Hot water cylinder: 120 litre heated with gas only vs 300 litre heated with gas + solar thermal

[vagrantly3893]

We are renewing our heating and hot water system and have the option of either a 120l hot water cylinder that would be heated only by gas, or a 300l hot water cylinder that would be heated by gas and solar thermal.

 

The house has 1 bathroom and 2 adults. My wife has 1 or 2 baths per week and showers daily. I shower every other day.

 

Considering the tank required for solar thermal has an extra 180l of water to heat that we probably don't even need, should we consider solar thermal as a solution? I don't know how to calculate it, but I think even with the solar thermal system, we would still consume more gas per year to heat 300l of water than to heat 120l of water.

Edited March 5, 2023 by vagrantly3893

[JohnMo]

Where are you in the stage of renewing, what are you replacing?  Are you replacing boiler radiators? Or just cylinder?

[vagrantly3893]

2 minutes ago, JohnMo said:

Where are you in the stage of renewing, what are you replacing?  Are you replacing boiler radiators? Or just cylinder?

 

At the beginning stage, having spoken with a company and received 2 offers.

 

Currently the property has 1 boiler for heating and 1 for hot water. We are replacing this with a new boiler and a cylinder. Radiators will have the valves replaced and the system will be balanced.

[Miek]

Usually the larger Solar thermal cylinder would be twin coil with the solar coil at the bottom to heat the whole tank and the gas coil half way up to heat the top 150l only. 

 

Used carefully Solar thermal can provide about 60-70% of your yearly DHW. That's what I found anyway. 

 

The huge drop in the cost of Solar PV in the last decade has largely seen Solar thermal relegated to history  as electricity is so much more useful than hot water. 

[vagrantly3893]

1 minute ago, Miek said:

Usually the larger Solar thermal cylinder would be twin coil with the solar coil at the bottom to heat the whole tank and the gas coil half way up to heat the top 150l only. 

 

Used carefully Solar thermal can provide about 60-70% of your yearly DHW. That's what I found anyway. 

 

The huge drop in the cost of Solar PV in the last decade has largely seen Solar thermal relegated to history  as electricity is so much more useful than hot water. 

 

Is this the system where you insert an extra electric immersion heater in to the hot water cylinder and detect when your PV is sending electricity back to the grid, and then switch on the immersion heater to use the excess electricity to heat the water?

[JohnMo]

So if you have options go with X plan for the control scheme (do web search). This allows you run two different temperatures from the boiler, one for central heating, so you can go with weather compensation, running the boiler at much lower temps and get plenty of condensing.  The other temp is just for heating the cylinder.  Also specify a heat pump coil, circa 3m2.  This will give good reheat times, future proof also.

 

As you say if there is no solar available you end up heating a 300L instead of a small cylinder.

 

This what I did.  The cylinder for DHW is dedicated solar. The boiler is a combi, if the solar water is above 45 it flow from the cylinder direct to tap.  Below 45 it is heated by the the combi boiler.

 

 

 

 

[vagrantly3893]

3 minutes ago, JohnMo said:

As you say if there is no solar available you end up heating a 300L instead of a small cylinder.

 

Once the 300L is heated and we are using around 120L per day, we just need to reheat the 120L, and keep the 180L from dropping in temperature, which I assume doesn't happen very quickly because of the tank insulation.

If we had 120L, we'd also be reheating 120L a day, but without keeping the extra 180L hot. I'm not sure how quickly the water temperature drops, but I think perhaps there isn't much difference between the 2 options.

[billt]

A 120l cyliinder will be perfectly adequate for your use, if correctly designed and operated. A 300l cylinder will just waste energy. However good the cylinders insulation you will inevitably have large losses from the pipework. Keep it as simple as possible.

 

Solar thermal is expensive and not worth doing these days.

 

My system has a 120l cylinder with a fast recovery coil heated by a 19kW boiler with hot water priority. The boiler runs at 70-75C when heating water and will heat the cylinder from cold in 20 minutes and much less time for reheating after some hot water has been used. Weather compensation is used for central heating water.

