No. of Tubes (Solar)

DOCARCH

Quick question. Sorry if this has been covered before!

Just looking for advice on the appropriate no. of evacuated tubes for a solar water heating system for a house with 3 occupants - 20 or 30 tubes?

The plumber/solar installer is saying either/or - not really making any suggestions either way?

Any advice or guidance appreciated as to how to decide?

ccsolar

Hi Docarch
We would need to know what size cylinder you are connecting the solar panels to
You also need to consider will the number of occupants increase over the next 10 years.
With 3 people in the house the 200 - 250ltr cylinder would be a good option with 30 tubes depending on the make of tube.
The heat dump would also need to be fitted with this size system and the roof orientation would also need to be considered.
We would need more information to size the system better.

Cc

DOCARCH

Thanks Cc

There will be a 300litre cylinder going in. There are 5 bedrooms in the house (and 5 bathrooms!). But the day to day occupancy will never be more than 3 or 4 max. The back of the house is SE facing (where the tubes will be located)

That's why I'm wondering might 20 tubes be more suitable than 30 - I'm worried about overheating of water and/or avoiding having to put in a heat dump.

homer911

I'm sure Quentin will comment, but with a 300 litre tank, 20 tubes would be far too little - at least 30 and possibly 40.

SE orientation will mean you miss the top heat in the afternoon, so you will want to maximise morning capability.

Its not so much the number of users, as the volume of water you are looking to heat and the temperature you are trying to get it to

Adding more tubes will also extend the system performance into spring and autumn

The heat dump is always a good idea - dont try and cut costs too much!

A site consult is usually best for system sizing

Also, you may not live in the house for ever, so sizing for potential use is a better option

ccsolar

.
Hi Docarch

(Its not so much the number of users, as the volume of water you are looking to heat and the temperature you are trying to get it to)

I have to disagree with Homer on this point , there is no point heating a large amount of water if there is not enough users.
The solar system will end up going into stagnation after a few days.
A good quality cylinder will have minimal heat loss.
In my opinion you could down size the cylinder and use just 30 tubes but if you go with the 300ltr then you will need the 40 tubes.
Best to get a site survey and ask for the heat dump to be included in the price.

Cc

quentingargan

Hi Docarch,

Agree with Homer911 there. As regards both cylinder and number of tubes, yes - you need to consider the needs of the household in terms of hot water use. I usually ask all sorts of personal questions about whether people prefer baths or showers, and how often. (I'm not allowed do house calls anymore...).

Also, you need to decide the length of season you want to cover. In a well insulated house, if the heating isn't running from March to October, you are trying to maximise production in spring and autumn. You just can't do that without having surplus energy in June. In terms of the overall cost of the project, a few extra tubes won't cost all that much, and you should fit a heat dump anyhow. So from May to July, the heat dump will run for slightly more time, but you won't be needing your boiler to kick in for more of the year.

You can use a smaller cylinder by the way, and bring it to a higher temperature. A 200L cylinder at 85 degrees stores the same amount of heat as a 300L cylinder at 60. However, you will have slightly higher heat losses. If you have limescale in the water though, don't try doing this - it will ruin the cylinder coils.

Cheers, Quentin.

championc

The less water you have to heat, the hotter it can get. The water generally heats up all together - so the whole 300l is being heated a degree at a time.

I have a 30 tube system with 180l store and I feel I got it about right. I just checked back at my data for last year for a sunny day in June. I found June 15 where I seem to have had a whole day of blue skies and my store temperature was raised up from 50 to 80 deg. A similar sunny Winter day may yield about 10 deg (with sun being lower and less sunshine hours too). My house faces due south. It is also important to have a well insulated store. Mine loses 1 deg every 3 hours so I can't complain.

So certainly 300l with 30 tubes is a waste of time. It will never yield what you are hoping for. Based on my experience, if you insist on going as large as 300l, you'd need 50 tubes to equal my experience ((30 / 180) * 300) = 50.

Hope this helps

DOCARCH

Thanks for the replies/advice.

Evergreen

DOCARCH wrote: »

Quick question. Sorry if this has been covered before!

Just looking for advice on the appropriate no. of evacuated tubes for a solar water heating system for a house with 3 occupants - 20 or 30 tubes?

The plumber/solar installer is saying either/or - not really making any suggestions either way?

Any advice or guidance appreciated as to how to decide?

You can't size a system based on the number of tubes, you have to look at the output per square meter and then match the output to your needs. The reason I say this is that different manufacturers use different tubes and differetn setups so thererfore the number of tubes required for the same heat varies wildly.

I have seen systems where 60 tubes are required for 300 L and I know that the system I use the manufacturer recommends 80L storage per 6 tubes - in other words 24 tubes. I have even seen systems where installers are only using 18 tubes for 300L.

My advice is to look further under the hood before making a decision, having too many tubes can cause just as much of a problem as having too little.

Hope this helps?

championc

Absolutely Evergreen. I was just about to post the same thing. My tubes have a diameter of 94mm whereas the "Chinese" tubes generally all seem to be 57mm. So my 30 tubes will have possibly the same surface area as a 40 Tube (or maybe more) of the other variety.

There is also likely to be a difference in the efficiency of the collector or heat from the sun too.

For reference, here's the data output from my system on a fully sunny day on 15 June 2010. So I had a temperature rise of about 30 deg C so if you had a 300L store, your gain based on the same collectors on the roof and the same aspect would be less that 20 deg C. So you can see how difficult it can be to heat the water especially given our lack of days with clear blue skies.

In my opinion, it may be an idea to try and get some sort of feeling as to how much water you ACTUALLY use and not just going for big for the sake of it. If you use showers rather than a bath, you could run the shower for one minute, shh how much water comes out and multiply by the average duration of a shower and then multiply by the average number of showers taken per week.