Heating engineers? Going electric

Solarchaser

Hi, looking for a bit of advice regarding central heating.

I currently have a combi on its last legs, and with my gas price tripling and electricity on my off peak period working out cheaper than gas, its finally time to go full electric and rip out the gas.

It's not just the price changes, I've been wanting to do this for a while, I'm aware gas could go down and electric up, but I'm still going full electric.

So to replace the combi I'm going with two tanks.
1 in the loft (3rd story)which will be a 210L thermal store, this will provide hot water (and a small top up for the heating via a heat exchanger.)
https://www.advanceappliances.co.uk/product/47-electric-combination-boiler-ecb-210/

This will let me replace my 2 electric showers with thermostatic ones as the electrics consume more power than my home batteries can cover, solar power will heat this tank.

2. A 500l hot water tank / buffer tank which will go in the crawl space under the floor, heated by two electric elements inside the tank, this will be the main central heating provider.
The hot water will sit in this until called for by the house.

I've done the calculations that say heating these tanks with solar and off peak electric will cover my heating needs (just about) in the dead of winter.

What I'd like experience of is how best to ensure smooth flow of the water around the system, and how best to get the heat out of the cylinder and round the central heating loop.

I was thinking of two pumps, one to pump the water around the main circuit, through a heat exchanger and one to connect to the heat exchanger to pump the hot water in and out of the 500l tank.

Another option would be to have only one pump, do away with the heat exchanger and just connect the tank into the heating system with a couple of one way valves to hopefully stop thermosyphon heat loss, though I'd think this has a larger capacity for the radiators in the house to quickly become very very hot.

I'm aware I need expansion vessels, is there a size guide based on tank size?

Please point out what I've missed

Also I was considering using a blending valve to go between the hit and cold main circuit so as to stop my kids being burned by 80c radiators, is this a daft idea?

ComicGeek

For starters, direct electric is a terrible idea. If you're going for all electric at least integrate an air source heat pump to produce the heat more efficiently. That would at least insulate you from future energy price changes. IMO those electric boilers have no place today, just the worst possible solution.

For how long do you think that your off peak elec tariff is going to be cheaper than gas? Are you on an Octopus Go tariff or similar? What happens in a couple of years when all these low off peak tariffs disappear? You should really run the calcs based on other other peak tariffs to see if it's viable - I would think not, based on those.

There is a danger of doing a massive alteration to your system only to find that the energy tariffs change again in a couple of years, and you've picked the most expensive option.

Buffer vessels are difficult to design, especially when you don't have a responsive boiler that can quickly kick in and raise the temp. Very rarely do I see ones that are designed and installed properly. 

Solarchaser

Hi, thanks for your response, but that really wasn't what I asked. 

You think direct heating is a terrible idea, cool, you may well be right, I'll see in a year or two.
Heat pumps not an option for me, and really don't solve the issue for the same reason solar doesn't solve the issue.

I asked about the flow from the buffer tank to the system, and anything related to that I've missed, whether I'd be as well to run a blending valve from the radiator return across to the radiator feed to stop the radiators getting 80c water. 

We are all going to have to ditch gas in the coming years, I'm just moving forward a bit earlier like I did with solar, an EV and with batteries,  definitely not the first, but before the hoarde.

Tou tarrifs are here to stay, more and more wind will be rolled out meaning more intermittency to production, so it will have to be overbuilt, meaning there will be more variability to supply, and more tou tarrifs not less.

In time we will have various types of storage to take up the excess and in theory mass v2g, but thats years and years away.

Gas production world wide is going to decrease, but middle England runs mostly on gas to generate electricity,  so uk will still require alot of gas, and so the gas price is going to stay high.

Is it a risk, sure, but it's a calculated risk and at absolute worst case scenario I can uncap the gas supply and refit a gas boiler, which could run as a system boiler with the buffer tank, so no big deal either way, but there is no point in fitting a new combi now when I have the option to go full electric.

ComicGeek

No, but that would be the answer to any of my clients who asked the same. It's a terrible idea, trying to heat lots of water with direct immersion heaters. You can buy air source DHW tanks that use integral heat pumps ducting to atmosphere to produce domestic hot water much more efficiently than direct electric - if you're using solar PV to run this (or even a battery) you can still do this much more efficiently and at the same price as an overpriced electric combi boiler.

Heating a thermal store with direct electric is completely the wrong approach IMO, and is going to cost a fortune to run. Trying to reuse the same rads means that you need to replicate the existing system temperatures - that means higher  standing losses, higher distribution losses etc.

I'm not going to help directly as I still think that it's a terrible idea. But you should look up thermal stores not buffer vessels, and you should look at manufacturers installation guides to see the typical schematics for pipework etc. Buffer vessels are used to store hot water to prevent large boilers having to turn on/off constantly - thermal stores are used to take heat from various heat sources (sometimes at different temperatures). They have multiple coils for heat input, so you could always fit a gas boiler or heat pump to one in the future if you spec the right one.

ComicGeek

Solarchaser said:

Heat pumps not an option for me, and really don't solve the issue for the same reason solar doesn't solve the issue.

Why aren't heat pumps an option for you?

If you want to disconnect the heating demand time from the energy purchase time to make the most of tou tariffs, then you can still use a heat pump with the thermal store. Surely better to be 300-400% efficient, and still reduce overall electrical consumption. Rather than using electric immersions to heat 5 bath fulls every day.

Money_Grabber13579

I’m curious as to how this works in practice. So, if I’m understanding you correctly, you would heat the two tanks using either solar power or off peak electricity. I get that bit and sounds like it might be quite cost effective. But you then want to pump that hot water around the house, presumably at times when electricity rates are higher. But how much heat will two tankfuls of water provide? Will you not be in a situation where there is an enough hot water for half an hour of heating and then you have to use peak rate electricity to reheat the tank?

