/specifier then a computer/web based calulator is good enough for most small domestic jobs.

 

[chris watkins]

I was taught to oversize on a combi and rads swap if the original boiler was not condensing. Because to set up the new boiler correctly and to maximize condensing capability the return temperature must be as you quite rightly say 55 degrees (dew point) this means that same size rads won't be jumping hot like with the previous boiler so to get the same btu output (not surface temperature) from the new rads the replacements
Must be oversized.
Sent from my GT-I9100 using Tapatalk 2

Sorry if it seem that I keep nit picking but there really does appear to be a lack of understanding on this condensing melarky I no it is very new & all that ha ha (what is it about 15 years old technology) The dew point is normally taken as 53deg C below this the water vapour in the flue gases can condent's back into a liquid & realising it energy the cooler the returning water back to the boiler the more effective this process is & the more of the trapped energy is released up to an extra 13% if the returning water is more than 53 then this 13% extra heat is lossed out the flue. So systems should be designed, including sizing rads & boilers, to maintain a return water temp of less than 53. If you have to heat an indirect HW cylinder then it is recommended that you design the system to work at 70 F & 50 R because you need a +65deg C F to heat the domestics but with a combi or with U/F system the F&R temps can be much lower to maintain the boiler in condensing mode for as long as possible. So, please no more of this oversizing boiler or rads.

 

[fuzzy]

I was taught to oversize on a combi and rads swap if the original boiler was not condensing. Because to set up the new boiler correctly and to maximize condensing capability the return temperature must be as you quite rightly say 55 degrees (dew point) this means that same size rads won't be jumping hot like with the previous boiler so to get the same btu output (not surface temperature) from the new rads the replacements
Must be oversized.
Sent from my GT-I9100 using Tapatalk 2

no, nothing should be oversized, it should just be sized correctly

rad schedules consider lower running temps so you just size the rad and pick one from the schedule that has the required output, therefore correct size

 

[MickBrown]

Not being a boiler engineer I do not fully understand everything discussed. However even if all the rads and the boiler are sized correctly the customer will turn off rads and also turn down thermostatic valves so less heat will be used. So I would assume that the return temperature will be higher and thus all the calculations would not mean a lot.

 

[chris watkins]

Not being a boiler engineer I do not fully understand everything discussed. However even if all the rads and the boiler are sized correctly the customer will turn off rads and also turn down thermostatic valves so less heat will be used. So I would assume that the return temperature will be higher and thus all the calculations would not mean a lot.

Not so Mick, heat or better to think in terms of energy used to heat the room up & then maintain it at the required temp is the same whether you are running the boiler at the traditional 82 Flow or at much lower temps. Its the rate at which the heat is exchanged between the boiler into the heating water then to the Rad & then into the air in the room, that is important.
You are right to say that people make adjustments but not all the time, TRV's & stat are there to make automatic adjustments (along with a number of other controls) to the water flow through rads or to switch the boiler on or off.
The main thing that changes is the outside air temperature we cant stop this, we have to work on something when we are designing (normally -3deg C down here), boilers, controls & shorty pumps are all designed to take account of this & keep the boilers in condensing mode for as long as possible.
What is absolutely certain is engineers should not be guessing at rad or boiler sizes.

 

[petercj]

Obviously anyone fitting a boiler / radiators within a commercial contract will be well advised to follow the accepted scales re sizing.

However, my boiler is well undersize by such ratings, being only 18.5 kw running 13 radiators (one being a double) and a HW cylinder, and it rarely gets maxed out - about twice in the last 5 years, which was slight rad-lag when heating the system from cold.

Having kept a watchful eye on the condensing aspect, it seems pretty limited, bearing in mind that the boiler is mainly topping up, rather than heating the system from cold, which is when the condensing side of things really comes into its own. But then with an ambient room temperature of 20c, which rises on the return as the boiler ramps up, the 20 degree differential required for condensing mode is short lived.

Although my hot-water is off between 10am and 5pm, the temperature prior to the boiler kicking in at 5 is usually 30c, so even more of a sprint for the boiler to catch the condensing sweet spot.

