System Boiler with weather comp

[msmithuk77]

Hi, 

 

I'm new on the forum and seeking some advice and confirmation on my approach to my heating system. 

 

We are replacing all the plumbing and heating in a recently purchased 80s house. 

 

What I wanted was the following: 

 

1. Domestic Hot Water Priority 
2. System Boiler with unvented hot water tank
3. Weather compensation
4. UFH Downstairs
5. Rads Upstairs

 

After some research and discussion with my plumber, I have the following details: 

 

 

• Valiant kit:
  • Boiler Valliant Ecotech Plus (new model) (I wanted Viessman, can get Vaillant and I’m happy with this):
  • Wiring centre
  • Weather Comp 
  • 2x SensoComfort Controllers (1x DHW and UFH, and 1x Rads)
  • Unvented cylinder 
  • VR10NTC temp sensors:
    • Unvented Cylinder pocket
    • Flow to the Rads (after pump group)
    • Flow to the UFH (after ump group)
• 2 Port Diverter splitting the flow from the boiler to A) the DHW (for hot water priority) and B) Rads and UFH
• Low loss header to manage the flow temps and reduce cycling
• Pump groups (1x Mixed for UFH and 1x non-Mixed for Rads)

 

The UFH will be on an open loop and only controlled by the boiler and flow temps at the manifold. The Rads upstairs will have TRVs (non-smart). 

 

What I'm looking to clarify are the following comments or suggestions to make sure I'm making the right decisions: 

 

• We should have Heatmiser Neo thermostats in each room/zone (as far as I’m aware they cannot modulate the boiler and defeats the points of WC)
• The Low Loss header is optional/unnecessary for my system but will make it more efficient. 
• The open loop UFH will make it very hard to manage the temperature in the individual zones and suggestion actuators with smart controls. 
• We have been suggested Vaillant ecoTEC Plus 635 35kW. Our house is detached, 4 beds, ~200 square meters, 2x bathrooms and WC. How do I make sure I’m not getting an oversized boiler or is this just about right? 

 

Thanks for any help/thoughts.

 

The diagram attached shows the system borrowed largely from a really helpful design found online.
 

  

System Gas Boiler.pdf

[JohnMo]

11 minutes ago, msmithuk77 said:

How do I make sure I’m not getting an oversized boiler or is this just about right? 

Heat loss calculation for house at your lowest practical outside temperature, normal to do it at about -3. But what you are being offered sounds huge.

 

26 minutes ago, msmithuk77 said:

The open loop UFH will make it very hard to manage the temperature in the individual zones and suggestion actuators with smart controls. 

Not true, you run UFH continually at a low flow temperature, if you find a room is too warm or cool you tweek the loop flow rate to compensate, once set it is self regulating to a large degree. Only time you need lots of control is if are trying to blast it with really high temps for short periods. WC is the opposite to that.

 

30 minutes ago, msmithuk77 said:

We have been suggested Vaillant ecoTEC Plus 635 35kW. Our house is detached, 4 beds, ~200 square meters, 2x bathrooms and WC. How do I make sure I’m not getting an oversized boiler or is this just about right?

Would suggest you find a different plumber.

 

Have you thought about increasing bedroom radiator size so you can flow all the house at the same low. temperature?

 

14 minutes ago, msmithuk77 said:

We should have Heatmiser Neo thermostats in each room/zone (as far as I’m aware they cannot modulate the boiler and defeats the points of WC)

With WC you don't really need them, but if you insist you have them as over temperature limit stops. So target room temp is say 21, set them to 23.

 

16 minutes ago, msmithuk77 said:

Low loss header to manage the flow temps and reduce cycling

Install the correct sized boiler and you don't need this.

 

Have a watch of this video on a Vaillant boiler install.

 

 

[msmithuk77]

Thanks for the info @JohnMo. All makes sense though, I'm going to do more research into the low-loss header. 

