I have an unvented Megaflow cylinder which until recently operated in a conventional manner using a system boiler. However, I have had an extension built which I intend to fit underfloor heating in (water-fed), and as a consequence of this I have now discovered that I have to uprate my boiler. Rather than fitting a new system boiler, and because my extension is south-facing, I wondered whether it would be possible (and worthwhile) heating the Megaflow using solar panels and using the pre-heated water from it at mains pressure to supply a combi?
Solar combi
- Thread starter southy1
- Start date
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At present not many combi boilers can accept pre-heated water, and the pipe work and control arrangements to do this are not as simple as your post seems to imply you think it is.
But with the correct thought and planning, as well as the right boiler it can be done.
But with the correct thought and planning, as well as the right boiler it can be done.
Thanks gas4you.
You're right, it does seem fairly straightforward to me. Why should it make a difference to the combi if water comes in at 10deg or 40deg, providing it's at mains pressure it's only going to heat it to the required output temperature.
However, accepting your advice, if it's not that straightforward can you suggest where I can source the correct 'thought and planning' and the information required regarding the correct boiler?'
You're right, it does seem fairly straightforward to me. Why should it make a difference to the combi if water comes in at 10deg or 40deg, providing it's at mains pressure it's only going to heat it to the required output temperature.
However, accepting your advice, if it's not that straightforward can you suggest where I can source the correct 'thought and planning' and the information required regarding the correct boiler?'
It is possible to do what you are suggesting -preheated water to a combi that is rated to accept hot water. However, don't forget the water in your cylinder could be in the range which legionella breed, especially in winter months when there is little sun. I think some pre-heated combi solutions are having problems with this regard.
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I would start by researching with boilers are capable of accepting pre-heated water, manufacturers web site would be a good place to begin.
Even the modern Vaillants that I fit cannot accept pre-heated water and in a very warm summer can even struggle withe the 'warmer' cold water coming in from the mains
Even the modern Vaillants that I fit cannot accept pre-heated water and in a very warm summer can even struggle withe the 'warmer' cold water coming in from the mains
Best way (depending on your budget) would be to use a thermal store. This way you keep your mains pressure hot water, are able to use solar panels and have additional 'top up' from a boiler.
example: http://www.consolar.co.uk/documents/Consolar Thermal Stores - how it works.pdf
example: http://www.consolar.co.uk/documents/Consolar Thermal Stores - how it works.pdf
Has anyone done a proper heat loss calculation to determine what power boiler you need.
Its very unusual for any house to need a larger boiler when the correct calculations are done. Its very common for builders to say a new boiler is needed though.
Supplying a combi with preheated water is not that simple at all. Even at the maximum flow rate the incoming water cannot be much over about 30°C because the boiler has a limited modulation ratio, typically 3:1.
Most current combis will need to cycle the burners on/off if the flow rate is less than a third of the maximum. With a high flow rate combi this minimum flow rate can be as much as 5 li/min which is more than some kitchen taps give.
Tony
Its very unusual for any house to need a larger boiler when the correct calculations are done. Its very common for builders to say a new boiler is needed though.
Supplying a combi with preheated water is not that simple at all. Even at the maximum flow rate the incoming water cannot be much over about 30°C because the boiler has a limited modulation ratio, typically 3:1.
Most current combis will need to cycle the burners on/off if the flow rate is less than a third of the maximum. With a high flow rate combi this minimum flow rate can be as much as 5 li/min which is more than some kitchen taps give.
Tony
Forget the combi.
A combi is an instantaneous water heater. It has no/little stored DHW and stops the space heating when there is a DHWS demand. That is why you will find 40 kW combis in buildings with a 10 or 15 kW space heating requirement. Combis have their uses, mainly in flats, where there is no space for a storage system.
A solar system is a storage system. The heat incident on the solar collectors at any instant is small and it relies on the heat being continuously accumulated in a store of some sort for later use.
There is no point in combining a non-storage combi system with a storage system.
If you try to store partially heated water without pasteurising it at 60 degC or above, at some time you will encounter Mr. Legionella; he is not a nice chap.
A combi is an instantaneous water heater. It has no/little stored DHW and stops the space heating when there is a DHWS demand. That is why you will find 40 kW combis in buildings with a 10 or 15 kW space heating requirement. Combis have their uses, mainly in flats, where there is no space for a storage system.
A solar system is a storage system. The heat incident on the solar collectors at any instant is small and it relies on the heat being continuously accumulated in a store of some sort for later use.
There is no point in combining a non-storage combi system with a storage system.
If you try to store partially heated water without pasteurising it at 60 degC or above, at some time you will encounter Mr. Legionella; he is not a nice chap.
A solar system that supplies the water input to a combi at 20C will give a useful saving on those models specified to use it.
The stored solar water is hot but cooled in a blending valve immediately before the boiler. It can reduce the gas consumed or it can enhance teh flow rate of the combi boioer.
BUT those are specialised systems and only a very few boiler manufacturers offer them. They are quite expensive and give limited saving.
A good solar storage installation can save about £100-£150 in an average hosehold.
Tony
The stored solar water is hot but cooled in a blending valve immediately before the boiler. It can reduce the gas consumed or it can enhance teh flow rate of the combi boioer.
BUT those are specialised systems and only a very few boiler manufacturers offer them. They are quite expensive and give limited saving.
A good solar storage installation can save about £100-£150 in an average hosehold.
Tony
Thanks for your suggestions but I'm still baffled. I understand that it is probably not the best set up for efficient use of solar energy but I need to replace my boiler and already have a Megaflow and a big roof facing south so it's a case of making the best use of what I have. The legionella point is interesting and something I was unaware of. However, I sent an e-mail off to Heatraesadia about the idea and they've replied saying that the Megaflow cannot be connected to a combi. I still don't see why?? I understand that some (most perhaps) combi's won't accept water above a certain temperature but I read that some do (Alpha). It just seems to make sense to me to pre-heat your water for free (after initial outlay) and store in an unvented tank which then feeds a combi boiler - giving you DHW on demand and I assume a good flow rate.
You could do what you want but the person you asked would not have understood the question!
Either you store the water at the use temperature of 20-30 depending on the system or you store it hot and feed the combi via a blending valve as in the Alpha system.
Tony
Either you store the water at the use temperature of 20-30 depending on the system or you store it hot and feed the combi via a blending valve as in the Alpha system.
Tony
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