What I have found to make a hell of a difference for lowering the gas bill is the use of a wireless room stat, which has meant that the boiler is off much longer than previously. 'Off' being the most economical mode of all. Wireless room stats may be more troublesome, but I reckon they are worth the extra hassle. Mine paid for itself in the first cold month of winter.

It seems to me that Valliant have created a high performance heat exchanger by employing the small bore pipe they use in the eco max pro, but unfortunately it is very vulnerable to restriction should any kind of debris enter the system.

I'm not aware of the formula they've applied when creating the sizing scales, but I suspect there is quite a discrepancy between theory and practice.

 

[chris watkins]

Well Peter where do you want me to start? As heating engineers whether domestic or commercial & I really do not see any difference between the two, (I have done both) they require exactly the same levels of knowledge, understanding & professionalism.
Systems in the past have being sized correctly to match the heat losses & then at least 10% has been added to allow a capacity to heat the rooms up from cold, known as intermittent heating (& not to hold a steady stat). Not having the benefit of the details of your house & system it is difficult to comment but I bet you leave your heating on longer than most when it gets cold & that's not a bad thing but you should know the why's.
Re the condensing please have a look at my earlier post, it is not the differential that is important but the returning water temp back to the boiler. (less than 53deg C)
I agree with you about the modern controls but it is not just the wireless stats you can get the same results if you exchanged your old mechanical / electrical room stat for an electronic one & they only cost a couple of £ more than the normal Honeywell ones.
All the manufactures use the same types of heat exchangers with small waterways (I would not install one without protecting it with a filter on the return before it) they are all trying to come up with boilers which will adjust there heat outputs automatically right the way down as low as possible to match the required heat output with the demand thus maintaining a cool return water temp & a boiler that is up to 13% more efficient that exactly the same one that not condensing. 13% saving!!!! in any other industry if I could offer that, people would be biting my arm off.

 

[petercj]

Well Peter where do you want me to start? As heating engineers whether domestic or commercial & I really do not see any difference between the two, (I have done both) they require exactly the same levels of knowledge, understanding & professionalism.
Systems in the past have being sized correctly to match the heat losses & then at least 10% has been added to allow a capacity to heat the rooms up from cold, known as intermittent heating (& not to hold a steady stat). Not having the benefit of the details of your house & system it is difficult to comment but I bet you leave your heating on longer than most when it gets cold & that's not a bad thing but you should know the why's.
Re the condensing please have a look at my earlier post, it is not the differential that is important but the returning water temp back to the boiler. (less than 53deg C)
I agree with you about the modern controls but it is not just the wireless stats you can get the same results if you exchanged your old mechanical / electrical room stat for an electronic one & they only cost a couple of £ more than the normal Honeywell ones.
All the manufactures use the same types of heat exchangers with small waterways (I would not install one without protecting it with a filter on the return before it) they are all trying to come up with boilers which will adjust there heat outputs automatically right the way down as low as possible to match the required heat output with the demand thus maintaining a cool return water temp & a boiler that is up to 13% more efficient that exactly the same one that not condensing. 13% saving!!!! in any other industry if I could offer that, people would be biting my arm off.

I appreciate that I should have mentioned the size of my house for my post to make any sense - thought of that after posting!

4 bedrooms, two bathrooms, 25' lounge, dining room, 30' kitchen, utility room, large hall with two staircases. It has been updated insulation-wise to the latest spec.

I know you're going to tell me that the boiler is too small chris, but in practice it works.

Previously, the room stat was in the hall, near the front door - totally bonkers.

The mobility of the wireless stat has allowed me to manage the heating in a very different way, and no, the boiler's not running a lot. My gas bill for last winter was 30% down on the previous year - which was somewhat colder, but then I was on a price fixed rate for the previous years - so I was well pleased.

The simple basis of it is that the stat is in the lounge for waking hours, and in the bedroom at nights. Because of the high level of insulation I can switch the double radiator in the lounge off at bedtime, and the temperature drop during the night is only a couple of degrees. The hall and stairs are kept at a background temperature. I have lowered the temperature in the kitchen because it's only used for cooking, which raises the temperature anyway, plus the kitchen retains heat pretty well too. The other thing I did was to fit a door closer on the lounge door.

