Oil boiler sizing - is bigger better?

TiredGeek

I'm going to have to fit an oil boiler this year and I'm wondering which way to go for the best running costs......

SAP report says the house is going to need 16kw to heat it, so, do I buy an oil boiler with a 16kw output and run it flat out or is it more economical to buy a 32kw (or bigger) boiler and run it at a lower level?

The cost difference between the two sizes really is quiet small so if the bigger boiler would be better then it's not that much of a stretch to get it

sk240

With the oil prices going up very quickly recently have you looked at an air or ground source heat pump.A good air source heat pump would cost about half to run than an oil boiler, and would probably be about the same cost to install?

Cardew

sk240 wrote: »

With the oil prices going up very quickly recently have you looked at an air or ground source heat pump.A good air source heat pump would cost about half to run than an oil boiler, and would probably be about the same cost to install?

I appreciate you are not to know the OP's history, but it might cause a wry smile if you read back on his history of posts.

He has just removed two Mitsubishi Ecodan ASHPs from his house because they have been a disaster.

You also might read some of the other threads on ASHPs to see how many other people have had very poor results - including loads on the Energy Saving Trust 12 month trial.

Incidentally people are paying well over £10,000 for ASHPs and taking pot luck if they are a disaster or work. That is far more than Oil CH

[Deleted User]

sk240 wrote: »

With the oil prices going up very quickly recently have you looked at an air or ground source heat pump.A good air source heat pump would cost about half to run than an oil boiler, and would probably be about the same cost to install?

Is this a wind up

[Deleted User]

Seriously though I would size the boiler to the house.

Cardew

To attempt to answer the OP's question, I have read that with modern condensing boilers they should not be oversized.

I believe this is to ensure that they stay in condensing mode as long as possible.

However nobody has satisfactorily explained to me, how a boiler that might run 15 radiators when a house is fully occupied, will stay in condensing mode when, say it is only running a couple of radiators.

TiredGeek

sk240, HAHAHAHAHAHAHA ROFL LMAO etc
Seriously, thanks for the replies guys

I'm just not sure of how to go about this, and plumbers are giving conflicting advise, if I put a big boiler in then surely it will manage to get the water temp up faster therefore run for less time.
A smaller boiler will obviously use less oil per hour but in my mind will run for longer and in the end use just as much oil.

So, 36kw boiler using "10" litres of oil / hour but running for 1 hour, or 16kw boiler using "5" litres of oil but running for two hours (amounts plucked out of air for ease of description).
In the end the same amount of oil used.
A big boiler will have headroom to take into account bad weather.
Do they fire up at full whack every time or are they variable?
What I'm trying to get at is will a 36kw boiler always kick out 36kw or can it kick out less and use less oil? (I'm not explaining this very clearly here, I just can't think of a better way to put it)

Bizarrely, we haven't totally given up on ASHP! Yes, we are bloody mad
But, some calculations have shown something interesting. Some fairly simple maths has shown that to use oil in a house with a 16kw heat load, taking boiler efficiency into account, the cost comes back at roughly £1.25 / hour to run.
Now, run that for 3 hours in the morning to take the chill off, and 6 hours in the evening and we're looking at about £11 / day.
My (very poorly) fitted ASHP system was managing to heat the house OK, when it worked, and costing about the same as above. However, that was for a 24hr run time.
I'm confident that we can engineer a system that runs far better than the abortion we have just ripped out, I'm confident it can get through a winter doing what we want it to do, and I'm pretty sure we can get that horrendous running cost under control.

Obviously the question of initial cost comes into it as there is a £10k difference in price, which frankly buys a whole load of oil to allow us to run it longer. With ASHP I can't have my oil pinched either.
Now if we can qualify for RHI...

TiredGeek

Cardew wrote: »

I believe this is to ensure that they stay in condensing mode as long as possible.

However nobody has satisfactorily explained to me, how a boiler that might run 15 radiators when a house is fully occupied, will stay in condensing mode when, say it is only running a couple of radiators.

Please correct me if I'm wrong, but doesn't the condensing mode rely on a relatively cold return temp of the water to the boiler?

If so, like you say, 15 rads will drop the temp nicely and it will condense, but if several turn themselves off with thermo rad valves then the water will be returning too warm for condensing to take place and therefore the efficiency will drop.
Not that I think we'll have that much of a problem with our cold old place and 35 rads!

albyota

Cardew wrote: »

I appreciate you are not to know the OP's history, but it might cause a wry smile if you read back on his history of posts.

He has just removed two Mitsubishi Ecodan ASHPs from his house because they have been a disaster.

You also might read some of the other threads on ASHPs to see how many other people have had very poor results - including loads on the Energy Saving Trust 12 month trial.

Incidentally people are paying well over £10,000 for ASHPs and taking pot luck if they are a disaster or work. That is far more than Oil CH

Just to let everyone reading this know......it wasn't the 2 x 14 kW Mitsubishi Heat Pumps that were the problem....it was the fact that they were fitted in an unsuitable house? and the plumbing was installed by 'workmen on horseback'.....am I right Tiredgeek?

LOADS......errm 29 nearly four years ago........!

albyota

I would go along with the SAP but get a Worcester Bosch 18/22 Greenstar condensing system boiler or similar.

That's if 28kW of heat pump can't hack it.

albyota

TiredGeek wrote: »

sk240, HAHAHAHAHAHAHA ROFL LMAO etc
Seriously, thanks for the replies guys

I'm just not sure of how to go about this, and plumbers are giving conflicting advise, if I put a big boiler in then surely it will manage to get the water temp up faster therefore run for less time.

and use EXACTLY the same amount of fuel

A smaller boiler will obviously use less oil per hour but in my mind will run for longer and in the end use just as much oil.

YES

So, 36kw boiler using "10" litres of oil / hour but running for 1 hour, or 16kw boiler using "5" litres of oil but running for two hours (amounts plucked out of air for ease of description).
In the end the same amount of oil used.
A big boiler will have headroom to take into account bad weather.
Do they fire up at full whack every time or are they variable?
What I'm trying to get at is will a 36kw boiler always kick out 36kw or can it kick out less and use less oil? (I'm not explaining this very clearly here, I just can't think of a better way to put it)

Bizarrely, we haven't totally given up on ASHP! Yes, we are bloody mad

Below you say you have ripped them out....???

But, some calculations have shown something interesting. Some fairly simple maths has shown that to use oil in a house with a 16kw heat load, taking boiler efficiency into account, the cost comes back at roughly £1.25 / hour to run.
Now, run that for 3 hours in the morning to take the chill off, and 6 hours in the evening and we're looking at about £11 / day.
My (very poorly) fitted ASHP system was managing to heat the house OK, when it worked, and costing about the same as above. However, that was for a 24hr run time.
I'm confident that we can engineer a system that runs far better than the abortion we have just ripped out, I'm confident it can get through a winter doing what we want it to do, and I'm pretty sure we can get that horrendous running cost under control.

Obviously the question of initial cost comes into it as there is a £10k difference in price, which frankly buys a whole load of oil to allow us to run it longer. With ASHP I can't have my oil pinched either.
Now if we can qualify for RHI...

A few mixed messages here and there......is it still in...is it ripped out.....its costing the same as your projected oil figure......;)