Understanding short cycling & UVC water temperatures?

[Ruggers]

I've two questions to understand?

1.If I'm looking to fit an unvented cylinder with a system boiler to a new self-build, well insulated air tight home, because of UFH ground floor and radiators 1st floor, I've been advised to run the heating water temperature at 60C. Someone in an older post also said that new builds need the DHW stored at 65C to prevent legionella and then have TMV installed to bring this down at the taps.

If the space heater water passing through the coil in the UVC heats the DHW around it, then how can 60 degree CH water heat the DHW to 65?

2. What exactly is short cycling of the boiler. I have a 1960 poorly insulated house with a combi boiler, in cold winter, the heating comes on and off on average every 1.5hrs for 15-20 minutes at a time from memory. I thought short cycling was when a boiler is having to come on little and often & so being inefficient.

In a well insulated new build, if you do a good job of it, surely the space heating will only come on & off a couple of times per day so there won't be any short cycling, but i keep seeing it mentioned, I must be looking at it the wrong way?

 

[muggles]

1. Get a Priority Hot Water system fitted, with OpenTherm or similar modulating controls. That'll give you a high flow temperature for heating the cylinder, and a much lower one for heating the home. It'll also vary the flow temperature for space heating to match what the rooms actually need rather than having a fixed one all year round, which is invariably set for the nominal worst case of it being -3°C outside.

2. Short cycling is when a boiler fires up, quickly reaches its target flow temperature, shuts down, cools down, re-starts and repeats. This can be avoided by having correctly sized radiators and pipework, selecting a boiler with good modulation which is correctly sized for the property, and then actually setting it up for optimum performative within that property via the installer parameters. This does rely on you buying a decent boiler which actually has installer parameters to alter, and choosing an installer who actually knows what they're doing with it

 

[Ruggers]

1. Are they two separate features thats built into some boilers, hot water priority and open therm? So open therm is just a variable/modulating feature that adapts the burner to cope with how much demand is required. Eg:Whole house calling for heat it ramps up more than if one zone was calling for heat?

2.So short cycling is usually caused by no modulation and/or an oversized boiler for the size of the system or circuit length? I'm trying to understand why it would re-start again so soon if it's reached it's temperature very quickly, unless the insulation was really poor and the system cooled quickly then demanding further heat shortly after. How often

 

[muggles]

1. Yes. Some boilers are capable of hot water priority, some aren't. Some have OpenTherm, some don't. A few lucky ones have both. There are alternatives to OpenTherm such as eBUS controls used by Vaillant and Worcester but they tend to be more expensive. OpenTherm is a communication protocol in essence but it does usually provide the option to have variable flow temperatures. For heating this means that if you're trying to heat your house from 20 to 21 it might only deliver water to the radiators / UFH at 35 degrees, but if you're looking to heat it from 17 to 21 it might deliver water at 60 degrees. It can also be varied according to the outside temperature using weather compensation.

2. Yes, short cycling is caused by the boiler delivering more power than the system can absorb / faster than the boiler can pump it away. The boiler will re-start as soon as the temperature within the boiler (not the system) falls a pre-determined number of degrees below the target flow temperature, but many boilers start on 50-70% of their maximum power until a stable flame is established, then modulate down. If the system can only absorb say 20% of maximum power when the boiler fires, it will quickly reach the target flow temperature, shut down and re-start once it has cooled a little

 

[Harry Bloomfield]

I've been advised to run the heating water temperature at 60C. Someone in an older post also said that new builds need the DHW stored at 65C to prevent legionella and then have TMV installed to bring this down at the taps.

Any temperature above around 55C is supposed to kill off the Legionella. My system (Vaillant) allows it to be stored at a lower temperature, but use a once per week Legionella program to raise the temperature to 60C. With no elderly, or infirm in the house - I like it stored at 60C, so no need for this program to be enabled. The only concession I have made to the hot being too hot, is to change the H and C taps on the small hand wash basin, to a single fixed temperature tap, fed from a TMV below the basin, simply for convenience - usually we wash hands under running water, rather than mixed in the basin. Easier to just turn one tap on and wash hands under that at an ideal temperature.

 

[Ruggers]

@muggles Sorry it took a while to reply, I watched this which backed up what you mention above.

I was just unsure if open therm was just a brand name or if it offered any differences to the function of modulating?

I would like to pair this system with weather compensation. Is the feature that learns how long it takes to heat your space and turned on slightly early or cuts off before the desired temp to prevent over shooting it, part of open therm or is this something different again?

@Harry Bloomfield - What's the benefits of having the tank at 60C all the time instead or just once a week? Is it not less efficient heating it to 60 to then blend down to high 40s? Do you need a mono tap for a TMV to work or can you have them on separate taps too?

 

[Harry Bloomfield]

@Harry Bloomfield - What's the benefits of having the tank at 60C all the time instead or just once a week? Is it not less efficient heating it to 60 to then blend down to high 40s? Do you need a mono tap for a TMV to work or can you have them on separate taps too?

Yes, it is less efficient, but I/we like our hot water to be good and hot for washing up, it is also faster to fill the bath - I have an open vented, so the hot is stored and both the hot and cold can be run at the same time, at full output.

