Questions regarding hot water storage tank (megaflo)

tallac

I'll be replacing the heating system for a new property. I'm currently thinking of getting a Vaillant system boiler with a megaflo storage tank.

Assumptions:

1. Let's assume I don't use more than a single tank per day.
2. Hot water is heated to around 65 degrees C

I have a few questions I wanted to ask to help me understand:

1. If I used a timed system to heat hot water between 12.30am and 4.30am (cheapest rate of electricity), will the hot water stay hot enough for showers and taps all the way into late evening?
2. If I wanted a hot water underfloor heating system, will it get hot water directly from the system boiler or will it use hot water from the megaflo tank?

Thanks in advance

Talldave

I don't understand - if you have a Vaillant boiler, why would you use electricity to heat water?

Underfloor heating is fed from a manifold that is connected to the boiler. They're just large horizontal radiators that run at a lower temperature than vertical radiators! 

matelodave

Most people with a boiler use it to heat the hot water tank as it only costs around 2.5p/kwh instead of 10-15p/kwh for leccy. so even cheaper leccy is much more expensive than gas.even at the cheap rate.

We only heat our tank to 45 degrees to about 1.5hours a day (just before we get up and have our morning ablutions)and it gives us sufficient hot water for a couple of showers and all the hot water we need until the next day (in fact at a pinch we can manage another two showers the next morning although the second one on the second day is only just about warm enough.We do have an efficient low volume shower (about 6 litres a minute) and we spend less that 3-4 minutes each in there. and most dishes are washed in the dishwasher. We dont waste hot water by running it down the sink when washing or rinsing stuff (including our hands) - bear in mind that every time you run the hot tap untill it runs hot, you can waste up to a gallon of cold water down the sink and leave a gallon of hot water in the pipework to get cold until the next time.

Most hot water underfloor systems get their hot water from the system boiler via distribution manifolds, frequently with a mixing valve to reduce the water temperature through the underfloor system. Are you intending to have some radiators in the system as well (eg. u/f downstairs and rads upstairs) Generally u/f systems only run at 40-50 degrees and radiators run at 60-70 degrees 

You really need to get proper advice and do some research to decide what you are trying to achieve and make sure that you have a coherent and efficient design rather than just rushing at it without knowing how it all goes together - that would be a recipe for disaster (and big bills)

MWT

matelodave said:

We only heat our tank to 45 degrees to about 1.5hours a day...

I may be out of date, but isn't the safety recommendation 60C for stored hot water to ensure Legionella is killed off?

tallac

Talldave said:

I don't understand - if you have a Vaillant boiler, why would you use electricity to heat water?

Underfloor heating is fed from a manifold that is connected to the boiler. They're just large horizontal radiators that run at a lower temperature than vertical radiators! 

Doh sorry... with all the things buzzing around my head, I forgot the hot water would be normally heated from gas. I also have off peak electricity so there's a bunch of stuff I'm optimising for that period of time.

With regards to hot water, I would like to get solar PV panels that can heat hot water when there's excess generation rather than exporting it to the grid. If there was sufficient excess solar energy to heat a tank of hot water, I was wondering if the heat from it could be useful until the next day when the solar energy kicks in again. Essentially using very little gas to heat the hot water.

matelodave

MWT said:

matelodave said:

We only heat our tank to 45 degrees to about 1.5hours a day...

I may be out of date, but isn't the safety recommendation 60C for stored hot water to ensure Legionella is killed off?

It gets a boost to 60 degrees once a week for that very purpose - you don't have to do it everyday (TBH once a week is probably more than it really needs).

matelodave

tallac said:

Talldave said:

I don't understand - if you have a Vaillant boiler, why would you use electricity to heat water?

Underfloor heating is fed from a manifold that is connected to the boiler. They're just large horizontal radiators that run at a lower temperature than vertical radiators! 

Doh sorry... with all the things buzzing around my head, I forgot the hot water would be normally heated from gas. I also have off peak electricity so there's a bunch of stuff I'm optimising for that period of time.

With regards to hot water, I would like to get solar PV panels that can heat hot water when there's excess generation rather than exporting it to the grid. If there was sufficient excess solar energy to heat a tank of hot water, I was wondering if the heat from it could be useful until the next day when the solar energy kicks in again. Essentially using very little gas to heat the hot water.

Yes you can do that as well but you really do need to do all your sums very carefully to work out the costs of all these "energy saving" devices and decide whether you are actually saving money or just energy (the two are not always compatible). You can spend a lot of money, but not save sufficient energy to to offset the cost or it might take, 10 years or more to break even. The other aspect to take into account are the laws of diminishing returns. You can spend a lot to save a lot, but once that saving has been made you won't save as much next time.

For example, I've got a heat pump and now the cost of heating my hot water for a whole year is around £100, so lashing out £1500 on solar to try to reduce my hot water costs even more would take 15-20 year to pay back, so isn't actually cost effective (especially for me as I probably wont actually live for another 20 years  - I'll be 93)