I was tempted to buy timers for the TRV's, but thought better of it, as they don't come cheap, and I'm cautious about their life span.

Have to disagree about staying with a fixed room stat, i.e. having a mobile stat makes a world of difference and opens up a range of options that are not possible with the fixed variety.

The Valliant handbook states that there needs to be a 20 degree differential between F & R for the boiler to be in condensing mode.

If the temperature in the house was allowed to drop right down in a cold winter, then the boiler would have a right sweat on (it's like a steam train when it gets going on a cold day as it is), but it never does.

The other choice in the Valliant range was 28 KW, and I thought long and hard about going for the bigger model, but having had the 18.5 for five years, I have no regrets.

But in theory it's the wrong choice.

 

[chris watkins]

Hi Pete
I am not going to tell you it is too small, its of an output that suits you & as most people know the harder a boiler works the more efficient it is likely to be (you told me "about twice in the last 5 years, which was slight rad-lag when heating the system from cold". If I sold a customer a system then they might well come back & say that I had miss sold it, if it didn't heat the house in the winter. Thats not to say it should be oversized either but because I have done the calc's I can show what is happening or not.
The Valliant MI's do state 20deg C temp diff as an indication of the flow rate through the boiler to carry the heat away, it has nothing to do with the boiler being in or out of condensing mode, this is basic science not rocket, please have a look on google.
I understand what you are saying about the wireless stat & if you are using it like that & it is working for you, great, but most people would not be so diligent in moving it around & this is not how it is designed to be used. The TRV's on each of the rads control the individual room temperatures by adjusting the amount of water going to each one, with the exception of one rad (normally the hall) which does not have one fitted but this is where the room stat is installed, so long as this rad is correctly sized it should give a fair representation of the rest of the dwelling & shuts the heating off or on giving overall control along with the programer. So you can see that if the room stat is in the same room as a TRV they could well be competing with each other.
So all in all right choice of boiler when you know the theory.

 

[petercj]

Hi Pete
I am not going to tell you it is too small, its of an output that suits you & as most people know the harder a boiler works the more efficient it is likely to be (you told me "about twice in the last 5 years, which was slight rad-lag when heating the system from cold". If I sold a customer a system then they might well come back & say that I had miss sold it, if it didn't heat the house in the winter. Thats not to say it should be oversized either but because I have done the calc's I can show what is happening or not.
The Valliant MI's do state 20deg C temp diff as an indication of the flow rate through the boiler to carry the heat away, it has nothing to do with the boiler being in or out of condensing mode, this is basic science not rocket, please have a look on google.
I understand what you are saying about the wireless stat & if you are using it like that & it is working for you, great, but most people would not be so diligent in moving it around & this is not how it is designed to be used. The TRV's on each of the rads control the individual room temperatures by adjusting the amount of water going to each one, with the exception of one rad (normally the hall) which does not have one fitted but this is where the room stat is installed, so long as this rad is correctly sized it should give a fair representation of the rest of the dwelling & shuts the heating off or on giving overall control along with the programer. So you can see that if the room stat is in the same room as a TRV they could well be competing with each other.
So all in all right choice of boiler when you know the theory.

Completely agree with your first paragraph chris: if I was thinking of selling this place, and it was time for a boiler change, I would fit the bigger model. Someone else could easily spend 3 times as much on gas as I do heating this place, and a bigger boiler would help them along the way. As it is, I begrudge BG every ruddy penny I pay to them. I've just change supplier after 8 years with BG, and I had to chase them to get them to pay me the money I was in credit with them.

Sizing might be an exact science, but human behaviour definitely does not conform to such predictable rules.

I don't know if you watch Grand Designs on Channel 4, but there was a guy on there last year who was heavilly into energy conservation, and he based his heating on recycling the air in the building rather than opening windows, so preserving body heat, cooking heat, etc, as well as the input from the heating system, and the results were surprising.

