Use electric hot water tank in series with gas boiler

[mafulynch]

Hi, we are building a new house and we are going to be using central heating and radiators with a gas boiler. We live in a remote rural place and we have to generate our own light. We have set up some solar power and generate more than what our batteries can charge and we can use during the day so generation is reduced. This is due to the difference in generation we have in summer compared to winter. So we were considering adding an electric water heater tank in series before the gas boiler, that would work only when excess solar power is available and preheat the water returning from the radiators so we could save some gas. We currently have about 6kW excess solar generation during a few hours of the day when it is not winter.

Do you think this is possible? we have never had a central heating system like this so I am not sure if this would be a problem or not.

Thanks in advanced

 

[Ric2013]

I suspect this is getting into the realms of alternative technology. The beauty of PV is you can just switch it on and off so overheating not a concern. So a small water tank would work rather than a large one. It would definitely be more conventional to use an accumulator, but perhaps there is cost or space vs benefit issue. If you definitely want to supplement CH and not the hot water side of a combi (another option) and have actually measured a lot of sustained excess power (and aren't relying on Pmax or Pnom output which may be hypothetical), I can't see what you suggest being impossible. A few small electric heaters might be an acceptable alternative, but I'll not blame you for rejecting them.

The only thing that bugs me if that if you put a small tank of some sort in with a couple of immersion heaters (or an electric boiler), in the boiler return you may find the gas boiler will view this as a reduced heating load and may not respond the way you want it to - you may find it then works unhappily or inefficiently, or it may just switch off altogether, which may not be so big a problem but would seem to defeat the object. How it woul respond would depend on the boiler and this may be a question to ask the boiler manufacturer.

On the other hand, if you're happy to install it with some kind of switching such that you EITHER heat with gas OR electricity, running the boiler flow or return through an electric heater is no worse than running it through a radiator as far as I can see.

Hold fire though, as others on this forum have more experience of incorporating renewables than I have.

 

[ShaunCorbs]

Use it to heat your hot water via twin immersion in summer you wouldn’t need to run the boiler

 

[mafulynch]

I suspect this is getting into the realms of alternative technology. The beauty of PV is you can just switch it on and off so overheating not a concern. So a small water tank would work rather than a large one. It would definitely be more conventional to use an accumulator, but perhaps there is cost or space vs benefit issue. If you definitely want to supplement CH and not the hot water side of a combi (another option) and have actually measured a lot of sustained excess power (and aren't relying on Pmax or Pnom output which may be hypothetical), I can't see what you suggest being impossible. A few small electric heaters might be an acceptable alternative, but I'll not blame you for rejecting them.

The only thing that bugs me if that if you put a small tank of some sort in with a couple of immersion heaters (or an electric boiler), in the boiler return you may find the gas boiler will view this as a reduced heating load and may not respond the way you want it to - you may find it then works unhappily or inefficiently, or it may just switch off altogether, which may not be so big a problem but would seem to defeat the object. How it woul respond would depend on the boiler and this may be a question to ask the boiler manufacturer.

On the other hand, if you're happy to install it with some kind of switching such that you EITHER heat with gas OR electricity, running the boiler flow or return through an electric heater is no worse than running it through a radiator as far as I can see.

Hold fire though, as others on this forum have more experience of incorporating renewables than I have.

Thanks for your help
Could you please explain abit more this,
"The only thing that bugs me if that if you put a small tank of some sort in with a couple of immersion heaters (or an electric boiler), in the boiler return you may find the gas boiler will view this as a reduced heating load and may not respond the way you want it to"

it is my understanding that the boiler is "renewable compatibel" and should sense the input and heat it accordingly. But was worried about having a small tank with inmersion heater affecting the flow, but didn't consider it would be like just a radiator more or even less resistance even.

The reason I wanted this and not just an electric heater is having it all in one place and not having to manualy manage it. My sistem works by limiting solar generation by slightly changing frecquency, so I am developing a device that will sense this change and apply the ammount of excess energy generated to the heating element. So this way I would always use 100% if the excess to heat since the load would be reduced if I am not under full generation.

And answering to the second comment, the reason I am going for helping the heating and not the hot water is that we actually use little hot water and our summers are kind of cold. A normal temperature would be 14°C or 57 F but with very long days, so many hours of productive generation.

 

[Murdoch]

op are you over complicating things? Why not just use the excess solar to heat hot water in your hot water tank? And I suspect unless you are in a very suny climate the UK won’t produce much actual hot water in the winter

6kw won’t heat much water

 

[mafulynch]

op are you over complicating things? Why not just use the excess solar to heat hot water in your hot water tank? And I suspect unless you are in a very suny climate the UK won’t produce much actual hot water in the winter

well, I do not have a hot water tank. The hot water will come direct from the boiler too. And I do not worrie much about winter because I know I won't get much out of it, but I have calculated I could get aprox 800kWh extra of generation per month in summer by doing this

 

[Murdoch]

well, I do not have a hot water tank. The hot water will come direct from the boiler too. And I do not worrie much about winter because I know I won't get much out of it, but I have calculated I could get aprox 800kWh extra of generation per month in summer by doing this

where in the world are you?