 

There's an immersion heater at the bottom of the cylinder which can be fed from excess PV production.

[vagrantly3893]

2 minutes ago, billt said:

There's an immersion heater at the bottom of the cylinder which can be fed from excess PV production.

This is what I am most interested in actually because the south facing portion of our roof is 100sqm. I need to ask the installing for a cylinder that has/accepts an immersion heater.

[JohnMo]

Typically the bigger the cylinder the more heat loss per day, the variables are energy rating and size

[Dillsue]

1 hour ago, vagrantly3893 said:

This is what I am most interested in actually because the south facing portion of our roof is 100sqm. I need to ask the installing for a cylinder that has/accepts an immersion heater.

You can have as many immersions and heating coils as you can physically fit in the cylinder. You need to work out the sources of heat you want and how much hot water you need before you tell your installer which cylinder to go for. 

I beleive you can get 2 channel PV diverters that will power 1 immersion until that cuts out and then switch to second immersion until that cuts out. If you want to maximise PV self use for water heating then get a cylinder with an immersion near the top and a second immersion at the bottom. Have a fast recovery/high efficiency coil fitted somewhere in the middle that will be compatible with a heat pump but can be used with your gas boiler. Speak to a cylinder manufacturer and they will guide you on sizes/capacities/positions.  If youve got the space get an extra inch or two of insulation to help keep the heat where you want it. There's a copper cylinder manufacturer in Newark that I found very helpful when we had a custom cylinder built

[Nickfromwales]

1 hour ago, vagrantly3893 said:

This is what I am most interested in actually because the south facing portion of our roof is 100sqm. I need to ask the installing for a cylinder that has/accepts an immersion heater.

Are you considering solar PV instead of solar thermal?

I've been in the heating and hot water industry for over 3 decades, and if I was given a solar thermal system for free, and could fit it myself, I'd sell it to someone and use the money to buy solar PV.

Your hot water use is low, but you're electricity costs are going north, so I would, personally, go with PV here and save yourself a lot of money long-term. 

Excess solar PV in summer will give you free hot water via the immersion, so your gas consumption in winter will be off-set by how "energy rich" the solar PV makes you in the 'sunny times' of the year.

If you go with PV, I would go with the bigger cylinder, but if you're sticking with gas then 180L should be the biggest you need.

[vagrantly3893]

5 minutes ago, Nickfromwales said:

Are you considering solar PV instead of solar thermal?

I've been in the heating and hot water industry for over 3 decades, and if I was given a solar thermal system for free, and could fit it myself, I'd sell it to someone and use the money to buy solar PV.

Your hot water use is low, but you're electricity costs are going north, so I would, personally, go with PV here and save yourself a lot of money long-term. 

Excess solar PV in summer will give you free hot water via the immersion, so your gas consumption in winter will be off-set by how "energy rich" the solar PV makes you in the 'sunny times' of the year.

If you go with PV, I would go with the bigger cylinder, but if you're sticking with gas then 180L should be the biggest you need.

Yes, PV would be my preference.

 

What are the arguments for the bigger cylinder if using PV? To get the most usage from the PV? I'm not really interested in using extra hot water just for the sake of it.

Edited March 5, 2023 by vagrantly3893

[Nickfromwales]

Just now, vagrantly3893 said:

Yes, PV would be my preference.

 

What are the arguments for the bigger cylinder if using PV? To get the most usage from the PV?

Storing more on a sunny day, where you may not then have such a sunny day the next. These are assumptions, as I don't know how big a PV array you'll adopt, but with the bigger tank, you then have the gs boiler only heating it to 55^oC which makes a big difference in the amount of waste / latent heat loss per hour. 