ComicGeek

The electric 'combi boiler' the OP has referenced is a thermal store which is heated by 6kW and 3 kW immersion heaters. The fully heated tank provides between 6-10 kW of heat for the initial heating period, with the replacement heat provided by the immersion heaters. Domestic hot water for showers, taps etc is provided instantaneously by an internal coil, but again is dependent upon reheating the thermal store once the heat is depleted, with the immersions operating throughout the heating periods. 

In order to minimise the ongoing operation of these immersion heaters, the OP wants to use an additional and larger thermal store of 500 litres. This also would be heated up during off peak, and would then feed into the heating system to provide a larger overall heat store. The electricity to run a circulation pump is very small.

How long the stores last before needing recharging depends on the storage temperature, design flow and return temperatures, and current heating demand. The risk is that the heating flow temperature continues to drop as the thermal store is depleted and you can't get enough heat output from the rads without bringing on the immersions during peak time. So a low design temperature with a blended supply is best, but may not work with the OP's current radiators.

It would take a lot of careful modelling to confirm if it could work - some manufacturers have good software that can look at this in detail. Failing to get it right means either large electric bills to maintain the store, a large retrofit cost, or no heat.

Solarchaser

Short answer about the heat pump, wife says no F'ing way.
Part of a harmonious marriage is knowing when you can push, and knowing when to stop, so heat pump is not an option.

The hot water tank / buffer tank for the colder months usage will be a very cheap way of doing things, somewhere between £1200-1500 including pumps, pipework and a rather thick layered box of insulation to go round it all to lessen the standing heat loss for the tank.

The thermal store will be really be just for the hot water, I'm only adding the heat exchanger as a backup to give extra headroom in the immersion heat department incase I need it, and I have a theory it might be adequate in the odd summer day when it's cold at night just to take the chill off the house.
It's gonna be around the £2-2.2k mark.
Though tbh I'm not fully sold on that, I may just go for the hot water thermal store version which is £600 cheaper 
https://www.advanceappliances.co.uk/product/153-electric-thermal-store-hot-water-only/ and go with a slightly larger hot water tank for heating,  not 100% decided on that.
I see merits for both

Also as the buffer tank will be right under the living room floor, the heat losses won't be all losses.
The tank I've been looking at is 3kwh of loss per day, with solar its "free".
This will not be heating 3 baths every day, but only in the dead of winter.
The tank is that size based on how much gas I use daily in the dead of winter, so *should* cover it, but I still expect to use some on peak electric in the coldest months to top up.
But as my gas standing charge is £80, id need to overspend rather a bit on those two months to make it more costly.

The heating loss of 3kwh on current tou is 13p, at current prices when tou increases it will be 21p. (Unless it increases of course)
My gas standing charge is more than that per day.

I wont be using an electric boiler, only using immersion heaters in the tanks themselves, I agree electric boilers are pretty expensive for what they are, their only good point would be opening up the selection of buffer tanks as I'd not be limited to a direct tank, and I suppose there would be a pump already there.

The tank size has been calculated based on being able to heat from 45c to 85c within my cheaper tou period and the reverse also, running from 85c to 45c as the lowest viable temperature to keep providing heat in the normal radiators.

That may not be enough, it may need to be 50c or even 55c, but if I don't try, I won't find out.

Attempted to test the current combi on 45c, but it seemed to bring its impending doom closer going by its noises 😀 

Where the buffer tank is going, there is nothing there at the moment, its essentially a crawl space, and I will have good access to it.
So if I stick with a 500l tank, and I find out this winter its not enough, then ill take the hit for using electric at the peak rate, as keeping the family warm comes first.

And after winter I'll maybe add another tank, which would be relatively cheap, as it would only be another 100-300l tank (guessing) so the retrofit wouldn't be so costly I don't think.... famous last words

Bendy_House

Hi SolarC.

A place we looked at a few years back had just been fully renovated, and - due to its rural location - had only LPG and oil as the usual heating methods. It also had a limited outdoor space, tho', mainly gravelled seating areas, so locating tanks would likely have been a problem/undesirable too.

Their solution was to have a large PV array - I can't remember its size, but it must have been at least twice what we have here, so I guess at least 6kW or more. This supplied a battery (I was surprised it was only one), and an electric combi. I don't recall there being a thermal store, which I thought would have been an obvious addition, but there may possibly have been one. The other point of note was that their leccy supplier was 'Smart Energy', and the arrangement included them being able to use any excess stored battery power during peak times, and they would recharge the battery at low rates such as overnight.

I never did get to the bottom of how much this all cost to run, and suffice to say that this uncertainty made mil nervous (it was going to be their house) and someone else stepped in and bought it.

Bendy_House

What's the difference between a 'Thermal Store' and a 'How Water Buffer', SolarC?

To keep things simple - ie being used in its intended and established way - surely the answer is 'thermals store' all the way? There's no mystery as to how they are used for CH and DHW - a heat-exchanger for the latter. And that's pretty much it?

Is your problem 'access'? Ie, you can only have one 210L TS in the loft, and none under the floor? If so, is there no way around this? Can't you reinforce the loft area to take two TS's, possibly each being larger? Or is there no room downstairs for this? Not even in a neat, well-insulated 'box' outside?

If you can get your idea down to TS's, then I'd have thought everything else would just fall into place.

How much PV capacity do you have?

Bendy_House

If you haven't already done so, I think it would also be worth joining and posting on here: https://www.electriciansforums.net/

I had great PV advice, and ended up going with a company on there - no regrets at all.