Boiler temperature for UFH and hot water cylinder

[Rob1992]

Hi, 1st post!

 

We completed our self build a year ago and I have been trying to find the most economical way of running our heating and DHW system but can't seem to find a conclusive answer, so hoping someone here may be able to help.

 

The set up we have is as follows:

 

Basement - UFH: barely required

Ground floor - UFH: main heating source of house. Blend valve set at 30 degrees

1st and 2nd floors - rads: barely required

Gas boiler- 30KW Intergas RF

Cylinder - Kingspan Ultrasteel 300L

MVHR system

 

The boiler is currently set to fire at 75 degrees to get the cylinder to 60/65 for legionella, but this seems a huge overkill when the UFH is blended to 30 degrees and the cylinder not needing to get above 50deg (?).

 

Other info, the boiler normally fires for about 4hrs a day in the morning to get the ground floor to temperature and but then needs no more until the next day. This normally raises the temperature a couple of degrees. The timer for the cylinder is set to come on once a day.

 

I am thinking it could be more economical to lower the boiler temperature to 55 degrees, and maybe run an immersion programme once a week (?) to get the cylinder to 65.

 

I really don't know the sweet spot at which a boiler is most economical so hoping someone can give me some honest science!

 

Many thanks in advance.

 

 

[TonyT]

Forget about the legionella issues if you have mains water.

 

Try 50 or just under

[JohnMo]

First thing to look at, is the boiler running in condensing mode.  For this to occur your return temp back into the boiler needs to be below 53 degs.  The lower the better.

 

Can you run the DHW and CH at different temperatures?

 

Otherwise as TonyT said.

[gravelrash]

As Tony T says. I presume your hot water is indirect on the tank. If indirect the hot water main will never get legionella.

[Rob1992]

2 hours ago, TonyT said:

Forget about the legionella issues if you have mains water.

 

Try 50 or just under

Can I ask why you say that and why do heat pumps have a procedure weekly to heat the cylinders up for a brief period?

[Rob1992]

2 hours ago, JohnMo said:

First thing to look at, is the boiler running in condensing mode.  For this to occur your return temp back into the boiler needs to be below 53 degs.  The lower the better.

 

Can you run the DHW and CH at different temperatures?

 

Otherwise as TonyT said.

I’m not sure to be honest, I don’t know what it’s doing for the four or so hours it takes for the house to warm. The flow pipe to manifold is generally scolding and the return pipe from the manifold mainly luke warm

[billt]

Boiler output at 75 is OK if it's just at that temperature for reheating the cylinder. A high efficiency coil should be able to absorb that energy and keep the return temperature low enough to keep the boiler condensing. It should be much lower than that in heating mode.

 

My Veissmann has an accessory box available which puts the boiler into maximum output mode for cylinder reheating, but reverts to lower temperature when the cylinder is satisfied. The return temperature is always less than 40C.

 

If the boiler is running at 75C all the time it's not going to be working efficiently so I'd investigate getting proper controls

[JohnMo]

19 minutes ago, Rob1992 said:

I’m not sure to be honest, I don’t know what it’s doing for the four or so hours it takes for the house to warm. The flow pipe to manifold is generally scolding and the return pipe from the manifold mainly luke warm

Had a look on the intergas website and it states the boiler is in condensing mode 100% of the time.

[Nickfromwales]

38 minutes ago, billt said:

My Veissmann has an accessory box available which puts the boiler into maximum output mode for cylinder reheating, but reverts to lower temperature when the cylinder is satisfied. The return temperature is always less than 40C.

I fitted Vaillant and Baxi mostly for a long time, and often wondered why they hadn’t adopted this type of approach. Really good idea ( and option ) to be able to define the output temps vs application. 

[ProDave]

11 minutes ago, Nickfromwales said:

I fitted Vaillant and Baxi mostly for a long time, and often wondered why they hadn’t adopted this type of approach. Really good idea ( and option ) to be able to define the output temps vs application. 

It makes the controls more complicated.  It would have to be S plan only NO Y plan and no mid position valves.  It would only be able to do heating OR hot water.

 

This is exactly how my ASHP works, but to achieve this, the motorised valves are controlled directly from the ASHP so the HP decides which to do and when and sets the flow temperature accordingly.

 

It is all perfectly possible and there would be many different ways to arrange the controls but it would be very different to what most present heating installers are used to doing with a standard system boiler and a wiring centre.

[Johnnyt]

I too have a Viessman 200

4 pipe system , One dedicated DHW, internal valve in boiler and seperate programmer and temp set to 55c

Other 2 pipes serve UFH, towel rads on own 2 way diverter timed  and 1st floor heating again own 2 way diverter and timed.

