Most efficient timetable for heating a unvented HWC

[Adsibob]

I have a 300L unvented hot water cylinder which is heated by a system boiler. The main demand on the hot water is:

• kids' bathtime which is almost daily, almost always at about 545pm (capacity of bath is 123L but we probably only fill it to about 85L);
• my wife's shower time which is most days at about 10pm (though some days she will shower at another time) (the shower is 10L a min flow rate, so she probably uses 120L)
• my daily shower which is usually at 7am (I probably use about 120L)
• random baths, which I may take once or twice a week (probably using about 200L)

 

Now all the numbers above are mixed water estimates, not hot water. Obviously in winter, with cold water supply being colder, hot water usage will go up.

 

But I'm just wondering, what the suggested heating timetable should be for our 300L tank. We currently have it come on from 635am for 25 minutes, then at 5pm for 40 minutes and then at about 930pm for about 30 minutes. This generally works well, but I sometimes wonder if it would be more efficient to:

1. have more regular, shorter bursts of heat. Like 5 minutes of heat once every two hours during the day; or
2. turn the thermostat down from its current temperature of about 50C to 44C and run the boiler a lot more frequently, on the basis that the heat loss from 44C water will be less than from 50C water.

 

We have an added complexity which is that we have a secondary loop going through a few hot water taps in the house and the HW cylinder. The pump is motion activated, but when people are in the house (varies day to day) it will come on for 2 minutes each time it's triggered and I've noticed it does cool the temp of the cylinder a little bit. The secondary loop is insulated, but not sufficiently so unfortunately. I try not to stress too much about that, as it is just leaking heat into the envelope of the insulated building.  

[Lofty718]

Your Viessmann should be running on PDHW, this is the most efficient way to run an unvented cylinder. It comes with this feature out of the box, has your installer set it up as a 4pipe hot water priority system?

 

 

Priority Domestic Hot Water (PDHW)

Central heating systems comprising of a gas boiler and hot water cylinder using a standard S or Y plan configuration are set to run at one flow temperature which will be the temperature for the cylinder at around 65°C - 75°C operating with a combined heat output for the hot water and heating demand. Running at these high flow temperatures limits the potential for the boiler to condense effectively, if at all, and that's probably for most of the year. 

Priority domestic hot water installations however control the hot water cylinder and central heating system at different flow temperatures maximizing its efficiency and creating a higher potential for the boiler to sit in condensing mode for longer periods of time. It also reduces the heat output for the whole system saving energy consumption.

How does it work?

This method of controlling the hot water and heating allows the hot water cylinder to take priority over the heating circuit when there is a demand from the cylinder sensor, the hot water is heated to a higher flow temperature in relation to the central heating flow temperature.

Combination boilers also use this method, when the hot tap is opened, the flow switch senses movement and diverts a valve over allowing the boiler to heat the mains water instantaneously producing hot water out of your taps taking priority over the heating circuit, when the tap is closed the boilers diverter valve reverts back to heating mode and reduces the boiler water temperature to the set level on the control panel of the boiler.

The Advantage of Priority Hot Water

PDHW technology allows the system to be run a two different flow temperatures. The hot water cylinder can be reheated at a considerably quicker time frame using a high flow temperature and operates at the boilers set maximum kW output with all the energy being used to heat the coil in the cylinder. Once the hot water is satisfied the boiler then reduces its flow temperature and kW output permitting the heating circuit to now operate at its designed heat output, the boiler can then start to condense.

Controlling and diverting the water would incorporate the use of either 2 port zone valves, a 3 port valve or an integral diverter valve depending on the boiler, set up and control options used for the specific system. Smaller gas boilers can be installed as only heating or hot water will be in operation at any one time.

Edited April 17, 2023 by Lofty718

[SteamyTea]

How about heating it up when CO2e emissions are lowest?

[Adsibob]

36 minutes ago, Lofty718 said:

has your installer set it up as a 4pipe hot water priority system?

How can I check this?

I do remember asking if he was using a different temperature for the HW heating to the UFH system and towel rads and he said it all had to be the same temperature as we have a low loss header (made by Viessman) as part of the setup and the manifolds will adjust temp for the UFH. If that's the case, I doubt i have that.

 

But else being equal, what's the most efficient timetable?

