Boiler sizing from cubic area, or actual usage

[Dan Robinson]

Are you referrring to the advertising ? If so I agree, and would add the word un-ethical

As I have said before, the figures are obtained from a test house not Intergas. Plenty of other manufacturers have quoted NETT figures to get to where they want.

Most marketing is booloocks in one way or another.

But principally I was referring to the thread in general. As once again there is misunderstanding, misinformation and an OP reading what he wants.

I should add.... Wildcat knows his onions about combustion. He is a combustion engineer.

Lifestyle play a part massively, and weather comp is but one way to control a system well.

Wooshitter controls, while improving are very limited. As will be their sales teams technical knowledge.

 

[bernardgreen]

and weather comp is but one way to control a system well.

Is it ? It might be but isn't it just one more ( over complicated ) thing to go wrong ? Could even be a sales gimmick rather than a real benefit.

 

[Dan Robinson]

It has benefits Maybe not as much some say.

It is nothing complicated. It is a 20p resistor in a weather proof container.

 

[bernardgreen]

It is nothing complicated. It is a 20p resistor in a weather proof container.

Plus an analogue to digital convertor and processing software ( on the PCB ) in the boiler.

 

[Kaymo]

But principally I was referring to the thread in general. As once again there is misunderstanding, misinformation and an OP reading what he wants.

Dan, I think it is actually you who have misunderstood this entire thread, and reading what you want. You have been down on this right from the start and unwilling to listen and understand the point I was trying to make about personal usage. With some help from some previous posters it turns out my original hunch was right, and the boiler that I have been quoted is over sized for my personal use, that was the original point of the post. The point about GCV and NCV came after I learned from the discussion that net values were being advertised to make the boiler sound more impressive.

Your analogy about cars also doesn't make any sense. If you were charged for kWh at the petrol pump you might be able to stretch a connection, but you are charged by volume for fuel. Gas is charged in kWh and boilers heat output can be measured in kW so there is a direct connection. Within the UK market gas is charged in kWh based on the complete calorific value of the fuel. That's fair enough, but knowing that and then coming up with a calculation that ignores the part of the calorific value of the fuel on the input, so that they can then add it back in again at the output to make the boiler sound like it is more efficient than it really is, is dishonest. I'm not naive, I know that most marketing is dishonest and the company will use what it can to sell, but the UK government should be regulating these things and not allowing these claims.

Take for example my situation. I start with an Ideal Mexico, 73% efficiency. I am quoted for a 30kW boiler which is advertised as 99.5% efficient. With a little investigation I find out that that boiler, fitted into my existing system will only ever deliver close to 30kW of heat output if it is run at ~75deg. If I am going to set it at that temp then the efficiency drops to 88%, but that's a net figure. No values are published for gross, but if around 10% is recovered by the condenser then can I presume that the real, or the gross efficiency is ~78%. All of a sudden that paints a different picture in terms of ROI on the original boiler. To top it all off I find out last night that WB themselves only forecast a 10-15 year lifespan on their new boilers, and there are a lot of reports of boilers of all makes needing replaced in shorter time frames. I only pay around £600 a year for gas, if I reduced my bill by an optimistic 20% then on current prices I would pay back the cost of the new boiler in about 30 years!

Look, I am not complaining. This thread has been really, really useful for me and I have learned a lot, I appreciate the time everyone has taken to respond. I now know that the savings I would make are around 10% less than suggested by marketing. Considering this I might decide to let the boiler run for a bit longer until it starts to develop problems. When I do replace it I would go for the 24 kW boiler, and run it at lower temps to ensure that it is in condensing mode most of the time.

 

[The831Bunny]

And after all this ******, what is your hot water requirement?

As to all this talk of combustion efficiency of 24Kw vs 30Kw boilers, it is so small that any small savings are wiped out if on/off controls are used, or there is a few days one year at -8C than the next year with a mild winter.

You’ve probably used more in time and electricity for your PC or whatever in this thread than what you will potentially save!

