2 boilers - will we need seperate cover for each?

silvercar

Interestingly, I've just checked the boilers on boilers.org.uk and the 3 year old one is rated D at 78% efficient, the 20 year old one is an E at 74% - so not much different!

Canucklehead

silvercar wrote: »

Interestingly, I've just checked the boilers on boilers.org.uk and the 3 year old one is rated D at 78% efficient, the 20 year old one is an E at 74% - so not much different!

Hi again...the Sedbuk website is an excellent resource.....seems to have been a lull in boiler efficiency over that time period until the early 2000s....no real incentives/legal requirements for manufacturers to make significant efficiency improvements in the UK market...same old, same old I guess;)

Canucklehead

Waterways_2

Cardew wrote: »

That of course depends on the size of your house, a combi is not sufficient for some houses; especially if you want plenty of hot water.

If you have existing plumbing and Hot water tanks it can be a huge job to modify the plumbing to fit a combi.

Personally I would NEVER have a combi, as a hot water tank is much preferable.

Incidentally what temperature is the water if you produce a flow of 19 litres a minute?

I really get amused at the claims of Combi manufacturers.
Even the Worcester Greenstar Highflow 440 Boiler which is their large floorstanding combi produces that flow rate with the water temperature at only 35C with the input temperature at 10C. e.g it is heating water by only 25C.

Even with the input water temperature at 10C, the flow rate is about 7 litres/min if you want water at 45C.

In winter input water temperatures can typically be below 5C.

Hot water tanks usually have their temperature at 60C.

I picked this old post up. This is incorrect. It is 35C temperature rise, not producing hot water at 35C. If 10C main temperature then it will produce 45C water at the stated flow rate. Reduce flow a little and 50C or 60C at the taps is possible.

Cardew

Waterways wrote: »

I picked this old post up. This is incorrect. It is 35C temperature rise, not producing hot water at 35C. If 10C main temperature then it will produce 45C water at the stated flow rate. Reduce flow a little and 50C or 60C at the taps is possible.

It is not incorrect!

Have you read the tech spec for the highstar 440? I suggest you do!

It is available from here:

http://www.worcestergreenstar.co.uk/greenstar-combi-boiler.asp?id=336

It very clearly state that at a flow rate of 20L/min with an input water temperature of 10C the out put water is 35C i.e. a rise of 25C.

There is a chart showing the temperature at various flow rates.

Indeed why would they bother stipulating the input temperature of the water if it was a 35C rise in temp?

can you come back on this point please.

P.S.
I am aware that it is normal to talk in terms you suggest i.e. flow rate xxL at 35C temp rise(my Worcester combi does) but Worcester do not on this model or the 550!

Waterways_2

Cardew wrote: »

It is not incorrect!

Have you read the tech spec for the highstar 440? I suggest you do!

It is available from here:

http://www.worcestergreenstar.co.uk/greenstar-combi-boiler.asp?id=336

It very clearly state that at a flow rate of 20L/min with an input water temperature of 10C the out put water is 35C i.e. a rise of 25C.

There is a chart showing the temperature at various flow rates.

Indeed why would they bother stipulating the input temperature of the water if it was a 35C rise in temp?

can you come back on this point please.

P.S.
I am aware that it is normal to talk in terms you suggest i.e. flow rate xxL at 35C temp rise(my Worcester combi does) but Worcester do not on this model or the 550!

Look at this. It clearly states 20 litres per minute at 35C temp rise.
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-highflow-440cdi/technical-data-table

Anyhow the new 550 Highflow is 25 litres/min. And this one sings.
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-highflow-550cdi/technical-data-table

The 550 is just under £2K, but that is it. Just pipe it up. It will do two bathrooms. OK a bit slow if two baths at once, but that is the case with a cylinder too. The probability of two baths being run at once is rather slim in 99% of homes. Two showers at once is a reality and the 550 can cope with two no problem as long as they are piped up correctly using pressure equalising shower mixers, which are what should be used on all mains pressure systems. One poster says his 13 year old 440 can cope with three showers.

The 550 gives 25 litres/min for 5.5 minutes delivering 135 litres of hot water. Then reverts to over 17 litres/min for ever when the internal heat bank is exhausted. That is 552 litres in 30 minutes. With reheat around 7 minutes.

A 550 unvented cylinder will take over an hour to re-heat using a decent sized boiler. A 550 litre cylinder is big and heavy and very expensive. and can blow up like this:
http://au.youtube.com/watch?v=pu3FwgIHsQA
http://au.youtube.com/watch?v=GF_Wrm-Ns0I
They can take the house down and require an annual service which costs.

The smaller 440 gives 20 litres/min for 6.5 minutes delivering 130 litres of hot water. Then reverts to around 13 litres/min for ever when the internal heat bank is exhausted. that is 435 litres in 30 minutes. With reheat around 7 minutes.

Why people keep carping on about unvented cylinders and system boilers is beyond me. It just doesn't make sense at all. All they have to do is do some basic sums instead of going on old wives tales. If you have a decent cold mains pressure and flow, then high flow combis are the way in pokey little UK homes. A system boiler and a 450 or 550 unvented cylinder costs far more than a 440 or 550 washing machine sized combi, and makes the house look like a school boiler room. I have seen 440's fitted in airing cupboards. Very neat, enough space for servicing, and lots of space above for clothes storage. And no boiler in the kitchen

If you have body jet showers, these high flow combis are the things to have. Many people who buy body jet showers have to pay a fortune in getting a larger cylinder as the existing cylinder lasts only a few minutes. It is best to sit back and reassess and instantly heated water is the way - the high flow combi, or high flow multi-point like a Rinnai, The Rinnai can be fitted outdoors on the garden wall taking no space at all in the house. A top quality Japanese unit.

