hi a friend has asked me the above but I have been out of that side for a few years now, so any help ?
Thanks
Thanks
Recommend a 24kw Combi
- Thread starter RobTi
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I wouldn't, I'd fit a 28kw for a few quid more.
Thanks he’s looking for the cheapest but what 28 is recommended, I will fit it except for gas and flue
Oh dear ... "ducks head and waits for the incoming" .....
hope you have a gas safe registered engineer willing to complete the installation for you, as you will have difficulty finding one if you don't.
never ceases to amaze me the level of stupidity on here, buy a Ferolli they are really cheap
If he wants the cheapest then we don't need to recommend anything - just go to Mr Central Heating and find the cheapest boiler they sell. It'll be a Vokera Easiheat I think.
If you want a decent boiler then get an Intergas, either the Rapid if you're on a budget, or the ECO RF if you want a 10 year warranty.
As for fitting it, in addition to doing the flue and gas, most Gas Safe Registered engineers will also want to hang the boiler on the wall. They don't know whether you've used suitable fixings or not, and whether the substrate is suitable for hanging a boiler unless they have done it themselves, and if the boiler fell off the wall it would be their responsibility even if you had hung it. Additionally, the boiler needs to be sited so that it complies with regulations, of which you may not be aware, so input from a qualified person at the start is very important.
Does your friend have to have a combi ? Maybe a heat only or a system boiler ( with hot water cylinder may be more suited to his / her needs ). Could also be more cost effective ( over all more efficient ) than a combi for his / her requirements for hot water supply.
I agree. A friend of mine runs a gas company and he gets 90% of his work from combi boiler failures. Also with a system boiler he'd get away with more like 15kW (depending on the house details) than 24 (or 28). The high output of a combi is needed to give a decent flow of DHW, rather than CH requirements.
Many will inspect and commission. No probs.
Well most boilers sold are combis that is not surprising. The first port of call is always a combi - a quality one. There are high flow models about.
Why is the first port of call always a combi ?
For a flat or a very small house a combi may be the right choice. But given the low over all efficiency of a combi ( actual use of gas and not the manufacturer's headline claims ) a system with hot water cylinder ( vented or un-vented ) can be the better port of call.
The advantages of combis are apparent, never runs out of hot water, massive space saving, cheaper to fit, etc, so no need to delve into that side in depth. For the average home in the UK a combi will fit the bill in 95% of situations. Many are very efficient indeed and with few moving parts; the Intergas and Ferroli Modena come to mind. No stored water losses for a start. Not cylinder put heat into the house in summer. Atag have a model with a hot water heat recovery section in the flueway, so very high DHW gas efficiencies. The model is 39 or the likes. If the cold water mains can deliver the flow and pressure, combis are the way. Many 15, 16, 17 litres per/min models are around; even above. 15/16 liters per/min can do a mini drencher shower. But like any appliance get a quality one. One which the internals are well laid out and easy to service. If the plate heat exchanger is scaled up after 10 years then just replace it. When scale builds up in a cylinder?? Wow look inside and then try lifting one. It is then replace the cylinder at much expense and inconvenience; that is if it did not corrode in the meantime.
A combi is is just a system boiler with a heat section added with many using the same parts as the system boilers in their ranges. When on direct heating of radiators they are just the same as system boilers. Some combis on DHW are more efficient than any cylinder, like the Atag I mentioned.
Combis have to be fitted properly, not having one 15mm cold mains pipe snaking around the house and the combi teed off it like it was just a sanitary appliance. It needs:
So:
A combi is is just a system boiler with a heat section added with many using the same parts as the system boilers in their ranges. When on direct heating of radiators they are just the same as system boilers. Some combis on DHW are more efficient than any cylinder, like the Atag I mentioned.
Combis have to be fitted properly, not having one 15mm cold mains pipe snaking around the house and the combi teed off it like it was just a sanitary appliance. It needs:
- its own 22mm cold feed from the maintap even if the mains pipe is 15mm
- a full-bore maintap
- split the combi feed and cold outlets at the maintap. Fit a tee for all cold outlets, except the shower. This can be 15mm in most cases.
- tee off the cold combi cold feed for the shower cold supply, just before the combi.
- have a dedicated hot pipe from the combi for only the shower.
- have the hot & cold outlets and taps on flow regulators (they are combined flow regulators and isolation valves) - you do not need firehose flow and pressure on bathroom basin, kitchen taps, toilet, dishwasher, etc.
- one flow regulator can do the dishwasher & washing machine which can get away with 6 litres per/min. In fact one regulator can do all the cold outlets
- the shower hot and cold pipes should have no tees off these pipes. The shower hot & cold needs dedicated pipes.
- no flow regulators on the shower hot and cold.
- Shower is king, all pressure and flow must be prioritized for the shower. Water takes the line of least resistance, make the shower hot and cold the lest resistance.
So:
- A quality combi
- A high DHW water flow rate combi
- Pipe up the house properly as above
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The losses from a combi supplying DHW on demand can be higher than losses from an insulated hot water cylinder heated once or twice oer day..
Turn a hot water tap on and the combi fires up. First the primary heat exchanger has to be heated along with the water flowing through it. Then this water heats the secondary heat exchanger and the water flowing to the taps begins to receive heat. Several ( many ) seconds of burning gas before the water leaving the boiler starts to rise in temperature.
Turn the hot tap off and the boiler shuts down. The heat in the primary heat exchanger, the secondary heat exchanger and the water between them and the pump now disipates into the air around the boiler with a significant amount leaving the building via the flu,
Run a hot tap continuously to fill a bath and the efficiency will probably be close to the claimed efficiency which may be high 90's %
Draw the same volume of hot water in several shorter draws with time between draws for the primary heat exchanger, the secondary heat exchanger and the water between them to cool and efficiency can be as low as 32% ( Building Research Establishment testing on 2010 ) click for report More recent testing and experience in a housing development have given very similar results. ( these are not yet in the public domain ).
Some combi boilers have done away with the secondary heat exchanger and instead have a dual circuit heat exchanger. Of these dual circuit boilers some have a very large thermal mass in the heat exchanger which in terms of wasted heat may be worse than the two heat exchanger system.
Also the wasted water while the heat exchangers are heating up can be 3% of the water used.
Strewth.
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