 

Edited April 17, 2023 by Adsibob

[Lofty718]

The installer sounds crap, Viessman 200  with a low loss header can be run at 3 different flow temperatures one for UFH, one for rads and another for DHW. Sending 70c water down to the manifold and having the blending valve mix it down is really old tech and highly inefficient.

 

If running at only one temperature you are wasting the boilers huge potential and could of bought something a lot more basic

 

i don't know the best timetable because this is my first unvented cylinder and mine runs as a hot water priority setup. But you should get someone in who knows their stuff about Viessmanns to rejig your system, I bet there's a lot of room for efficiency gains by the sounds of it.

Edited April 17, 2023 by Lofty718

[SteamyTea]

Have you looked into the actual heat losses of your DhW system.

You may find they are quite low, so you could heat it up almost anytime.

[Adsibob]

27 minutes ago, SteamyTea said:

Have you looked into the actual heat losses of your DhW system.

You may find they are quite low, so you could heat it up almost anytime.

Well I think the water circulation secondary loop is the weak point in terms of losses. Hence why I was toying with the idea of heading the water more regularly, but to a lower temp.

[SteamyTea]

2 minutes ago, Adsibob said:

Well I think the water circulation secondary loop is the weak point in terms of losses. Hence why I was toying with the idea of heading the water more regularly, but to a lower temp.

Maybe improve the insulation on the relevant pipework/valves.

At a lower temperature, you get lower losses, you may find that the final 2⁰C stored in the water soon drops in temperature.

You could just try playing with the temperature first, see how it responds.

Ground water is starting warm up a bit now, so can confuse a bit.

 

Shallower baths, shorter showers is the most effective way to save energy.

[Adsibob]

49 minutes ago, Lofty718 said:

The installer sounds crap, Viessman 200  with a low loss header can be run at 3 different flow temperatures one for UFH, one for rads and another for DHW. Sending 70c water down to the manifold and having the blending valve mix it down is really old tech and highly inefficient.

 

If running at only one temperature you are wasting the boilers huge potential and could of bought something a lot more basic

 

i don't know the best timetable because this is my first unvented cylinder and mine runs as a hot water priority setup. But you should get someone in who knows their stuff about Viessmanns to rejig your system, I bet there's a lot of room for efficiency gains by the sounds of it.

He may be crap, but this was just one of those things that happen in life which is nobody’s fault really. I went to a specialist Viessmann design company and asked for a design, they never got back to me. I also consulted a gas engineer and plumber who only did Viessmann, but he was reluctant to get involved because he was worried about stepping on my contractor’s toes. Inexplicable really. It proved difficult to find anyone else. I was v. busy, and didn’t appreciate the scope for getting things wrong. My builder was in a  hurry and his installer was an approved Viessmann installer (albeit he has only recently been certified) and I let him get on with it. For all I know, it’s a competently designed and installed system, though I doubt it has all the bells and whistles. Our expenditure on gas isn’t too bad at £90 a month on average from mid December to mid April (before the government subsidy). Given we consume quite a lot of hot water and it’s an upgraded 1930s 5 bed house, as opposed to anything near passive standards, I think that’s not too bad, so not sure whether it would have been worthwhile spending several £k on a fancy design and extra on more Viessmann kit, which tends to be pricy.

 

 Appreciate your concern, but unless it’s cheap and easy to check and improve someone else’s HW and heating design, I really just want to tweak the variables I can.

Edited April 17, 2023 by Adsibob

[Adsibob]

I should also add, to put things into perspective, that we regularly heat our ground floor to 22C or even 22.4C, and we only have 100mm of underfloor insulation so £90 a month during winter seems pretty good. But obviously compared to all the passive build builders, that’s a horrific running cost.

[Lofty718]

Your gas usage seems crazy cheap compared to mine especially with current prices

 

A knowledgable Viessmann installer would of installed it without room thermostats, zone valves, actuators or a mixing valve on the manifold so it's not much more expensive to do it the german way

 

The main cost is the boiler which you already have, Viessmann have it all built in out of the box other than a few items.  I had the same thing with the builders plumber wanting to install a high temp running UFH system but I ripped it out. It doesn't work out too much more expensive because you don't need zone valves, manifold mixing valves, actuators, thermostat or a ufh wiring centre.

 

It would probably only save 10 percent in energy, but will give the house and mainly ufh much more comfortable temperatures and a much more functional how water system, like a combi.

