new oil boiler size

spector11

Hi i want to size a new grant vortex boiler for my house. i know a little about plumbing but not alot. its about 15 years old and has rolled top barlo radiators. ive measured all the radiators and cylinder size and im looking at their radiator sizing chart. do i go by the T50 or T62 radiator sizes to calculate.
ive been told this is not an accurate way to size a boiler for an existing house and i dont know why it is to be honest as im sizing the boiler to heat a certain heat demand. i also realise i should undersize the new boiler slightly.
there are no zones in my house but i will get that updated in the next couple of years. Ill be getting a reg plumber to carry out all works but i like to have some involvment myself so i want to size it and purchase it myself
Can anybody advise me please

Thanks
jim

Wearb

There used to be sizing calculators online and probably still are if you dig around.
As for buying the boiler yourself; I would advise against it. The installer may wash his hands of it if it gives trouble. By all means price it, but I would let the plumber supply it.

scudo2

What sq/ft is your house,?

that's what I calculate on.
Zoning is also a major factor for efficiency and boiler size, so the sooner you do that the better.

spector11

the house is 3000 square foot with standard insualtion from when i built it. what i really need to know is why isnt the above method that i mentioned of adding up rad sizes be deemed accurate?? do i use T50 or T62 from the chart? I dont understand why calculating the total rads btu/KW value for the whole house and then subtract say 10-15 percent to allow for undersizing. Can somebody tell me why this cant be done in laymans terms??
thanks for all your patience
jim

Wearb

spector11 wrote: »

the house is 3000 square foot with standard insualtion from when i built it. what i really need to know is why isnt the above method that i mentioned of adding up rad sizes be deemed accurate?? do i use T50 or T62 from the chart? I dont understand why calculating the total rads btu/KW value for the whole house and then subtract say 10-15 percent to allow for undersizing. Can somebody tell me why this cant be done in laymans terms??
thanks for all your patience
jim

The reason its not normally done the way you suggest is that it assumes that the original sizing was accurate. That no house improvements have taken place or no extensions, sun rooms etc. When spending so much money, you should make sure you get it right. Be careful with under-sizing also. You don't want to be undersized during a severe cold spell like we had a few years back.

Wearb

spector11 wrote: »

the house is 3000 square foot with standard insualtion from when i built it. what i really need to know is why isnt the above method that i mentioned of adding up rad sizes be deemed accurate?? do i use T50 or T62 from the chart? I dont understand why calculating the total rads btu/KW value for the whole house and then subtract say 10-15 percent to allow for undersizing. Can somebody tell me why this cant be done in laymans terms??
thanks for all your patience
jim

Forgot to answer the other part of your question in my last reply.

http://wiki.diyfaq.org.uk/index.php?title=Central_heating_radiators

The above will give you an explanation about radiators and suggests using Delta 50 for HE boiler.

spector11

Thanks guys
forgive my slowness but im still not grasping it fully.
1 What if my original rads are added up using T62 chart and they add up to say 150,000. Then what if i measure my house size and it says i only need 100,000 btu for my house. If i fit a 90,000 boiler to suit the house demand instead of the rad sizes, will my 90,000 boiler seriously struggle to heat the 150,000 radiator load???
The same could be said if i got my current houses insulation upgraded and the house only needed say 75,000btu to heat up.
If im talking nonsense please let me know.
thanks
jim

ps sorry for the delay in getting back

scudo2

spector11 wrote: »

Thanks guys
forgive my slowness but im still not grasping it fully.
1 What if my original rads are added up using T62 chart and they add up to say 150,000. Then what if i measure my house size and it says i only need 100,000 btu for my house. If i fit a 90,000 boiler to suit the house demand instead of the rad sizes, will my 90,000 boiler seriously struggle to heat the 150,000 radiator load???
The same could be said if i got my current houses insulation upgraded and the house only needed say 75,000btu to heat up.
If im talking nonsense please let me know.
thanks
jim

ps sorry for the delay in getting back

120,000BTU Grant vortex ( minimum ) size.
It is a 3000 sg/ft house

spector11

thanks scudo2.
The reason im asking HOW it is done is because my brother and cousin are both thinking of upgrading their boilers also.
their houses are also different in size. Plus, Im genuinley interested at this stage
so back to my previous post, can anybody explain it to me as to how its not an accurate way to calculate boiler size
thanks
jim

littlemissfraze

Jim

It is not an accurate way of sizing the boiler because it is assuming the rads were sized correctly in the first place. It also is not taking into account heat loss from the pipework and morning startup. For the hot water cylinder you need to factor in how long you want to be able to heat up the volume of water.