Size the boiler for the hot water requirement foremost, then confirm whole property and heating system load. All modern boilers can all be range rated to suit the heating load as required on commissioning, adding load compensation or weather compensation controls ensures you comply with the current building regs and runs the boiler at the highest efficiency. Have a look at Vaillant ambiSENSE controls or Honeywell evohome to control your seldom used rooms.

 

[Kaymo]

And after all this ******, what is your hot water requirement?

My hot water requirements are met by either the 24kW 30i or the 30kW 29Cdi, as both have around 30kW hot water output and around 11l/m - that was simple so I didn't mention it.

As to all this talk of combustion efficiency of 24Kw vs 30Kw boilers, it is so small that any small savings are wiped out if on/off controls are used, or there is a few days one year at -8C than the next year with a mild winter.

You’ve probably used more in time and electricity for your PC or whatever in this thread than what you will potentially save!

Doubtful, regarding the electricity, but I don't disagree about the differences in efficiency between the two boilers being small, most of the time. I was working on the premise that the smaller boiler would modulate down to a lower range. My point however is that I don't believe I would ever use more than 24kW, which why I didn't see the point in getting a larger boiler - I just wanted to confirm that.

weather compensation controls ensures you comply with the current building regs and runs the boiler at the highest efficiency.

I looked into this, but unless I have misunderstood how it works I couldn't see the point. I figured that I would get the highest efficiency by setting the boiler to a low temperature setting and leaving it there, if it got really cold I might turn it up for a few days, then back down again. As I said I may have misunderstood, but my impression was that this was all that these controls did.

 

[Mottie]

My current 26 year old heat only boiler is 60,000 BTU’s. It probably doesn’t produce that anyway as it’s more than likely all scaled up. It provides more than sufficient heat for my heating and hot water needs. Even so, since new, I have never been able to have it set at more than around 80% on the boiler temperature scale as that would pop the button underneath and require it to be reset. I’m assuming that it was possibly too big for my needs? I’m currently looking to replace it as it’s kettling now and I don’t want to chance another winter with it. Would I be correct in thinking that I can fit the same size boiler and that a new type boiler will modulate if things got too hot? Any point in getting an even bigger boiler? The reason I ask is that I’ve got a BG 'heating engineer' coming to assess my boiler needs this afternoon and I’d like to have a rough idea before he comes so that I can tell whether he's spinning me any fibs.

 

[Dan Robinson]

Plus an analogue to digital convertor and processing software ( on the PCB ) in the boiler.

All built into the boiler already.

 

[fixitflav]

Even so, since new, I have never been able to have it set at more than around 80% on the boiler temperature scale as that would pop the button underneath and require it to be reset. I’m assuming that it was possibly too big for my needs?

I wouldn't think so. The control-stat should stop the boiler before the (linit-stat) button pops. Maybe the temperature continued to rise after the boiler and pump had stopped, if so a pump run-on timer might have helped, but you have clearly managed OK for 26 years!

Would I be correct in thinking that I can fit the same size boiler and that a new type boiler will modulate if things got too hot?

I think most modern boilers modulate gas flow, but you could check to make sure.

The reason I ask is that I’ve got a BG 'heating engineer' coming to assess my boiler needs this afternoon and I’d like to have a rough idea before he comes so that I can tell whether he's spinning me any fibs.

As they said about Harold Wilson, you can tell the BG engineer is lying, as his lips are moving at the time

 

[Mottie]

Well, he’s been and gone. See the quote here. https://www.diynot.com/diy/threads/what’s-your-opinion-on-this-bg-boiler-quote.501410/

 

[fixitflav]

I looked into this, but unless I have misunderstood how it works I couldn't see the point. I figured that I would get the highest efficiency by setting the boiler to a low temperature setting and leaving it there, if it got really cold I might turn it up for a few days, then back down again. As I said I may have misunderstood, but my impression was that this was all that these controls did.