My TMV it like a T - hot in, cold in and an output pipe, with a temperature adjuster on top. The output pipe can go to one or more taps, but obviously the flow will reduce if more than one is turned on. It perfectly suits the use of the downstairs toilet wash-basin I fitted in on and fixed an annoyance, as well as a safety feature. When I first fitted it, youngsters were using the toilet and being taught to wash their hands after using the toilet - what better way than a single tap, with water flowing at a perfect temperature for hand washing, plus avoiding the risk of scalding?

 

[Ruggers]

I agree Harry, I just wondered if your limited to mono tap only incase i wanted twin taps somewhere. They often give better flow rates on a bath but basins a single tap is usually fine it's just hand washing or dishes, we're the same with young kids so if storing at 60 will need TMV to cover all taps. I need to learn how many and where to hide them for when using a small wall hung sink without a pedestal or storage unit underneath.

I think I'd much prefer the weekly program cycle of heat but I think new build will still require TMV to cover all. 2 bathrooms, a kitchen sink, utility sink and an ensuite sink (No shower)

 

[vulcancontinental]

You're living in a house with a combi but planning to move into a new build with unvented?

The easiest way is to do what muggles suggested, buy an Opentherm boiler and put controls on it that allow high temp for DHW reheat then modulated flow for CH. The easiest way would be to use the system boiler version of the one in the video (only fair to tell you I work for that company) and EPH controls wired for OpenTherm. They can be arranged DHW priority over CH reheat or run simultaneous heat up of both at the higher temperature demanded then drop to lower temps when the DHW reheat ends.

Short cycling should be avoided if possible as it can impact efficiency quite markedly. Tests showed a 12% hit on overall efficiency if the boiler cycled every 10 seconds (very unusual) but it drops to below 1.5% if the burn time exceeds 3 minutes, far more palatable. If your burn time is 15-20 minutes as above I wouldn't let it worry me. With modulating controls (OT or WC or both) your burn time could be longer but at lower temperatures and therefore marginally higher efficiencies.

 

[Harry Bloomfield]

I agree Harry, I just wondered if your limited to mono tap only incase i wanted twin taps somewhere.

No, you are limited to a single tap, you can still have both taps or even three is necessary - hot, cold and a temperature controlled one. My TRV has a cap which can be locked with a screw if necessary, to prevent it being tampered with.

 

[Ruggers]

You're living in a house with a combi but planning to move into a new build with unvented?

The easiest way is to do what muggles suggested, buy an Opentherm boiler and put controls on it that allow high temp for DHW reheat then modulated flow for CH. The easiest way would be to use the system boiler version of the one in the video (only fair to tell you I work for that company) and EPH controls wired for OpenTherm. They can be arranged DHW priority over CH reheat or run simultaneous heat up of both at the higher temperature demanded then drop to lower temps when the DHW reheat ends.

Yeah current house is a renovation with a combi and whole new pipes and system when fitted some years ago. New home will be a self build with a gas boiler and more than likely a UVC but also looking at thermal stores. radiators upstairs and wet UFH down with 150mm pipe centres.

The way things are heading now with new 2022 June regs recommending CH 55C flow temps, a lot of radiators will require over sizing. I still don't understand how you can store DHW at 60-65C to kill legionella if your CH coil only has a flow rate of 55C?

What does EPH stand for or is it a brand name for controls? So the sole benefit of priority DHW is to heat the tank quicker rather than it trying to heat both CH & DHW simultaneously? That sounds back to how a combi works with the diverter valve.

 

[vulcancontinental]

Yeah current house is a renovation with a combi and whole new pipes and system when fitted some years ago. New home will be a self build with a gas boiler and more than likely a UVC but also looking at thermal stores. radiators upstairs and wet UFH down with 150mm pipe centres.

The way things are heading now with new 2022 June regs recommending CH 55C flow temps, a lot of radiators will require over sizing. I still don't understand how you can store DHW at 60-65C to kill legionella if your CH coil only has a flow rate of 55C?

What does EPH stand for or is it a brand name for controls? So the sole benefit of priority DHW is to heat the tank quicker rather than it trying to heat both CH & DHW simultaneously? That sounds back to how a combi works with the diverter valve.

Pendantic point first, the radiators will not be oversized if they're sized for a 55°C flow, they'd be the correct size.

EPH is a brand I use myself with OpenTherm and PDHW. Not sexy looking but does the job..

If you're having radiators and underfloor you'll need a system that can offer two flow temps for heating with an electronic mixer or a mechanical thermostatic blending valve for the underfloor.

Yes, DHW reheat can be done priority as the coils within are large area and give rapid reheat at high temp so short lived lack of heat to space heating is not noticed. Just like a combi.

The other way to do this is with better electronic controls but that's more expensive.

Not sure just what a thermal store can offer that a boiler, cylinder etc cannot unless you plan to incorporate a heat pump as well.


There are Vokera boiler, heat pumps and controls that can do all this.

 

[dilalio]

(only fair to tell you I work for that company)

You must know the chap in that video then @vulcancontinental

 

[dilalio]

I still don't understand how you can store DHW at 60-65C to kill legionella if your CH coil only has a flow rate of 55C?

Usually, this is done via an immersion heater, scheduled to operate once per week.

 

[vulcancontinental]

You must know the chap in that video then @vulcancontinental

Seen him before, he's looking old now.