I am about to have a new heating system installed in my house. The choice of boilers seem to come down to either a Worcester-Bosch Highflow 400 or a Vokera Linea Max. As both of these boilers are about the same price and have very similar specifications is there much to choose between them? I would be interested in any independent reliability surveys or comparative maintenance costs.
I know neither of these boilers are condensing but I can't find a similar condensing boiler and also the vent will be very close to a boundry so a plume might be unpopular with the neighbours.
From feedback the Highflow is pretty reliable and it has been around for a quite a few years now. Assuming a condesning boiler is out because of the plume, try an Alpha CB50. Approx the same performance as the High but wall mounted.
The Worcester Bosch 40kW Greenstar condensing combi gives about 16 litres/min flow whereas the Highflow is about 18, as is the CB50.
The Highflow, Vokera and CB50 are all two stage flow rates. High flows when the store is full of hot water and low flow, about the same as an avearge combi, when exhausted. The recovery rate on all is v good; a matter a few minutes.
Look at the Potterton Powermax
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A boiler and unvented cylinder all in one floor munted case. As with the Gledhill Gulfstream 2000 is similar except it uses a heat bank like the Highflow and Vokera.
Both are reputable makes, but neither is an exciting product from the performance point of view. Although marketed as "suitable for the larger house", really the power levels of 24-28kW are very average these days. That is not to say that you necessarily need anything larger but rather that there is plenty of choice in this range and above.
Both are right at the very bottom of the legal limit on what is allowed for new boiler installations - 78% on the SEDBUK seasonaly efficiency scale. Condensing boilers achieve around 90-91% efficiency, and from my own experience, the cost savings line up with the efficiency difference. Bear in mind that gas prices are in the process of increasing and the trend will undoubtedly continue. If you plan to remain in the house for a while, you will recover your costs.
Pluming on newer designs of condensing boiler are nowhere near the level of earlier designs and on many models it is possible to use flue systems constructed from 50mm high temperature PVC waste pipe. These can be run some distance - even internally - and hence exhaust to a more convenient place.
You haven't said much about your requirements.
Since it is a large house, do you have two bathrooms/showers and do you need to use them concurrently?
Are you using a storage system currently?
Do you desperately need to save the cylinder and tank space?
There are some things to be careful about here.
Mains pressure hot water can be good, but it's essential to check that the mains can deliver a good flow rate. Try measuring with a bucket and stop watch at the kitchen tap. As a rule of thumb, if you get 20 litres/minute or more then a mains system can perform well, if it's in the 9-12 range then it will be very poor and you may need to look at (can be expensively) having the service pipe from the road increased in size.
Having the water heated instantly in a simple combi means that the hot water flow is limited to the instant energy is limited to that provided by the burner. With the 24 and 28kW figures that you have mentioned, this would be 9 and 11 litres per minute when the water is cold in the winter, delivered at bath/shower temperature of 40 degrees. This is poor for one bath/shower and hopeless for two.
To get around it, both products you mention have a small heatbank store of around 50-60 litres. This energy is applied rapidly to the cold water passing through the boiler and allows the manufacturers to quote impressive flow rates of 18 litres per minute. At 60 degrees output and mixed with cold water, this will provide good flows for two showers.
However, there is a catch. That is that once the little heatbank runs out, the performance drops to the instantaneous rate and you are very limited. With the volumes listed, you will be able to fill a bath, not very deeply before the store runs out - approx 100 litres total. A reasonable bath needs closer to 150 litres. Two showers will run for a very few minutes on it if run together.
The manufacturers have not been dishonest if you read the small print of the data sheets - they do say that the flow drops - but they also use an average to optimistic mains temperature of 10 degrees. In the winter, around 5 is more typical and that is probably when you want a greater HW production rate. So the reality is not as good as the brochures imply.
The only ways around this are to have a much larger boiler - you can get 46kW instantaneous ones which will deliver around 18 litres/minute at 40 degrees - or you need to increase the storage.
If you like the idea of mains pressure water, and can spare the cylinder space, then a heatbank can be a good idea. These replace the cylinder and you can get rid of the loft tank. The concept is roughly like that in the Heatflow, but you can have a store of say three times the size and obviously that makes a huge difference if your demands are for two bathrooms. This idea also allows you to have a wall mount or otherwise smaller boiler and gain space where it is installed. For that matter the boiler could easily go somewhere else entirely.
A pressurised hot water cylinder is another approach, and that would have a similar improving effect.
Otherwise, if the mains water rate is poor, a storage system with loft tank will perform better than a mains one and you can increase pressure for a shower with a pump.
There are plenty of boilers around to cover all of these cases, so don't get sucked into something that is not a good match for your requirements.
18019 litres/min is not bad at all amnd will fill a bath pronto.
Thye both use heated stored water, a heat bank, to boost performance.
He said he has doubts about the condensing plume.
He is after a combi, no condensing combi I know eliminates the plume.
He stated two stored water excellent flow combi's, not simple low flow low models.
He is running a house not a school. It will very rare will drop into the low flow stage of the combi's. But he will "never" run out hot water. The recovery rate of these models is only a few minutes.
100 litres is the average bath size. Any more water and it fill at the lower flowrate of combi. this can be running while people are actually in the bath, as many people,do anyhow. They can in when there is only a few inches of water and leave the taps on.