The "correct" way of sizing a boiler is first calculating your heat losses, both fabric and infiltration. You can add a margin for morning boost. You then must add in you hot water cylinder. If done correctly this is very long winded as you have to consider each construction in each room (floors, walls, windows etc.)

The usual way is to take a rule of thumb to calculate your heat loss - 35W/m3
Then add your cylinder load - H = m x Cp x ( t hot - t cold )/Hours heat up x Efficiency x 3600

(So for 100litre cylinder and 90% efficient and 1 hour heat up) - H = 100x4.187x(60-10)/1x0.9x3600 = 6.5kW

Then heat loss + cylinder = heat load

heat load x 1.2 (20 % margin and morning boost) = boiler load

You should definitely oversize rather than undersize to allow for future use in extensions etc.

Wearb

Most if not all of boilers have more than one rate if fire, so after working out your requirements, choose one that can be set higher and lower (or just higher) than your requirements. Remember also that a lot if the time not all zones will be calling for heat. Therefore a lot of the time your capacity will be under utilised.

Dtp1979

littlemissfraze wrote: »

Jim

It is not an accurate way of sizing the boiler because it is assuming the rads were sized correctly in the first place. It also is not taking into account heat loss from the pipework and morning startup. For the hot water cylinder you need to factor in how long you want to be able to heat up the volume of water.

The "correct" way of sizing a boiler is first calculating your heat losses, both fabric and infiltration. You can add a margin for morning boost. You then must add in you hot water cylinder. If done correctly this is very long winded as you have to consider each construction in each room (floors, walls, windows etc.)

The usual way is to take a rule of thumb to calculate your heat loss - 35W/m3
Then add your cylinder load - H = m x Cp x ( t hot - t cold )/Hours heat up x Efficiency x 3600

(So for 100litre cylinder and 90% efficient and 1 hour heat up) - H = 100x4.187x(60-10)/1x0.9x3600 = 6.5kW

Then heat loss + cylinder = heat load

heat load x 1.2 (20 % margin and morning boost) = boiler load

You should definitely oversize rather than undersize to allow for future use in extensions etc.

You should never oversize a condensing boiler.

littlemissfraze

Dtp1979 wrote: »

You should never oversize a condensing boiler.

Why?

If it is to do with keeping it in condensing mode as often as possible the best way of doing this would be to reduce your F&R temperatures so that the return falls below 55oC.

If you are feeding a hot water cylinder this is not recommended as the risk of legionella increases if the water is not stored above 60oC and also the water coming out of your taps may not feel hot enough.

Dtp1979

littlemissfraze wrote: »

Why?

If it is to do with keeping it in condensing mode as often as possible the best way of doing this would be to reduce your F&R temperatures so that the return falls below 55oC.

If you are feeding a hot water cylinder this is not recommended as the risk of legionella increases if the water is not stored above 60oC and also the water coming out of your taps may not feel hot enough.

If you oversize an oil boiler it will short cycle

littlemissfraze

Dtp1979 wrote: »

If you oversize an oil boiler it will short cycle

All boilers are sized to be able to heat the building on the coldest day of the year right?

So in essence they operate at part load for most lot of the year. Short cycle occurs due to poor controls and poor plant selection. The turndown ratio of the burner comes into play.

Dtp1979

littlemissfraze wrote: »

All boilers are sized to be able to heat the building on the coldest day of the year right?

So in essence they operate at part load for most lot of the year. Short cycle occurs due to poor controls and poor plant selection. The turndown ratio of the burner comes into play.

Well I would never oversize an oil boiler. I've seen them short cycle and the efficiency would suffer because of this

littlemissfraze

Turns out it is a single stage burner so short cycling could occur if the boiler was oversized.

Wearb

Some threads break my heart and this is one of them. I often wonder if half of the replies are only visible to some and all of them to me. :pac::pac::pac::pac::pac:

Wearb

. Someone having fun. .

jimf

Wearb wrote: »

. Someone having fun. .

