Hot water priority question

[Murdoch77]

I've had lots of useful advice on here before and I'm now hoping for a bit more. We're currently in the process of getting quotes to replace our system boiler. Somebody suggested we should get a boiler capable of hot water priority that could keep the boiler in condensing mode most of the time.

I did a quick Google search which seemed to suggest that since 2005 all new gas boilers installed in the UK have to be condensing boilers.

So my questions are:
Are some boilers better at condensing than others? Which ones?
Can somebody please explain (in simple terms) what hot water priority means. (I have no technical knowledge at all).
What are the main advantages and disadvantages of hot water priority? Thank you in advance.

 

[muggles]

Condensing boilers only condense (and thus achieve maximum efficiency) when conditions are right for them to do so. It's not an on/off switch type effect either, it's a sliding scale. The water returning to the boiler from the system needs to be a maximum of 52°C for condensing to begin, with the flow at 70°C. Full condensing is achieved at around 50 flow / 30 return.

Hot water priority allows the boiler to run at a much higher temperature to very quickly reheat a cylinder. Once the cylinder is satisfied, the boiler will revert to a lower temperature as determined by the modulating controls connected to it, which is more efficient.

Without hot water priority, the boiler has to be run at high temperatures all the time so that it will be hot enough to heat a cylinder as required.

 

[Madrab]

Condensing boilers are such that when the return CH temperature is correct <= ~ 50deg (dew point) - the boiler can extract more energy from the exhaust gases as the water vapor condenses out of the exhaust gases, that makes the boiler more efficient. It's all about control and balancing of the CH system to ensure that the system water temps are correct and the return water temp @ the heat exchanger is maintained below that dew point.

Priority Domestic Hot Water (PDHW) is a CH system setup that allows stored HW (cylinder) to be heated up as quickly as possible. In effect, when HW is called for, the boiler switches a valve(s) over so it sends the heating water solely to the cylinder and ramps the boiler up to max temp to heat that water up as fast as possible. Not all boilers can do that - it needs to be able to have it's flow temp adjusted automatically so it can change between the cooler radiator/UFH temp and the max output temp for the cylinder - this can be managed by a system called Open Therm (other protocols are available) but the boiler needs to be compatible, not all are.

That'#s what happens when you forget to press post - wot he said too ^^

 

[Harry Bloomfield]

Can somebody please explain (in simple terms) what hot water priority means. (I have no technical knowledge at all).
What are the main advantages and disadvantages of hot water priority? Thank you in advance.

Your boiler has only one flow/output pipe, and so only able to output at one temperature at one time. Best fuel efficiency, condensing, works best when the returned water temperature is as low as possible. CH demand temperatures, are usually much lower, than that required to heat a cylinder of HW, so to satisfy the two, often conflicting flow temperature demands, it is better if the heating system is designed to serve one demand at a time, but the boiler has to be designed to know which it is serving, and have two flow temperature settings.

My own system is set to heat the cylinder to 65C, the boiler is set to a flow of 70C, whenever called to heat the cylinder - the flow has to be hotter, otherwise it would never be able to satisfy the demand.

My system is optimised - it is able to decide what flow temperature it needs to set, to be able to perfectly match the flow temperature, to the demand for heat in the house, based up present temperature, outdoor temperature, and set temperature. It thus saves fuel, by running at the lowest flow temperature which is just hot enough to meet the demand.

 

[JonathanM]

I'm just a DIYer, but here are my thoughts

Are some boilers better at condensing than others? Which ones?

I think the differences are very small, at least according to the manufacturer published data.

There has been some debate as to whether aluminium heat exchangers lose some of their efficiency over time compared with stainless steel heat exchangers, but this is disputed. I believe that many system boilers have stainless steel heat exchangers, but I'm sure that Worcester Bosch and some Ideal boilers use aluminium, and maybe other brands do as well.

What are the main advantages and disadvantages of hot water priority?

Advantages are that it should be more efficient as you can run your radiators cooler and maximise condensing. Also, some people prefer the gentle heat you get from running radiators cooler.

Disadvantages might be if your cylinder has a small coil that doesn't heat it very quickly. In this situation, when it is very cold, the house might cool down too much whilst only the cylinder is being heated. Also, it doesn't work if your radiators are too small to give out enough heat at a cooler temperature.

 

[JonathanM]

Can somebody please explain (in simple terms) what hot water priority means.