Not that I go that far - lol - but the ten pounds I spent on a door closer, plus the wireless stat, was definitely money well spent. Seems strange to think that something so simple can make such a difference.

 

[chris watkins]

Not that I go that far - lol - but the ten pounds I spent on a door closer, plus the wireless stat, was definitely money well spent. Seems strange to think that something so simple can make such a difference.

Simple is most certainly the best, that's for sure, have you met the wife !!!

 

[chris watkins]

If anybody whats to find out more about heat loss, design, controls or just about anything else about this subject, I can highly recommend this book- Domestic Heating Design Guide 2007 ISBN = 1 903287 40 5. You may have to get it through CIPHE or a few others but it has taught me even thing I know (well almost)

 

[Nostrum]

Totally agree the Chris Watkins on all aspects.


People get hung up about doing a few calculations that take a few minutes a room and give you an accurate, approved and logical reasoning for sizing the radiators, something that will make you look much more professional, experienced and can provide you with something to fall back on if you need it.

Radiators in 90% of UK households are well oversized, partly because of this ft2x5 business thats been going on for years, and partly because houses were not insulated like they are today.

I newish house can have a 450x400 single panel in a single room, something that you'd find in most people airing cupboards but works just fine.

Petercj, your 18kw boiler sounds like it could be a touch oversized according to your information and levels of insulation. I certainly wouldn't put a bigger boiler in until you sized it correctly. You should go through the calc and you may be very surprised of the outcome.

Bigger rad's will mean an oversized boiler if your adding up the emitters to size the boiler, more expense, less space and potentially increased installation difficulty. It's not so much of a problem for gas boilers as they modulate (except for the gas supply requirements) but if you oversized a pellet boiler or similar you could be looking at thousands more.

Also moving your thermostat round really doesn't make sense, the hall is usually the best place as it is subject to the most air changes per hour, if you put it in your front room with the gas fire on etc the heating will be off for hours regardless of how warm the other rooms are, but that's another matter!

Come on lads, we have got to earn the 'engineer' in heating engineer some how, other wise the term 'pipe monkey' becomes a little more accurate!

P.S no offence intended to anyone 🙂

 

[chris watkins]

People get hung up about doing a few calculations that take a few minutes a room and give you an accurate, approved and logical reasoning for sizing the radiators, something that will make you look much more professional, experienced and can provide you with something to fall back on if you need it.

Radiators in 90% of UK households are well oversized, partly because of this ft2x5 business thats been going on for years, and partly because houses were not insulated like they are today.

I newish house can have a 450x400 single panel in a single room, something that you'd find in most people airing cupboards but works just fine.

Come on lads, we have got to earn the 'engineer' in heating engineer some how, other wise the term 'pipe monkey' becomes a little more accurate!

P.S no offence intended to anyone 🙂

Well said Sambotc, could not have put it better myself, how could they take offence it is a requirement for installing new systems as well as replacing a single rad that it has to be sized correctly, it is not difficult to do or learn, it does not take a long time to do, either, so there are no excuses if you are going to call yourself a professional Plumbing / Heating ENGINEER.

P.S. We have modulating boilers that will not work correctly because they are oversized there is a limit as to how low they will go because the boiler controls are designed for the Kw rating stated, if it is connected to a much smaller system it will not work correctly.

 

[SimonG]

No offence taken, and as a professional plumber and heating engineer I will continue to use the method that is proven and works for myself. 🙂

 

[Nostrum]

Yes I think we have talked about the minimum rated output problems before. My own house is a typical example. Thermostatic shower so wanted a good flow rate, ecotec 831 combi rated down to 8kw on the boiler but min output is 6.7kw I think. My house heat loss (mid terrace 2 bed 1970's) is about 5kw at minus 3 so once the house is up to temp it's bouncing of the bottom limit all night. Not much I can do about it as there's no room for a cylinder and the boilers don't yet modulate low enough.

In reality, it works and who's to know any better, but potentially there are savings to be made and ultimately emissions could be lowered (save a few polar bears)

Didn't you say the boiler manufacturers were looking into twin gas valve designs to combat this problem?