Weather comp seems like a good idea in principle, how much it saves in practice I wouldn't know. For best results you want plenty of rad area relative to heat output needed, so the water temp can be turned down a good bit and still keep warm. From your description, that sounds like the situation you're in. You might be able to retro-fit it to your existing system, if you can source a strap-on pipe stat (for your boiler flow pipe) with variable switch point set by the outdoor sensor. Wired in series with the existing control-stat.

 

[fixitflav]

Have a look at Vaillant ambiSENSE controls or Honeywell evohome to control your seldom used rooms.

Do these devices, and things like working CH from a smartphone, actually save anything significant? I'm not claiming they don't, just curious about whether any reliable studies have been done.
When I and the now ex moved into first house in February 1973, Honeywell W-plan, set CH timer to start at about 0630h, for when we got up for work, off 0830h, on 1600h off 2300h. When we got home from work the boiler was on, and stayed on till about 2130h when it stopped on room-stat. After about 20 mins it started again, and ran till timer switch-off. And we weren't very warm then!
When we had kids, left heating on all day and the whole system was much more relaxed. Boiler only started on room-stat now and then, and we were warm with room-stat setting 1-2°C lower than before. Didn't notice any bigger gas bills. As I'm sure we all know, it's because if the walls, floor, furniture etc are cold, need to have warmer air to feel comfy. Started leaving the heating on 24/7 and have done that in both houses since.
So I'm a bit sceptical whether these latest controls pay for themselves.
I can see it's a good idea to have seldom-used rooms set a bit cooler, either by trimming manual valves or better, with TRVs.

 

[fixitflav]

Your analogy about cars also doesn't make any sense. If you were charged for kWh at the petrol pump you might be able to stretch a connection, but you are charged by volume for fuel. Gas is charged in kWh and boilers heat output can be measured in kW so there is a direct connection. Within the UK market gas is charged in kWh based on the complete calorific value of the fuel. That's fair enough, but knowing that and then coming up with a calculation that ignores the part of the calorific value of the fuel on the input, so that they can then add it back in again at the output to make the boiler sound like it is more efficient than it really is, is dishonest. I'm not naive, I know that most marketing is dishonest and the company will use what it can to sell, but the UK government should be regulating these things and not allowing these claims.

To go off on a bit of a tangent, engine specs use NCV when quoting thermal efficiency, shaft power out/heat in. 44MJ/kg for the usual liquid fuels. I suppose that's reasonable as the water goes out the exhaust as vapour. It's possible to recover heat from the exhaust gases, but that doesn't affect the shaft power.

 

[Kaymo]

Started leaving the heating on 24/7 and have done that in both houses since.

There is a guy called Jeff Howell who I think writes in the Daily Telegraph, who argues that leaving the heating on at the lowest setting saves money. His argument is that when a room cools water vapour condenses in the fabric of the building, the walls etc. When you then heat the room back up it takes additional energy to convert that water back into vapour. Every time you heat and cool the house this process repeats. He also makes a separate argument that the cooling of the house and condensation, and the damage it does to window sills and other components, outweighs any energy savings.

This is of course the opposite to the conventional argument made by the energy saving trust and just about every other related body, that leaving the heat on costs more. I think that both are right and both are wrong, depending on the circumstances. I suspect that for people who are in the house most of the day it makes sense just to leave it on, rather than let it cool for a few hours while you are out and have to ramp the heat up to get it comfortable again, but for people who are only getting back to the house from say 6pm I doubt it. The greater the difference between inside and outside temp the greater the heat loss, so I can't see how for people who are away for 8 hours plus each day, it can make sense to leave the heat on. I suspect many of Jeff Howells readers are pensioners, which is probably why it works for his audience who are only popping out for the bingo for a couple of hours...

I've always turned it off when its not being used and I plan to get a remote operated stat regardless of whether I replace the boiler. When I'm away in winter I'd prefer to leave the heating off, but with the option of being able to get it fired up a few hours before I get back.