combi boiler recommendations for 30c flow rate and 3500watt heating load

[gravelrash]

combi boiler recommendations for 30c flow rate and 3500watt heating load?..im struggling struggling to find information from boiler companies for what I need and they dont want to email back about min load, min temp settings or modulation rates.

[Radian]

Vokera go on quite a bit about their 10:1 modulation The 30^oC shouldn't be a problem for any boiler.

[gravelrash]

looked at Vokera but shocking reliability and low warranty on recent reviews

[JohnMo]

Assume your heat load is at lowest temperature.  So it will also need to 0.5kW when it's about 10 degrees.

 

So basically you need a buffer.  A decent combi of your choice with a good sized buffer.  My heat demand is just over 3kW, boiler lowest modulation at low temp is 6.7kW.  Have 180l buffer, with a thermostat set at 34 deg.  UFH just draws it water from the buffer and pumps around UFH at 30.  Currently getting 94% efficiency from the system - Gas in to heat water to heat taken from UFH via a heat meter.

[JohnMo]

I have a Atag A325ECX, super quite

[Radian]

5 minutes ago, gravelrash said:

looked at Vokera but shocking reliability and low warranty on recent reviews

Dodged a bullet then 😆 'Twas the first one to come up when I googled 10:1 modulation. I knew a few do this but couldn't remember the makes.

[gravelrash]

1 hour ago, JohnMo said:

I have a Atag A325ECX, super quite

Whats the min output and temp on the atag?

[JohnMo]

Will do low 20's

 

Only goes down to 6.7kW at low flow temps, 6kW at high flow temps.  Hence the need for a buffer.

 

But DHW performance is why a bought it, 95.1% efficiency with flue gas heat recovery built in, 18.8l/min flow rate, using the buffer with a preheat coil in it.  Will run two showers with ease and third if you are careful with temperature setting.

 

It's not peak load that kills you with CH, it is low load short cycling. 

 

Have reduced last years gas consumption, by a considerable amount.  Using big buffer is key.

 

October 800kWh reduction (561kWh this year, 1370kWh last)

 

November 600kWh reduction (1138kWh this year, 1753kWh last year)

 

December looking to save over 1200kWh

[gravelrash]

2 minutes ago, JohnMo said:

Will do low 20's

 

Only goes down to 6.7kW at low flow temps, 6kW at high flow temps.  Hence the need for a buffer.

 

But DHW performance is why a bought it, 95.1% efficiency with flue gas heat recovery built in, 18.8l/min flow rate, using the buffer with a preheat coil in it.  Will run two showers with ease and third if you are careful with temperature setting.

 

It's not peak load that kills you with CH, it is low load short cycling. 

 

Have reduced last years gas consumption, by a considerable amount.  Using big buffer is key.

 

October 800kWh reduction (561kWh this year, 1370kWh last)

 

November 600kWh reduction (1138kWh this year, 1753kWh last year)

 

December looking to save over 1200kWh

What size buffer are you using? Was hoping to avoid buffer as difficult to size without having wasted heat.

[JohnMo]

I am actually using a combi thermal store, mainly because I got it new very cheap.  It is indirect with a coil inside.  The coil is way too small for low flow temps (designed 80 deg flow temp), so have added a large 40 plate PHE and pump.

 

In the summer it is charged by excess PV and goes a long way to reducing DHW gas usage.

 

Photo prior to and after insulation 

 

 

[Radian]

21 minutes ago, JohnMo said:

It's not peak load that kills you with CH, it is low load short cycling. 

 

I'm coming round to the idea of buffering for my radiators as well as our single UFH zone (I use a hare-brained scheme to use our HW cylinder to buffer the UFH which works like a charm). The use of TRV's always leaves the boiler short cycling for ages and no amount of tweaking setpoint and balancing rads seems to get round it. As with my UFH hack, any surplus water seems to get used by natural HW demand.

[gravelrash]

1 hour ago, JohnMo said:

I am actually using a combi thermal store, mainly because I got it new very cheap.  It is indirect with a coil inside.  The coil is way too small for low flow temps (designed 80 deg flow temp), so have added a large 40 plate PHE and pump.

 

In the summer it is charged by excess PV and goes a long way to reducing DHW gas usage.

 

Photo prior to and after insulation 

 

 

Is your house timber frame? what U values have you got on the build?

[JohnMo]

ICF build, except the top of wall behind the cylinder which is timber frame.

Walls 0.14

Floor 0.09

Roof 0.11

Windows 0.6 to 1.1.

[JOE187]

I have Ariston Evo system 18kw rated  down to about 8kw it runs dual temperatures the  rads at between 36 to 46 degree C depending on weather compensation and room stat, hot water for unvented cylinder is set to 56 degree C. 

All components controllled by Ariston Sensys

  I installed this system in 2012 Self build and have not touched it other than cleaning condense trap and a yearly gas safe check!

[Nickfromwales]

22 hours ago, gravelrash said:

Was hoping to avoid buffer as difficult to size without having wasted heat.

It won't be wasted heat   It'll be advantageous heat input within the heated envelope, and the sum of all inputs = your required heat demand met via whatever source ( albeit slightly indirectly from the buffer ). Lose the buffer into an airing cupboard and keep your under-crackers fresh.

[JohnMo]

I can remember the days, an airing cupboard with a copper vented cylinder - no insulation to be seen.

 

If the buffer was in the shed you worry about wasted heat loss. The energy actually saved by a buffer by letting the boiler run for decent time should not be under estimated.

 

Now in a position to compare the effects of having and not having a buffer, now have a heat meter installed and all the usage figures of gas, as referred to above

 

November 2021 compared to 2022 average daily usage of gas was 20kWh per day lower in 2022 than in 2021. 

 

Efficiency of gas to heat conversion in 2021 was 61%, this year 98%.

 

(although I said 95% in an earlier thread that wasn't correct)

 

 

[tuftythesquirrel]

On 11/12/2022 at 17:05, JohnMo said:

So basically you need a buffer.  A decent combi of your choice with a good sized buffer.  My heat demand is just over 3kW, boiler lowest modulation at low temp is 6.7kW.  Have 180l buffer, with a thermostat set at 34 deg.  UFH just draws it water from the buffer and pumps around UFH at 30.  Currently getting 94% efficiency from the system - Gas in to heat water to heat taken from UFH via a heat meter.

Nice one @JohnMo

For my own benefit, how do you calculate the boiler efficiency?

[JohnMo]

I am taking the meter reading, popping it in to OVO (my gas supplier) app, this give me an exact metered and calorific value converted kWh consumption since last meter read.

 

I have an energy meter at the UFH, it measures, flow rate and supply and return temps and gives instantaneous kW heat rate and the accumulated kWh.

 

This seems to be most accurate way I could think of and heat losses are included in the total.

 

Divide gas consumption by energy meter read over the period and multiply by 100.  Ideally this will be over a period where no DHW is being used, as I have the UFH on 24/7 as a single zone, overnight is best. Or do it over a few days and take my typical 5kWh off for DHW.  On a really cold day and low DHW day, even without adjusting for DHW the energy meter is reading the same or lower than gas consumption.