It is a way of providing hot water from the boiler to the radiators and the cylinder at two different temperatures. Very hot water to heat the cylinder, and cooler water to heat the radiators. Many boilers have this functionality built in. On others, though, it can only be done by using third party controls. It's complicated, because each manufacturer does it differently.

 

[Harry Bloomfield]

Disadvantages might be if your cylinder has a small coil that doesn't heat it very quickly. In this situation, when it is very cold, the house might cool down too much whilst only the cylinder is being heated. Also, it doesn't work if your radiators are too small to give out enough heat at a cooler temperature.

What needs to be avoided, is both demanding heat at the very same time - no point setting it to heat water at 7am, if you get up at 7am and will need a warm house.

 

[Harry Bloomfield]

Advantages are that it should be more efficient as you can run your radiators cooler and maximise condensing. Also, some people prefer the gentle heat you get from running radiators cooler.

In my case, apart from when there is a sudden increase in demanded room temperature, my boiler modulates gently up and down, so it completely avoids the creaking of pipes, as they heat up and cool down again.

 

[Exedon]

We have now done 2 small sites and a number of private jobs using ideal vouge system boilers and X plan control with weather compensation including my own.
On all the new builds and most of the others we used pressurised cylinders with high performance coils.
Another advantage is boiler can be sized just to heating system as the dhw never runs at same time.
I have my own dhw timer on constant so in effect only runs on demand. Once cylinder is hot typically reheat times are few minutes less than 10 even after a bath is filled.

 

[Madrab]

isn't quite funny the way the same question can be answered a multitude of different but ultimate similar ways.. Gotta love this forum lol

 

[JonathanM]

I agree, and I think that's what makes this particular forum so good. An OP can get the information they require in different ways, some more technical, some more for the layperson, some by professionals with huge experience of actually working on these systems, others from homeowners who share their own experiences of living with them. It's a great mix.

 

[ericmark]

The problem is the 11% extra efficacy gained from the latent heat, to gain the extra 11% we need to have an analogue control, using thermostatic radiator valves (TRV) we have an analogue control. However, in a lame attempt to force people to have a more economic system, there has been a push to have motorised valves which are on/off, which in the main results in less economic systems being installed.

The problem in the main is cost, a typical TRV head which connects to a central controller/thermostat costs around £60, this house has 14 TRV's so that mounts up. So we try to find some half way system, with some TRV's linked, and others simply a programmed unit but not linked.

The exist from the EU resulted in prices to change, in 2019 I was buying programmable TRV heads for £15 each, today they cost a lot more, so one has to decide what level to adopt.

There are other considerations, my DHW is heated by any excess solar power, not ideal, as off-peak is less than I should be paid for export, so a simple time switch would be better.

The debate over gas price v off-peak electric is not simple. As we have to include losses in pipe work, so although gas is cheaper than off-peak electric, once one includes the losses, it is not so cut and dried. I have rather a poor installation where the pipe run means the losses are high.

The debate can go on until the cows come home, and in Wales mainly sheep, so that is some time. It is easy to say I get free DHW with solar, but since my pay in tariff is more than cost of oil, if I ever get pay in? Then the argument electric v gas or oil is hard to work out.

 

[ianmcd]

Your boiler has only one flow/output pipe,

not all boilers

 

[Mister Banks]

PDHW can be setup with either a 2 pipe or 4 pipe system.

 

[dilalio]

In more simple terms to add to those above...

PdHW usually requires a boiler with 2 demand switches, usually 1 x 240v and 1 x low voltage.
Most but not all are capable of this.
With a 3 or 4 pipe boiler, the diverting of primary flow to heating or cylinder, happens within the boiler.
With a regular 2 pipe boiler (flow and return) an external valve is required to divert the primary water.
This is wired to changeover outlets at the same time as the boiler switches between KW outputs.
It can be done with only one boiler switched live if using opentherm or another proprietary protocol, as OT controls have the ability to communicate with boiler which will recognise the demand and "know" what it is heating for.

Again, as mentioned above, it is important to have a cylinder coil capable of absorbing the very hot water produced by the boiler and quickly transferring this heat to the domestic hot water surrounding it.

Personally, nowadays, I would recommend you get a system all by the same manufacturer... Boiler, cylinder and controls. They will cover the lot under warranty and they will be designed to work harmoniously and more efficiently together.

Vaillant would be my recommendation, Viessmann also do similar but are not great to deal with as an organisation.

Look up Urban Plumber on YouTube (he used to be on here a lot but doesn't post much lately), for some good info regarding PDHW systems.

The proprietary controls will also offer you weather comp for the heating side.