:eek::cool::P:confused::pac::D;):p:):rolleyes::o:mad::(

now are you happy

esox28

The usual way is to take a rule of thumb to calculate your heat loss - 35W/m2

[/

any house built befor 2011 should be using 50w/m2.

Then add your cylinder load - H = m x Cp x ( t hot - t cold )/Hours heat up x Efficiency x 3600

sorry I can't follow this !

(So for 100litre cylinder and 90% efficient and 1 hour heat up) - H = 100x4.187x(60-10)/1x0.9x3600 = 6.5kW

littlemissfraze

Esox28,

I had 35W/m3, so about 100W/m2 for an average height room. I'm starting to think I err a bit to much on the cautious side though!



cylinder load - H = m x Cp x ( t hot - t cold )/Hours heat up x Efficiency x 3600

The amount of heat (H kW) you supply to your cylinder is dictated by
1. the amount of water the cylinder holds (m = mass of water, 1litre=1kg)
2. the specific heat capacity of water (the amount of energy required to raise the temperature by 1oC = Cp) (use 4.187)
3. the amount you want to heat the water by, so assume you want the water to be 60oC and assume the cold water entering the cylinder is at 10oC. (t hot = 60oC , t cold = 10oC)
4. hours heat up is how fast you want it to get from cold to hot
5. efficiency - there'll always be losses to the hotpress etc. so if its lagged I'd assume 90% efficient
6. 3600 - is to convert hours to seconds

Hope this makes sense if you're interested.

househero

littlemissfraze wrote: »

Esox28,

I had 35W/m3, so about 100W/m2 for an average height room. I'm starting to think I err a bit to much on the cautious side though!



cylinder load - H = m x Cp x ( t hot - t cold )/Hours heat up x Efficiency x 3600

The amount of heat (H kW) you supply to your cylinder is dictated by
1. the amount of water the cylinder holds (m = mass of water, 1litre=1kg)
2. the specific heat capacity of water (the amount of energy required to raise the temperature by 1oC = Cp) (use 4.187)
3. the amount you want to heat the water by, so assume you want the water to be 60oC and assume the cold water entering the cylinder is at 10oC. (t hot = 60oC , t cold = 10oC)
4. hours heat up is how fast you want it to get from cold to hot
5. efficiency - there'll always be losses to the hotpress etc. so if its lagged I'd assume 90% efficient
6. 3600 - is to convert hours to seconds

Hope this makes sense if you're interested.

None

esox28

househero wrote: »

None

Yea same but different

Vol x specific heat x temp rise x eff x time

Just worded different.

100w/ mtr2 is very high alright, we are using 30w/mtr2 at present for ufh calcuations for new builds.

Even the meres calculator is grossly over sizing rads atm, with the lowest wall u-values being .65w/mtr2 k and the building regs .19w/mtr2 k.

What's a plumber to do?

househero

esox28 wrote: »

What's a plumber to do?

Guess & look professional while your doing it

Is 15,000 BTU (3 rads) overkill for a Sunroom (Northfacing) with insulated roof, windows on 3 sides. 20m Sq, built 09.

esox28

househero wrote: »

Guess & look professional while your doing it

Is 15,000 BTU (3 rads) overkill for a Sunroom (Northfacing) with insulated roof, windows on 3 sides. 20m Sq, built 09.

Depends on what the required heat load is!

spector11

hi guys
apologies for the delay in getting back as i was on holidays.
im afraid im as confused as ever now. i was talking to a couple of plumbers and they were agreeing with me. they said they do heat loss calculations for new heating installations where all radiators are being replaced but when sizing a boiler for existing radiators they add up the btu values for each and size the boiler accordingly. they gave me some examples.
one said, he was just at a job in a 2500sq\ft house where some rads were undersized. the total btu`s for all rads was 75,000. he said most houses would need a 90\110 boiler for that house size but he said if he fitted at 90\110 itd be way oversized and therefore short cycle.
The other guy said, think of it this way. If you have 10 radiators at 1,000 btu each then youre gonna need a 10,000BTU boiler to heat those rads irreguardless of insulation in the house. This all makes perfect sense to me but im reluctant because of the advise on here telling me the oppisite. HELP!!!!!
ANYBODY??