[Adsibob]

4 hours ago, Lofty718 said:

Your gas usage seems crazy cheap compared to mine especially with current prices

And we fixed prices well after the Russian war. On octopus, so great service but not cheap.

 

4 hours ago, Lofty718 said:

 

A knowledgable Viessmann installer would of installed it without room thermostats, zone valves, actuators or a mixing valve on the manifold so it's not much more expensive to do it the german way

we have about 12 different zones, 12 thermostats, 4 manifolds and 4 wiring centres. A lot of kit, but I doubt we would get as much control as we do with just everything controlled by Viessmann kit. 

4 hours ago, Lofty718 said:

 

The main cost is the boiler which you already have, Viessmann have it all built in out of the box other than a few items.  I had the same thing with the builders plumber wanting to install a high temp running UFH system but I ripped it out. It doesn't work out too much more expensive because you don't need zone valves, manifold mixing valves, actuators, thermostat or a ufh wiring centre.

 

It would probably only save 10 percent in energy, but will give the house and mainly ufh much more comfortable temperatures and a much more functional how water system, like a combi.

I’m curious to understand how and why.

[Radian]

On 17/04/2023 at 18:57, SteamyTea said:

Maybe improve the insulation on the relevant pipework/valves.

At a lower temperature, you get lower losses, you may find that the final 2⁰C stored in the water soon drops in temperature.

The losses from DHW cylinder and immediate pipework are also dependent on the temperature of the environment that they're housed in. It occurred to me that while it's difficult to add extra insulation directly to our cylinder, I can easily add insulation to the ceiling and inside the partition walls that form the airing cupboard. Reducing the temperature differential should result in a linearly proportional reduction of standing losses.

[Lofty718]

1 hour ago, Adsibob said:

And we fixed prices well after the Russian war. On octopus, so great service but not cheap.

 

we have about 12 different zones, 12 thermostats, 4 manifolds and 4 wiring centres. A lot of kit, but I doubt we would get as much control as we do with just everything controlled by Viessmann kit. 

I’m curious to understand how and why.

The idea with weather compensation is that you don't mess around trying to control it, you leave it open loop and have everything running at a low flow temperature with the heating constantly running and condensing, the only time it doesn't condense is when there is a call for hot water, which is what hot water priority is, the boiler will go into high temp mode and heat the cylinder as fast as possible, shutting off all flow to the heating circuits and once it is satisfied it will then resume heating the house. The temperature is regulated by a weather curve that is set on the boiler, your Viessmann if you go into the settings will have various curves that can be adjusted.

 

A system boiler not set up like this, run at one flow temperature (minimum dhw temp) will rarely be in condensing mode. I hope what I said makes some sense, I am no plumbing expert but these are things I have learnt. Most boilers are set up incorrectly and not many installers do this.

 

It is the same principle of how a good heat pump system is designed which should always be with weather compensation, you NEVER split a heat pump into loads of different zones and people that do that are the ones that had crap installers and say that heat pumps don't work.

 

12 zones in anything that isn't a mansion is truly insane. It must of been a very expensive install, it would be cheaper to have done it the way i suggested.

 

Watch this video, it is how i would of set a VIessmann up and he explains exactly the points I'm trying to make about why it would be cheaper

 

 

Edited April 18, 2023 by Lofty718

[SteamyTea]

7 hours ago, Radian said:

can easily add insulation to the ceiling and inside the partition walls that form the airing cupboard. Reducing the temperature differential should result in a linearly proportional reduction of standing losses.

It is what I have done.  It made a huge difference.

[JohnMo]

9 hours ago, Adsibob said:

I’m curious to understand how and why.

WC allows the lowest temp flow at all times. So the amount of condensing within the boiler flue increases.  More flue condensing more efficient the boiler runs.

 

Our efficiency went from high 80s% to around 107 to 114% efficiency measure by direct consumption and a heat meter.

[Adsibob]

I remain satisfied that the setup we have, with 12 zones, gives us many useful advantages. The house has three exposed walls (being a semi detached). The front faces East, the back faces West, and the side faces South but is mostly in the shade. Accordingly, different rooms have different solar gains. We also like having pretty different temperatures in different areas of the house, and also have a wood burner that we run a couple of nights a week in winter. So it is quite usual for the heating schedule in each of the five bedrooms to be pretty different to each other (one bedroom being empty, another being my office, the two kids’ bedrooms being on opposite sides of the East/West divide and wanting our master bedroom to be cooler than the kids’). Similarly, each of the three bathrooms also has its own requirements. So maybe the reason I’m only spending £90 a month on gas is that I have each room set up to come on only when I absolute need it. I don’t think I could achieve that with no zoning and WC, since this would not account for the microclimate in each area of the house.