Average shower of 7 - 8 litres/minute at approx 40C. So 2 showers will take
15 litres of mixed water. Of the 15 litres 2/3 to 3/4 will be hot and the rest cold of the 60C draw-off temperature, so only 10 litres/min will be drawn off the 60 litre 80C temperature stored water heat bank. Then you have to take into account that on draw-off the heat bank always fires up providing approx 11 litres/min. In effect the stored water supplies one shower, the instant side doing more than the other. As the instant side is infinite and never runs out, the stored water alone will provide one of the showers. The 60 litres at 80C will keep one shower running for about 13 minutes. So, two average shower will run simultaneously for approx 13 - 15 minutes, not a few minutes.
Please do not give opinions of equipment you have even seen never mind seen them in action. Those I have met who have these sort of combi's love them and would never go back to tank/cylinder/power shower pump dross. The poster is attempting gain information to make a reasoned decision. The last thing he needs is misinformation from an amateur.
Highly cost effective and gives a high flowrate and should always be considered. Two Worcester Juniors can be had for approx £1100, not far from he cost of one Highflow or one Vokera. Combined they will give approx 21 litres/min flowrate. They give natural zoning with no complex wiring or zone valves. One does upstairs CH One does the downstairs CH. each has its own programmer stat.
A doddle to fit, a doddle to wire up, cheap, CH zoning, high flowrates and backup if one goes down. A win, win, all the way. Must be considered in these situations.
Why not talk to you neighbours and explain about condensing boilers and plumes, they might not mind after all. A condensing boiler condenses out most of chemicals in the flue gases, unlike a conventional boiler. If it does turn out that they don't like it, the flue can be resited easily with a lot of boilers.
It would be outstanding, but not all that likely from a 24kW boiler and domestic water supply. Are you going to contact Thames Water to arrange for a 60cm supply pipe?
I covered that.
There is, however a range of effects.
Even a conventional boiler will produce a plume on cold dry days.
I'm aware of that, which is why I said "in a" and not in "the ones that were mentioned". I then went on to discuss the effect of adding a small store.
The storage capacity is very limited as was demonstrated last week.
No specifics on what "large house" meant, or indeed the numbr of bathrooms and pattern of use. I therefore described what could happen under patterns of use that are entirely possible in a large, but not that large house. it is possible that two bathrooms are in use simultaneously and peak demands are high. I certainly have that situation, and it is not at all unusual.
Obviously if the OP's needs are less, then the situation will be better and a small store combi may be OK.
I have simply stated what could happen under quite possible circumstances, not hoped that it won't.
It's a small bath. We already covered that, and I am not getting into another pantomime argument about it.
That makes no sense in English at all.
That is not an average shower at all. It is in the range of a poor electric shower. We covered that already.
It needs at least twice that.
That is optimistic if you do the actual calculations as we did only last week.
So even in the best case that you describe, it lasts 6 minutes. In reality, it lasts for closer to half of that.
In the 28kW model. In the 24kW model, it's 9.
In reality, it is still way short, because the premise of 7-8lpm is erroneous in the first place.
At what usage rate?
The 80 degree water is used via a heat exchanger, but to simplify matters, let's assume 100% efficiency and "mix" that with 5 degree mains cold water.
Using
Vc[Tf-Tc] = Vh[Th-Tf]
where
Vc = cold volume Vh = hot volume Tf = mixed water temperature Th = hot temperature Tc = cold temperature
produces
Vc[40-5] = 60[80-40]
Solving for Vc gives
Vc = 68.6 litres
Adding Vh back in to give the volume at 40 degrees gives 128.6 litres.
To achieve a 13 minute run before freezing ones bits would require a flow rate of no more than 9.8 lpm maximum.
At the more realistic 15lpm, the run time will be 8.5mins max.
If there are females in the house who wash their hair in the shower, there are going to be tears.
Both can run at a poor rate of 8lpm or one at that and one for 8 minutes properly. This is a few minutes.
It also pays no account of what would happen if a bath is taken in one bathroom. Then the situation is dire.
In English?
Before buying anything, one should look at the technical specification and certainly past the marketing. It is only possible for a product to perform as well as physics allows.
I haven't said that these products are totally useless or inapplicable, simply that as a result of the design limitations, there is a limit to the performance.
That may be acceptable to some people, but it does need to be pointed out that there can easily be scenarios where the results will be disappointing. In some households people are willing to alter their patterns of use to fit in with the limitations of the equi[pment. Others can't, and then a limited solution like this is not going to work for them. It's completely irrelevant whether I have one of these or not. The performance is totally predictable with simple arithmetic.
You should look at the actual specifications and not the marketing hype.
I am sure that there are. Either their usage patterns fall within what this type of device can do or they have altered their usage pattern to fit the system.
Some people won't find that acceptable, and it is more likely that that will be in a larger property with multiple bathrooms and lots of people.
I'm sure. So your suggestion of over 18000 lpm is perhaps a touch on the high side?
It can be only slight with a modern design of condensing boiler especially on a longer flue, and in any case I suggested ways to address any pluming issue.
Would you like me to post a link to the thread where the numbers were calculated?
It may be for you, but as I say, I am not getting into a pantomime argument with Widow Twankey.
My Icos plumed a lot. It was actually quite annoying. The WB Greenstar hardly plumes at all. However, this may have a lot to do with the Icos having a short horizontal flue just below the bedroom window, whilst the WB flue is longer and vertical through the roof, out of sight!
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