[vagrantly3893]

4 minutes ago, Nickfromwales said:

Storing more on a sunny day, where you may not then have such a sunny day the next. These are assumptions, as I don't know how big a PV array you'll adopt, but with the bigger tank, you then have the gs boiler only heating it to 55^oC which makes a big difference in the amount of waste / latent heat loss per hour. 

 

Me neither regarding PV size. I'm more focused on future proofing our system. Do you have some information on the amount of water heater by the immersion Vs gas? E.g 80% gas, 20% electricity. I assume this depends on the kW of the immersion and the gas?

[Nickfromwales]

17 minutes ago, vagrantly3893 said:

I'm more focused on future proofing our system.

Could you elaborate?

 

A pv design will work out your net generation vs known self-consumption ( base loads ) from your bills. The excess ( what % is deemed as export ) will go towards hot water via immersion.

Most of my clients have completely free hot water for at least 6 months of the year, so the boiler 'hibernates' for summer.

[vagrantly3893]

6 minutes ago, Nickfromwales said:

Could you elaborate?

 

A pv design will work out your net generation vs known self-consumption ( base loads ) from your bills. The excess ( what % is deemed as export ) will go towards hot water via immersion.

Most of my clients have completely free hot water for at least 6 months of the year, so the boiler 'hibernates' for summer.

We haven't moved in to the property so we don't know what our consumption will be. But I think you're meaning is, if we have a bigger tank, we'll need a slightly bigger PV design than if we had a smaller tank. My goal now is to ensure the cylinder we buy is compatible with a PV system. I.e. it has an electric immersion heater.

[Nickfromwales]

1 minute ago, vagrantly3893 said:

But I think you're meaning is, if we have a bigger tank, we'll need a slightly bigger PV design than if we had a smaller tank.

Not quite. What I was saying is that if you have lots of excess one day and not the next, you'll will have stored as much as you possibly can each day and have some 'spare' capacity to see you through. Quite coarse 'maths' here as I'm flying blind  .

 

4 minutes ago, vagrantly3893 said:

My goal now is to ensure the cylinder we buy is compatible with a PV system. I.e. it has an electric immersion heater.

All cylinders will come with an immersion, but you may benefit from adding a second one, by choice, so the PV can be fed in as low down in the cylinder as possible, eg to heat the whole tank with any excess. With a large tank, the PV will be topping you up vs hating you from cold, and heating from cold ( recovery ) is a big ask, whereas topping you up is a doddle.

 

[Nickfromwales]

If you want to buy a cylinder, contact Trevor from cylinders2go and mention my username and the forum for a discount . He can price you for an additional immersion too. Many on here have bought from him ( Telford cylinders ) and the stainless steel ones have a lifetime warranty. Their all I have fitted for over 20 years, with zero issues, ever.    

[vagrantly3893]

Thanks for the info. I actually live in Germany now however many manufacturers, system setup and products seem to be the same as the UK.

[Nickfromwales]

6 minutes ago, vagrantly3893 said:

Thanks for the info. I actually live in Germany now however many manufacturers, system setup and products seem to be the same as the UK.

Ok, yes, most much of a muchness tbh. Just look at stainless steel for longevity. You may get a good price for Solarwatt PV panels then lol.

[Miek]

20 hours ago, Nickfromwales said:

 if I was given a solar thermal system for free, and could fit it myself, I'd sell it to someone and use the money to buy solar PV.

 

This is exactly the conclusion I came to. My solar thermal kit is still in the shed and the space on the roof I had earmarked it for is now filled with PV. Even though I love tinkering with solar thermal....

[Nickfromwales]

14 hours ago, Miek said:

This is exactly the conclusion I came to. My solar thermal kit is still in the shed and the space on the roof I had earmarked it for is now filled with PV. Even though I love tinkering with solar thermal....

It gives more energy per m2 but is just a ball-ache and a one-trick pony. In summer it just gives you stupid amounts of excess heat energy which you then either cannot use, or you have to run a pump to dump it to a heat loss radiator.

Still cannot get over how many new builds have a mix of PV and ST on the roof!!!