[JohnMo]

2 minutes ago, Adsibob said:

microclimate in each area of the house.

This is done with balancing the water flow through the different areas. Not saying you need to do this, as you are happy with your setup; but for others reading this who maybe aren't sure.

[Adsibob]

1 hour ago, JohnMo said:

This is done with balancing the water flow through the different areas. Not saying you need to do this, as you are happy with your setup; but for others reading this who maybe aren't sure.

That’s interesting. But would you still get remote control of the flow balancing so that you could amend it as and when you see fit. I will give you an example: most nights a week, my kids sleep in their own rooms. But since we got my daughter bunk beds, she’s been quite excited about the prospect of having her brother come over for a sleepover. We normally decide whether the sleepover is happening at dinner, which gives me enough time to pull my phone out and turn off the heating in the unused room. Similarly, they can both stay over at my parents unexpectedly, I can work from home in my home office (bedroom 5) unexpectedly and we can have guests in our guest bedroom unexpectedly. So the zone control needs to be quite dynamic.

Edited April 19, 2023 by Adsibob

[Lofty718]

With a weather compensated system it's a totally different form of heating. The heat in the house is determined by the outdoor weather sensor which is linked to sensors on the flow pipes, usually one on the common flow pipe (low loss header or CCT) and other sensors  for every ufh/radiaor zones that all talk to the boiler and make it modulate accordingly

 

This means that the issue you raise is largley redundant and as mentioned earlier there should never be a need to turn a thermostat up or down once the correct weather compensation curves for the property are set because every room will be very comfortable and you'll barely notice the heating is on because it is constantly providing small inputs of heat at low flow temperatures that exactly matches the heat loss of the house, where as a high temperature system is a lot less linear. e.g room gets cold. the temperature drops and thermostat blasts the room with heat until it's satisfied.

 

Another benefit to a low temperature weather compensated running boiler is that there will be a lot less wear and tear on the boiler, a system with many zones will inevitably mean the boiler is always cycling turning on and off, this will wear the internal parts of the boiller much quicker.

Edited April 19, 2023 by Lofty718

[Lofty718]

@Adsibob as you say your system works and the bills are low so thats all that matters, but thats part of the issue is that a badly installed boiler

will still work but as @JohnMo says the only way you will acheive the 100% plus effciency figures on the boiler sticker is with proper controls and weather comp, without it will be 85-90%  at best.

 

That's why heat pumps are a problem, because a badly installed heat pump will not work for the customer. Where as a boiler will.

[Adsibob]

Makes sense. Thanks @Lofty718 for the explanations, wish I’d got you to commission the system in the first place! 
It’s incredible how there isn’t more education in the industry. My plumber showed me his recently obtained Viessmann certifications, and so I didn’t probe much further. But maybe it was poor communication: it’s possible that because I told him I wanted various zones, each with their own thermostat, he just ignored his training and did what I asked.

 

Back to my original question, would it make sense to time my hot water heating to coincide with when the heating of the UFH is on. At least in winter, we can pretty reliably predict those times. In that way, the boiler would come on and off less frequently.

[JohnMo]

26 minutes ago, Adsibob said:

told him I wanted various zones, each with their own thermostat, he just ignored his training and did what I asked.

The other thing "the customer always right".

 

Recently obtaining his cert, maybe not enough confidence to push back. Or was he told to plumb the British way - not the correct as designed way?

[Lofty718]

It's mainly a lack of education, they are just doing what they know and is easiest monkey see, monkey do as they say. Being a Vaillant master tech installer, Viessmann certified or gas safe means nothing. But the Viessmann training courses do suggest setting up with weather comp and variable temperature mixing valves so he must not have been paying attention in the course or read their design manuals.

 

 

With regards to your original question @Adsibob I don't think it will make a huge difference really, could you take a photo of your boiler/cylinder setup? has it been set up as a 4 pipe system? let us know.

Edited April 19, 2023 by Lofty718

[SteamyTea]

Was it Charles Bukowski's quip that got rewritten to;

"Every family needs a lawyer, a doctor and a plumber in it"