Mixergy and combi boiler integration?

[bontwoody]

Im looking for some advice/opinions on how to green up the heating system in my new house, a mostly stone built, 2 bed bungalow. Obviously I will improve its insulation before doing anything else.

It currently has a newly installed gas combi and some solar PV. I intend to increase the amount of PV and install a heat pump and batteries at some point, but I am trying to firm up the best way to provide hot water and heating interim.

Since the combi is new, Im thinking that I will go down the route of minimising gas consumption for a few years before going the whole hog and installing an ASHP.

The combi is situated in a cupboard that has room for a small cylinder underneath. Although more expensive, Im thinking of getting an unvented Mixergy cylinder installed, rather than a standard cylinder. My reasons for choosing Mixergy, are that I can use a smaller cylinder, less energy to heat it for my needs and integrate it into a monitoring system easily.

My idea is to get the Mixergy fitted and connect the combi to it as a system boiler. This will be a backup to using a PV diverter to power an immersion heater in the cylinder. To convert a Mixergy cylinder for a heat pump, a flat plate heat exchanger is bolted on, and Im wondering if it might be a good idea to fit this in advance, and heat the cylinder when needed via the heat exchanger from the combi. I imagine this wont be quite as efficient but will make the change over to a heat pump very simple when the time comes.

For heating, Im thinking about keeping the radiators in most of the rooms, but oversizing them and testing in the winter with the combi set to a low output temperature. Im also considering a split air to air heat pump/ac in the bedrooms, which I have used before abroad and will provide cooling when necessary.

All opinions considered and that’s in advance for any input.

[JohnMo]

Have a similar system installed. I'm using an open vented 180l thermal store. Basically I have the following.

 

Combi boiler, with a large buffer, with domestic hot water coil.  Basically a Thermal store.  Run in two modes or can also run modes 1 and 2 together.

 

Mode 1 heating season, a 2 port buffer for the central heating.  But any water heading towards the combi is preheated by the buffer, leaving combi less to do for DHW.

 

Mode 2 off heating seasons, solar PV heat heats cylinder via a diverter.  To get the best out of the system you need a solar diverter valve and thermostatic valve.

 

Mode 2 works like this.

If water from cylinder is above 43 deg, it is sent directly to the thermostatic valve and on to the taps, if below 43 deg, it is forwarded to the combi before going to the thermostatic valve.

 

Prior to installing the solar valve, the boiler would still fire any time you opened the tap, as it saw a cold slug of water, so gas consumption was still a couple of kWh with a hot cylinder.  Since valve installed gas consumption is 0 to about 0.5kWh most days a poor solar day a couple of kWh.

 

Simple sketch, it's the same cylinder in both drawing just split the system to make it simpler to understand.

 

 

 

[bontwoody]

15 minutes ago, JohnMo said:

Mode 2 works like this.

If water from cylinder is above 43 deg, it is sent directly to the thermostatic valve and on to the taps, if below 43 deg, it is forwarded to the combi before going to the thermostatic valve

Ok, I think I see what you are doing, so you are using the cylinder as a source of preheat before sending it to the combi? I had envisaged the opposite kind of system where the combi just heats the cylinder and hot water is taken from there. Apologises if this isnt the case, I have reasonable hands on plumbing skills but lack system experience. Im off out in a minute so Ill have another look at it later to try and get my head around it better. 🙂

[JohnMo]

I have tried various layouts. It either preheats, but if above 43 it does not go to the boiler at all.

 

Last week I set it up so the cylinder was heated by the boiler (same as system boiler), but abandoned the trial as I used way more gas compared to the system as described previously.

 

My average gas consumption was around 4kWh per day even with the sun out, when heating as per a system boiler. 

 

The last 3 days I have used 0.6kWh in total as preheat, as per mode 2 above.

[DamonHD]

The way my system is now set up the heat store is a smalish Thermino (~2 day's DHW in a small unit under the combi) currently heated by PV diversion, but can be boosted from mains at appropriate times.  The Intatec Intasol valve automatically draws water direct from the Thermino if hot enough, else via the combi (mixed down to 28C since my combi doesn't like hot input) as preheat.

 

https://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html#2022-03-09

 

Gas demand has dropped from 4kWh/d to almost nothing (less than 0.5kWh/d).

 

Rgds

 

Damon

[JohnMo]

Pretty much the same set up as me, I'm using a Watts solar valve.

 

https://wattswater.eu/catalog/safety-control-accessories-for-heating/valves-for-solar-applications/thermostatic-kit-solarkit/

 

My heating buffer operates at 25 to 30 degree range during the heating season, so a phase change store is no use to me. Plus my cylinder only cost £150.

[bontwoody]

15 hours ago, JohnMo said:

My average gas consumption was around 4kWh per day even with the sun out, when heating as per a system boiler. 

 

The last 3 days I have used 0.6kWh in total as preheat, as per mode 2 above.

Wow, thats an impressive difference! I think I have my head around your system now. Am I correct in thinking the cylinder is heated purely from the immersion which heats the whole tank to between 25-30 degrees most days?

 

The mixergy works on a different principle, only heating the top of the cylinder as needed. I suspect that I should be able to provide virtually all my hot water needs from solar, outside of deepest darkest winter when I might need the combi backup.