 

[SJB060685]

If you install an additional heat source on the return to the boiler this will increase the return temperature, which as mentioned above the boiler will assume there's a reduced load and start to modulate its output, which can cause problems like radiators not producing enough power or longer reheat times for any hot water cylinder. It could also cause condensing issues.
I'd be more inclined to use this extra energy to preheat/top up stored hot water, be it a hot water cylinder or a buffer tank.

 

[Ric2013]

Argentina!

Hi Mafulynch,

I get your point about not wanting to preheat water for washing - I suspect others are thinking of the British climate and not realising you're in South America. High altitude, I'm guessing: cold but sunny?

No, I don't think a small heated tank will restrict flow. I meant that there's not a problem having a small tank when the electric heating is off. Ignore my radiator comment - it was badly worded.

I don't know what your boiler manufacturer means by 'renewable compatible' - you'd have to ask them. Is it a combi boiler? If so, they may mean it will accept preheated water on the secondary side - which you aren't interested in.

SJB06085 is spot on with guessing my concern. IF you have a boiler that is a modulating condensing boiler it may be monitoring the return temperature and trying to adjust the pump speed and burner to maintain a set temperature difference between the flow and return. It would take some thinking to work out the possible effects of putting a 6kW gain into the boiler flow or return under varying conditions and heat requirements and how the boiler may respond. I would imagine the boiler would work to some degree, but it may cycle an awful lot and never actually get a chance to go into condensing mode. It might even detect the situation as a fault.

Some ideas of the problems you might face: assuming you get a 20°C rise off your electric, you could try to setup the flow to account for full preheat conditions (run the emitters at, say, 60/20 and then preheat back to 40°C, so the boiler runs 60/40) but the radiator output will be reduced. Or you could run the boiler 60/40°C and then superheat the flow to 80°. This will give hotter radiators and if you run the radiators 80/40, it's not bad.

Problems might arise with both experiments when the electricity goes off as the boiler is now running on a 40° drop it isn't designed for and the radiators may be too cool to be of much use. Unless you can make the tank designed to restrict the system flow and use some kind of modulating control to bypass the tank and so increase system flow as the output reduces. It would seem very complicated though and you'd risk having two systems trying to compensate for one another.

Rather than running in series, have you considered running through a low loss header if an accumulator is not an option? LLH are not without their effects on boiler efficiency, but would at least work.

 

[mafulynch]

where in the world are you?

I am in south Argentina. I know it is abit of a over complication at first, but it has it reason. Because of the weather we get a gas subsidy, and if we go over price rises 30 times. In winter we are allowed to use more, so that is not a problem. But in summer we are abit on the limit, so we are looking for ways to lowering the ammount of gas we use for heating

 

[mafulynch]

Argentina!

Hi Mafulynch,

I get your point about not wanting to preheat water for washing - I suspect others are thinking of the British climate and not realising you're in South America. High altitude, I'm guessing: cold but sunny?

No, I don't think a small heated tank will restrict flow. I meant that there's not a problem having a small tank when the electric heating is off. Ignore my radiator comment - it was badly worded.

I don't know what your boiler manufacturer means by 'renewable compatible' - you'd have to ask them. Is it a combi boiler? If so, they may mean it will accept preheated water on the secondary side - which you aren't interested in.

SJB06085 is spot on with guessing my concern. IF you have a boiler that is a modulating condensing boiler it may be monitoring the return temperature and trying to adjust the pump speed and burner to maintain a set temperature difference between the flow and return. It would take some thinking to work out the possible effects of putting a 6kW gain into the boiler flow or return under varying conditions and heat requirements and how the boiler may respond. I would imagine the boiler would work to some degree, but it may cycle an awful lot and never actually get a chance to go into condensing mode. It might even detect the situation as a fault.

Some ideas of the problems you might face: assuming you get a 20°C rise off your electric, you could try to setup the flow to account for full preheat conditions (run the emitters at, say, 60/20 and then preheat back to 40°C, so the boiler runs 60/40) but the radiator output will be reduced. Or you could run the boiler 60/40°C and then superheat the flow to 80°. This will give hotter radiators and if you run the radiators 80/40, it's not bad.

Problems might arise with both experiments when the electricity goes off as the boiler is now running on a 40° drop it isn't designed for and the radiators may be too cool to be of much use. Unless you can make the tank designed to restrict the system flow and use some kind of modulating control to bypass the tank and so increase system flow as the output reduces. It would seem very complicated though and you'd risk having two systems trying to compensate for one another.

Rather than running in series, have you considered running through a low loss header if an accumulator is not an option? LLH are not without their effects on boiler efficiency, but would at least work.

Thank you for your help. You are right, very sunny days but cold weather in general.

I will contact the manufacturer and ask about what they mean by renewable compatible. Good thing is we haven't purchased anythong yet since we are still working on the exterior of the house, so there is still time to do all modifications needed and buy equipment accordingly.
And yes, it is a combi boiler and in deed my original idea was to place it before to give a temperature rise of about 20°C and save this way on gas. Will check with manufacturer about this.
I do not know what a low loss header is, so will look more into it since that would be much better that making a system that would bypass the electric. Altough doable with PLC or something of that sort, even the same heat controller I am developing, I would like to avoid it for simplicity.

Again, thanks for your help