DHW, Gas boiler - cylinder or combi

[JohnMo]

Question

 

Do you use less gas heating cylinder to produce DHW or direct from a combi gas boiler?

 

My thoughts are - A combi will start up quite a few times and have a short run times, worst efficiency.  Filling a cylinder will be longer run time, so should be more efficient.

 

But I am not sure, what are your thoughts?

[PeterW]

Burner efficiency there is little in it - usually 2-3% tops. You get into the vagueness of heat loss and other things which muddy the water along with pressure and flow discussions. 

[kommando]

Our water pressure can get low so went DHW as if the combi pressure was low we could not rely on using mains pressure to replenish pressure. Now the DHW is heated by solar diverted excess it's advantage is enhanced.

[Nickfromwales]

PV is inevitable, so if you fit a combi you’ll be stuffed for diverting excess into DHW all summer long. No free hot water for you then!!
“Efficiency” needs to be elaborated upon, and is an overall summary vs just what that appliance choice will provide you with. 
Do not be short-sighted here   

[markocosic]

Combis materially more efficient than boiler plus cylinder due to standing losers; especially where usage is low and/or most usage is long drawoffs and heat lost from cylinder and pipework isn't useful space heat.

 

Can't run PV into a combi though.

 

I'm waiting for heat pump cylinders to come of age (be available at sensible cost) in the UK. Where a dedicated heat pump heats the cylinder instead of a dumb immersion. Common as much stateside.

[JohnMo]

16 minutes ago, markocosic said:

Can't run PV into a combi though.

 

You can if you have cylinder upstream of the combi.

 

Just set up to do some experimenting, as we have quite a few days of overcast skies coming up/

 

Already have a buffer (thermal store) on the heating system, which has a DHW coil in it, this is used to preheat water going in to a combi in the heating season and heated by PV immersion in the summer.  Downstream of the DHW coil is solar diverter valve (to a mixer valve or to combi).  On a good solar we have free hot water.

 

Have a good idea of gas used for DHW, when there is little or no solar.   Have just set the heating system to charge the buffer at 47 degrees, 4 degrees hotter than the diverter is set at.  Will see how gas consumption goes, over the next week or so.

 

Have already noticed the draw of volume at the kitchen sink, is much lower with the heated buffer, than with the combi.

[markocosic]

You can; but that's then a cylinder not a combi 😉

 

Conventional cylinders work well for high hot water loads in houses with high heat loads where the significant losses from the primary pipework and the cylinder itself are "useful" space heat most of the year and small compared to the overall use.

 

There an energy saving trust study kicking about somewhere that yielded heat input: output efficiency of 30-70% for most cylinders. Reduce that further to compensate for gas boiler inefficiency. Increase it if for part of the year that heat is useful. Combis were more like 70-80% efficient overall.

 

Best imo will be this kind of thing; but executed well with integrated pv divert via heat pump etc

 

https://www.ariston.com/en-me/products/heat-pump-water-heaters/domestic/

 

https://www.ariston.com/en-me/products/heat-pump-water-heaters/domestic/nuos-plus-uae/

 

 

[markocosic]

https://www.tesy.co.uk/heat-pump

 

https://www.vaillant.co.uk/for-installers/products/arostor-domestic-hot-water-heat-pump-58880.html

 

Or perhaps more relevant to houses with a wet heating system:

 

https://www.auer.fr/en/products/heat-pump-water-heaters/edel-water-heat-pump-water-heater/

 

They need to be PV divertable though so that either the compressor only runs when there's ~300W+ available or it is inverter driven and can run when there's very little excess available etc. One for @Radian next week perhaps! 😉

[Radian]

2 hours ago, markocosic said:

Or perhaps more relevant to houses with a wet heating system:

 

https://www.auer.fr/en/products/heat-pump-water-heaters/edel-water-heat-pump-water-heater/

 

They need to be PV divertable though so that either the compressor only runs when there's ~300W+ available or it is inverter driven and can run when there's very little excess available etc. One for @Radian next week perhaps! 😉

 

This caught my eye:

 

And cools the floor as well. Hmmm...

[markocosic]

It's french but not a daft solution 

[joth]

2 hours ago, markocosic said:

It's french but not a daft solution 

Needs some integrating to the main UFH controls to ensure the primary circuit continues to circulate via the UFH loop even when there's no call for cooling or heating on the ASHP. But... Yeah it does make sense 

[JohnMo]

Think it makes good sense in the summer when it's hot, as you can use to cool the slab.  But at almost any other time, you are taking heat out of the floor provided by another source, i.e. a boiler, heat pump, even solar gain etc.  So if you take a kW away out of the UFH return flow you need to add kW at the other end to keep things in balance.

 

So would it be more cost effective just heat DHW from the normal source of heat.

[markocosic]

Not if the slab itself is heat pumped.

 

The alternative; for much of the year except winter; in houses larger than shoeboxes where space heat kWh per dhw kWh is decent; is an exhaust air heat pump.

 

See the manual in the tesy link for the various operating modes of the air units.

[markocosic]

These were the other solution that makes some sense:

 

https://www.ebay.co.uk/itm/224649548830

 

I'd prefer the water water unit chilling a ufh loop though.

 

Cop of an air source heat pumps feeding that ufh loop with top-up heat at 25c will be huge in the shoulder season so not materially worse than air source units there.

[Radian]

52 minutes ago, JohnMo said:

Think it makes good sense in the summer when it's hot, as you can use to cool the slab.  But at almost any other time, you are taking heat out of the floor provided by another source, i.e. a boiler, heat pump, even solar gain etc.

 

For anyone like me with a UFH loop in a garden room, outside the heating season, the floor may be a significant thermal resource. The sunlight that falls directly on the floor is re-radiated and trapped by the low-E glazing so becomes one of the most significant contributions to the room's air temperature. In turn, reducing the floor temperature would reduce this re-radiation so reducing the source of the unwanted air temperature rise. The fact that this would also be sourcing the DHW, potentially through solar PV is a win-win.

[markocosic]

So when are you trying this out for me @Radian😉

 

Potentially useful resource if interested - also some debate about alternatives:

https://gathering.tweakers.net/forum/list_messages/1941248

 

UK market will get interesting if they *actually* ban gas and get slightly more imaginative than like for like replacement of boilers with monoblocs.

[JohnMo]

We I did a bit of a trial. 

 

Normal summer use buffer cylinder (180l) acting as a pre heater for combi boiler.  With immersion connected to PV diverter. 

 

Trial 1.

Combi boiler directly connected to buffer hot water coil.  Daily gas usage one day at zero, mostly 2 to 4kWh

 

Trial 2.

Solar diverter valve between combi and buffer hot water coil.  Daily gas usage  zero, 2kWh kWh, but mostly in the region 0.5kWh.  note had an issue when outlet from buffer is above 45 at start of shower and goes below 45 during the shower and water is diverted to combi, shower would run cold.  The fix for this to to turn combi water temp to max.  Which seems to work.

 

Trail 3.

 

Closed the central heating lines out of the buffer and used the central heating side of the boiler to heat the buffer to around 55 deg.  Solar diverter valve installed and PV diverter active.

 

Heated buffer to 55 deg.  Typical daily usage 4.5kWh.  Buffer would get to circa 60 - 65 during the day, we would shower and it needed to be recharged to get it back to 55 the next morning.

 

So as it stands the best method for me, is preheated water from solar PV via a solar diverter valve.  A bit more PV would bring the summer usage down to zero all summer.  Or move house and views to England for more solar intensity, from NE Scotland (no).