Second circuit set at 48 and ufh blended down.

I can have various flow temps at different times of day.

Seems to work well but early days, only fitted in Nov so still tweeking and noting gas usage which the boiler also tells me for both  heating and DHW

 

[Nickfromwales]

1 hour ago, ProDave said:

It makes the controls more complicated.  It would have to be S plan only NO Y plan and no mid position valves.  It would only be able to do heating OR hot water.

 

This is exactly how my ASHP works, but to achieve this, the motorised valves are controlled directly from the ASHP so the HP decides which to do and when and sets the flow temperature accordingly.

 

It is all perfectly possible and there would be many different ways to arrange the controls but it would be very different to what most present heating installers are used to doing with a standard system boiler and a wiring centre.

Dave, stop being a dinosaur  

If an ASHP install cam go into a domestic residence ( like yours ) then there is no reason why the same principals of operation cannot go into any other residence with the same simplicity. It’s just a W-plan ( hot water priority ) at the end of the day so not exactly “new technology”  There’s no reason whatsoever why gas / oil / electric boiler manufacturers should not have adopted this sooner imo. I can remember having this conversation with Vaillant tech support about 9 years ago and they thought I was a nut job. 
This country is still getting used to just having started walking upright……

[Nickfromwales]

53 minutes ago, Johnnyt said:

I can have various flow temps at different times of day.

Seems to work well but early days, only fitted in Nov so still tweeking and noting gas usage which the boiler also tells me for both  heating and DHW

Please report back here as the info comes in.

[joe90]

2 hours ago, Rob1992 said:

Can I ask why you say that and why do heat pumps have a procedure weekly to heat the cylinders up for a brief period?

I have no legionella cycle on my ASHP install, with mains water and a sealed system it’s the general  consensus here it’s not required.

[billt]

12 hours ago, ProDave said:

It makes the controls more complicated.  It would have to be S plan only NO Y plan and no mid position valves.  It would only be able to do heating OR hot water.

 

I wouldn't say that it's more complicated. My system still has the mid-position valve, but it's turned round so the normally open port feeds the heating and the wiring has to be changed so that it becomes, in effect, a two way valve which feeds the cylinder when water heating is demanded. The controls are exactly the same as before; you just have to think about the wiring a bit.

 

The cylinder reheats so quickly that lack of heating for a few minutes is unnoticeable.

[ProDave]

14 hours ago, Nickfromwales said:

Dave, stop being a dinosaur  

If an ASHP install cam go into a domestic residence ( like yours ) then there is no reason why the same principals of operation cannot go into any other residence with the same simplicity.

Yes absolutely, it is not rocket science, but you DO have to be able to read a manual.

 

I keep finding what should be simple system boiler installs here that are not working, and have never been right from new.  There are plenty of installers that have no actual understanding of things work, can't think through the flow of water through valves or electricity through switches and other controls. and just wire a system "the way they have always done it" turn it on and leave without making sure everything actually works properly.

 

Concepts such as pump over run, frost thermostats etc completely baffle them, so not much hope for getting an ASHP install right.

[SteamyTea]

1 hour ago, ProDave said:

Yes absolutely, it is not rocket science, but you DO have to be able to read a manual.

 

I keep finding what should be simple system boiler installs here that are not working, and have never been right from new. 

You have highlighted why the term 'Engineer' needs to be protected.

Nothing wrong with being a technician, but there is a huge difference between one and an Engineer.

[JohnMo]

1 hour ago, SteamyTea said:

You have highlighted why the term 'Engineer' needs to be protected.

Nothing wrong with being a technician, but there is a huge difference between one and an Engineer.

Have to agree, everyone wants to be an engineer, some that I would not employ as a technician.

 

Best comment during our build (by someone calling them self as a engineer) was 'why are you insulating the floor, heat rises, you are just wasting your time and money."

[PeterW]

On 16/02/2022 at 19:50, Rob1992 said:

Can I ask why you say that and why do heat pumps have a procedure weekly to heat the cylinders up for a brief period?

Heat Pump software is designed for a multi region market. In a lot of countries, the hot water tanks are fed by open cistern arrangements or the tanks have air spaces directly above the water in the tank so it is possible for contamination to occur. Lack of use of chlorine in the water treatment process also means that water quality is not guaranteed so the manufacturer includes a standard weekly 65°C cycle that is designed to kill off bacteria in the tank. 
 

In the U.K., the predominant installation is one with a mains pressure unvented cylinder fed directly from a rising main. There is no air space, no ability for contamination and the water supply is chlorinated therefore the need for a legionella cycle is removed. 
 

There are no recorded cases of legionella from a domestic water supply in the past 15 years in the U.K. to my knowledge.