 

I think the results you got with the system boiler configuration are because your are essentially heating the whole tank up to a usable temperature instaed of just what you need.

 

Thanks for the diagrams, I was considering something like this when I was thinking about using solar thermal panels as well, but in the end I dont think the extra expense added up.

 

[bontwoody]

11 hours ago, DamonHD said:

The way my system is now set up the heat store is a smalish Thermino (~2 day's DHW in a small unit under the combi) currently heated by PV diversion, but can be boosted from mains at appropriate times.  The Intatec Intasol valve automatically draws water direct from the Thermino if hot enough, else via the combi (mixed down to 28C since my combi doesn't like hot input) as preheat.

Thats definitely an alternative way to go, my feeling is that this would be more expensive that a mixergy cylinder and I cant see an option to add a heat pump later, unless Im missing it?

 

Your blog is interesting, Ill spend some time looking over it. We were living on Gozo Malta before the pandemic hit.

[PeterW]

16 hours ago, bontwoody said:

Apologises if this isnt the case, I have reasonable hands on plumbing skills but lack system experience.

You can’t fit an unvented cylinder without a G3 certificate or sign off. 
 

A smaller tank will be false economy as you’re using the Mixergy at 65-70°C with gas but ASHP will be 45°C so you’ll need much more capacity to get the same usable amount of hot water.

 

 

[JohnMo]

Cylinder heated purely by solar, when the heating is off.  It got up about 65 degrees yesterday.

 

I also intend to add solar thermal, have everything in place just need to find time to hook it all up.

[bontwoody]

12 minutes ago, PeterW said:

You can’t fit an unvented cylinder without a G3 certificate or sign off. 
 

A smaller tank will be false economy as you’re using the Mixergy at 65-70°C with gas but ASHP will be 45°C so you’ll need much more capacity to get the same usable amount of hot water.

Yes, I knew that and wasnt intending to fit it myself.

 

I was still thinking I might need an immersion top up, perhaps using an off peak tariff, but an ASHP can easily reach 55C and some of the newer ones more

[bontwoody]

12 minutes ago, JohnMo said:

I also intend to add solar thermal, have everything in place just need to find time to hook it all up.

Its something I would love to do but Im finding it hard to justify it since I could buy a lot of extra PV for the same price. It might come down to whether the DNO ups my solar feed or not

[SteamyTea]

55 minutes ago, bontwoody said:

an ASHP can easily reach 55C

But at a CoP of not much more than 1 for a lot of the year.

 

I have an ordinary, vented, 200lt, E7 cylinder. This year, for the first time, I am only heating the top third. The temperature is higher, but the volume less, saving at least a kWh a day. Secondary timer cost me less than 20 quid, so the experiment has already paid off the investment.

[bontwoody]

8 minutes ago, SteamyTea said:

But at a CoP of not much more than 1 for a lot of the year.

Well I have to say that wasnt my experience using a Ecodan, but i guess it depends where you live, it rarely drops much below zero in Swansea. Also the output temperature only reaches 55 right at the end. However a COP of anything above 1 is better than an immersion heater.

 

11 minutes ago, SteamyTea said:

I have an ordinary, vented, 200lt, E7 cylinder. This year, for the first time, I am only heating the top third. The temperature is higher, but the volume less, saving at least a kWh a day. Secondary timer cost me less than 20 quid, so the experiment has already paid off the investment.

That is a low tech version of the Mixergy I think, much cheaper 🙂

[JohnMo]

1 hour ago, bontwoody said:

Its something I would love to do but Im finding it hard to justify it since I could buy a lot of extra PV for the same price. It might come down to whether the DNO ups my solar feed or not

I just picked lots of bargains when available, Alpha boilers brain back system, end of line solar panel, end of line insulated flex pipes, spent about £450 in total and have everything I need except a couple compression joints and 4m of copper pipe.

[bontwoody]

20 minutes ago, SteamyTea said:

But at a CoP of not much more than 1 for a lot of the year

https://static1.squarespace.com/static/5894a3b0be659481ff6f152c/t/606c6192a19ef13fa60b3732/1617715603447/Mitsubishi+Ecodan+Spec+Sheet+Single+Phase+Oct+2020.pdf

 

Just found this, im not sure at what ambient temperature the SCOP is quoted but it looks more promising than you suggest?

[bontwoody]

2 minutes ago, JohnMo said:

I just picked lots of bargains when available, Alpha boilers brain back system, end of line solar panel, end of line insulated flex pipes, spent about £450 in total and have everything I need except a couple compression joints and 4m of copper pipe.

I'll keep my eyes open then and save some space on the roof 🙂

[JohnMo]

Forgot to add a note about my system, your combi has to be able to accept pre heated water at around 43degs.

 

Most modulating boilers will, but some don't.  So you need to check, not always that easy to find the info.

[bontwoody]

On 04/08/2022 at 12:59, JohnMo said:

Forgot to add a note about my system, your combi has to be able to accept pre heated water at around 43degs.

 

Most modulating boilers will, but some don't.  So you need to check, not always that easy to find the info.

Thanks.