The Ethos 54C combi delivers around 23 litres/min for ever, again great for body jets and two bathrooms.

Both specs are here:
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-highflow-550cdi/the-greenstar-highflow-series-at-a-glance-

gasbag1602

george1939 wrote: »

It is not necessary to have two boilers I get all central heating and 19 litres of hot water per minute'forever' from my one combi {worcester}Highflow please check each boilers specification [easy enough to couple in hot water pipes and dispence with one of the boilers.Only one wear and tear then and maintenance

Worcestor highflows are nothing but trouble, avoid this boiler like the plauge!

mech_2

Waterways wrote: »

Look at this. It clearly states 20 litres per minute at 35C temp rise.
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-highflow-440cdi/technical-data-table

Then it must be a mistake. Using a calculator, the rated output of the boiler and the volumetric heat capacity of water, I come to the conclusion that the figure Cardew quoted is right and the figures are for a 25 degree rise.

Cardew

mech wrote: »

Then it must be a mistake. Using a calculator, the rated output of the boiler and the volumetric heat capacity of water, I come to the conclusion that the figure Cardew quoted is right and the figures are for a 25 degree rise.

I hadn't seen the table that Waterways posted that clearly shows a 35C rise. and, as I posted, my old Worcester combi tech spec is also in that format showing a 35C rise.

However the chart in the tech spec for the 440 shows temperature of the water and the time taken and specifies an inlet water temperature of 10C - 130 litres of 35C water in 6.5 minutes.

If the chart shows the temperature rise - what is the relevance of the inlet water temperature. If it can deliver 20L/min at 35C rise, 10C would be raised to 45C.

So some confusion.

However looking at the technical specification in more detail it would appear that there is a internal thermal store where a large quantity of water is pre-heated. This will enable it to produce higher temperature water at 20L/min.

So it is in fact some sort of hybrid - and loses some of the advantages of a Combi.

However it is not accurate to say that it can produce 20L/min at 35C rise - once that tank is emptied of hot water used to supplement the HW flow it will revert to ??L/min

Indeed it can only produce 50litres of 42.5C water at 20L/min with 10C inlet water.

That indeed supports Mech's contention. Also the fact that my 24kW combi(the 440 has 29.5kW) and 65% efficiency(440 = 90%) can only produce 7L/min at 35C rise.

Waterways_2

Cardew wrote: »

I hadn't seen the table that Waterways posted that clearly shows a 35C rise. and, as I posted, my old Worcester combi tech spec is also in that format showing a 35C rise.

However the chart in the tech spec for the 440 shows temperature of the water and the time taken and specifies an inlet water temperature of 10C - 130 litres of 35C water in 6.5 minutes.

It is clear you are a little confused. The chart shows the temperature at Delta T (Delta is difference). So at 35C delta T a 550 will deliver 135 litres in 5.5 minutes. 25 litres/min. The chart is clear on that:
http://www.worcester-bosch.co.uk/homeowner/products/gas-fired-boilers-and-systems/greenstar-highflow-550cdi/versatility

However looking at the technical specification in more detail it would appear that there is a internal thermal store where a large quantity of water is pre-heated. This will enable it to produce higher temperature water at 20L/min.

So it is in fact some sort of hybrid - and loses some of the advantages of a Combi.

It is combination boiler. That is....it combines all the hot water and heating into one box. It uses stored water for an initial burst of 20 litres/min on the 440 or 25 litres/min on the 550. After it reverts to an infinitively continuous combi output of around 13 litres/min for the 440 and 17 litres/min for the 550 - what the burners can heat directly. They are two stage flowrate combis. The first high flowrate stage is more than adequate for most homes. They are rated as two bathroom units.

The engineers have carefully matched parts for high efficiency. The least efficient hot water heating is a system boiler heating a low mounted immersed coil cylinder. The highflow heats the store of water directly - no coil or heat exchanger at all.

However it is not accurate to say that it can produce 20L/min at 35C rise - once that tank is emptied of hot water used to supplement the HW flow it will revert to ??L/min

Read above and my previous posts. It is accurate. W-Bosch do not claim it will deliver 20 litres/min or 25 litres/min indefinitely. The Ethos 54C can deliver around 23 litres/min indefinitely - this has no stored water. Rinnai make multi-points than deliver high flowrates still using a domestic U6 gas meter. MAN have a 78 kW wall mounted model (special order) that delivers around 35 litres/min indefinitely, however you will need the next size up gas meter for it a U16.

The W-Bosch tables are correct. Even one poster has given real life experience of these units running three simultaneous showers and many come in to counter with no experience of them. I find that amazing. The figures speak for themselves.

As to one post saying the Highflow is a poor unit, well it has been around for about 20 years in various stages of development. As for efficiency, the earliest HighFlow was 72% and they have climbed steadily to 92%. The non-condensing 440 model mentioned on this thread is around 80%.

I hope this clears it all up.

Cardew

Thanks - yes it clears it up and with further reading of the tech spec and your earlier post I had realised that the 'trick' was to pre-heat stored water to enable a 35C rise.

It is relevant to point out that the pre-heating of the thermal store to some extent means that is is slightly less economical than a combi heating the water directly as there will be losses from the store.

Agreed they might not claim that the 440 can deliver 20L/min indefinately, on the other had they don't qualify the claim of 20L/min!!

However I would agree with you that such a boiler would comfortably meet the needs of most housholds.

Just a final question.

As the chart shows temperature change(and not temperature) what is the relevance of quoting the input water temperature as 10C?


If the input water temperature is 5C or 15C will it not raise both by, say, 35C to give output at 40C or 50C?

Or have I got that wrong too?