Indirect Thermal Store pre heating to Combi?

[Elliotpea]

Hi,

I'm having Solar PV installed in the next few months, at the same time we need a new boiler.

I'm looking at the DHW flow rate is important as i like a powerful shower as possible, so i'm thinking of the Vaillant EcoTec Plus 938. The property has 2 bathrooms, water pressure is not a problem (extremely high).

With the Solar PV i'm thinking of using the excess power generated to heat the water in a thermal store using an immersion. Then using the thermal store to pre-heat the water before it goes into the combi, with the aim of increasing hot water flow and reducing gas use. There will be a blending valve in front of the combi to stop the water going in too hot. I have some questions:

1) Is this a good idea (is it normal??)

2) What type of thermal store do i need? Indirect Vented with a heat exchanger?

3) Any makes and model recommendations of thermal store?

Thanks,

Elliot

 

[Dan Robinson]

I'd be more worried about the choice of boiler.

 

[Elliotpea]

Do elaborate, i'm open to suggestions

 

[Dan Robinson]

Not all Combi's can take preheated water. The 938 is also totally pointless.

Look at the Intergas, they can take preheated water directly.

 

[Elliotpea]

I'll take a look at the Intergas, meanwhile, why do you say the 938 is 'totally pointless'? No wonder you have over 41,000 posts if you only ever do half replies!

If anyone could advise on the thermal store side?

Thanks,

Elliot

 

[Dan Robinson]

Because it's store is not that large, and is very quickly depleted turning the boiler into a normal combi that is going to be massively over sized for the property.

As for thermal stores - it all depends on much information you have not provided.

You might see my replies as half answers, but they are geared to encourage people to look for the rest. Especially if I;m posting them from a mobile whilst doing something else. Very rare is that a question of this nature has not been asked before.

 

[Phill Peck]

If you've got the space for a thermal store and a boiler. Why on earth are you considering a combi?

System boiler, unvented cyl and "immersun" type device for your solar pv.
Immersun will hold off boiler while PoV array powers immersion. When not enough power boiler will top up temperature

 

[Agile]

Your first need is to calculate the heat loss of your property.

Add 2 kW for hot water heating and you have your required boiler size. probably about 18 kW but you say nothing about your house type, size or insulation levels.

Then an unvented cylinder, ideally with two immersion elements with one half way up and using the lower one to be heated by the solar PV output. I expect a 300 li will be a good size if you are not more than 6 people.

Boiler also heats the cylinder as required. Best if it also has a heating coil half way up so the boiler can only heat the top half.

Tony

 

[BigSnoopy01]

i agree, go with unvented hot water.

 

[Elliotpea]

Thanks for your replies.

I had a good search on the forum for this question, there wasn't anything applicable, and a lot of the results were 8 years old or more. I've also done a hell of a lot of reading on the subject where i can find information applicable. I do of course value yours and anyones time in replying.

With regards to the 938 store - i didn't envisage using that part of it, i just wanted the highest flow rate possible from a combi, as well as reliability. I thought the Vaillant was good for this?

The reason i'm trying for a combi is efficiency and continuous hot water. I don't like the idea of keeping hot water to temperature when it's not being used, i'm sure there must be a lot of waste energy in that? There is 4 of us in the house, kids and wife has a bath at night mostly, the Mrs' hates it when we run out of hot water after she tops the kids bath up.

If using excess solar energy to a thermal store, then preheating before going into the combi to maximise the flow rate and minimise gas usage, i'd have the best of both worlds.

The house is a 5 bed, will be UFCH throughout once the renovation is finished. In total would be 15 UFCH circuits, totalling 190m2. House is fully insulated, cavity wall and the loft has been doubled up. Tripple glazed.

Thanks again,

Elliot

 

[Phill Peck]

Tony you really are mad. 18kw boiler and 300l cyl based on what?

Op system boiler and unvented. Is the way.
I still don't say how many people are in the house. The 938 won't maintain that flow rate for long.
Small unvented and hot water priority will be a much better system

 

[flameport]

The reason i'm trying for a combi is efficiency and continuous hot water. I don't like the idea of keeping hot water to temperature when it's not being used, i'm sure there must be a lot of waste energy in that?

Combis being more efficient is a myth - most losses of heat are in the pipework, which will be the same regardless of how the water is heated or stored.

Losses from a modern high quality hot water cylinder are next to nothing - here are some: http://www.osohotwater.co.uk/domestic-products/delta-coil.html
typically around 1kWh per day, or in terms of gas cost, about 4p per day or £15 per year. In any case, a thermal store will also have losses.

Also note the reheat times - minutes rather than hours so the old 'put the hot water on for a few hours before using it' is long gone. As the cylinder can and should be heating while in use, not having any hot water left won't be happening.

I'm looking at the DHW flow rate is important as i like a powerful shower as possible

If you want that, then forget about any combination boiler.

 

[Elliotpea]

Thanks for the feedback again, after reading this and looking at the heatloss figures on the cylinders, it looks like this will be a better way to go.

Can anyone recommend an A rated horizontal cylinder to go in the loft (boiler will be on the gable) and a suited boiler for the house. As mentioned above, the house is a 5 bed, will be UFCH throughout once the renovation is finished. In total would be 15 UFCH circuits, totalling 190m2. House is fully insulated, cavity wall and the loft has been doubled up. Triple glazed.

Thanks,

Elliot

 

[Dan Robinson]

Joule cylinders. Intergas boiler - quite possibly SB18 but hard to tell from that information.

 

[Elliotpea]

Thanks, i've read up on the Intergas and it looks like it could be the right choice.

With regards to the cylinder, i've checked Joule out, and had a look around - it looks like i'll only get an Erp C rating on Horizontal, i can't seem to find anything better rated. However, i also read that the difference between a C and an A is about 1.5kwh heat loss per day, which the solar will soak up anyway.

Unless there are any other suggestions, so far this looks good, just need to work out the exact boiler.