 

Another QQ if you don't mind. I'm going to use 2x Flamco Pump Group for the UFH (mixed) and Rads (non-mix). I have a quote from an UFH company and they have included Grundfos EuP Pump (22kw) and a Grundfos Pump Station. I'm assuming I don't need these with the Flamco Pump Group as I'll have these in the system already. Correct? 

[JohnMo]

Correct

[JohnMo]

10 hours ago, msmithuk77 said:

2x Flamco Pump Group for the UFH (mixed) and Rads (non-mix)

I would spend that money replacing the high temperature radiators with radiators designed for dT 15. That would allow you run the whole system at a single flow temperature.

 

Install limiter trv's on the radiators. Run the whole lot as a single zone with a 2 Deg setback if you want at night. So all heating is either on or off. Everything on a typical winter day will be running a very low flow temps 25 to 30 and to a large degree self modulating. If room temp increases output decreases to keep room temp quite stable.

 

No additional pumps required, no LLH or mixing sets, run the whole system from the boiler pump on WC run from the Vaillant controller, no additional thermostat required.

 

If it looks or sounds complicated, it usually is with little or no advantage.

 

Your shopping list reduces to

 

Valiant kit:

Boiler Valliant Ecotech Plus (new model) sized to suit heat loss - definitely not just a big one.

Wiring centre

Weather Comp 

Unvented cylinder - install a heat pump cylinder with 3m2 coil. This will give super quick recovery at low flow temps. Size cylinder the same as a heat pump and store water at about 50 degs. So 210L to 300L for 3/4 bed.

VR10NTC temp sensors:

Unvented Cylinder pocket

2 Port Diverter splitting the flow from the boiler to A) the DHW (for hot water priority) and B) Rads and UFH

Some radiators and plumbing.

 

The above will give continuously condensing running conditions and efficiency of around 105 to 110%. Compared to closer to 80 to 85 with the other design mixing down from hough temperature.

 

The first step is a heat loss calculation for whole house and the radiator rooms. This will allow a suitable boiler and radiators to be specified.

[msmithuk77]

12 hours ago, JohnMo said:

Correct

Excellent - thanks again!

[msmithuk77]

11 hours ago, JohnMo said:

I would spend that money replacing the high temperature radiators with radiators designed for dT 15. That would allow you run the whole system at a single flow temperature.

Could you expand on what to look for? 

 

I suspect this will be a step to far for my ambition

[JohnMo]

1 minute ago, msmithuk77 said:

Could you expand on what to look for? 

 

I suspect this will be a step to far for my ambition

First whole house and room by room heat loss calculations. From the room calculation you find the size of the radiator required for each room. dT 15 means the flow temp I'm the radiator is 15 degC hotter than room target temperature.

 

This will get you most of the way

 

https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

 

 

[Alexphd1]

Little bit different (new build high insulation) We plumbed upstairs radiators with a ufh manifold so no trvs.  We also run cable to each room for adding in a thermostat at a layer date but tbh the rads are never on. 

[JohnMo]

For radiators use this to get a decent radiator size

 

https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-for-a-heat-pump/

[JohnMo]

Another read 

https://www.heatgeek.com/viessmann-vs-worcester-boilers/

[Lofty718]

@msmithuk77 strange it seems like your plumber is 80% there with his knowledge, but a 35kw boiler is insane, an 18kw would do the job undoubtedly

 

I'd do as @JohnMo suggested and run everything at the same flow temperature, this will provide maximum efficiency, reduce the amount of equipment needed, reduce electricity (pumps etc) and also reduce the amount of things that can go wrong in the system.

[msmithuk77]

12 hours ago, JohnMo said:

For radiators use this to get a decent radiator size

 

https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-for-a-heat-pump/

This is great thanks!

[marshian]

If you can read a tape measure and know the construction of your house and the std/age of the double glazing you can do the heat loss calcs for a days work using https://heat-engineer.com/home for a one off fee of £12

 

I've done it myself early 80's 4 bed detached house (with some 90's insulation upgrades - SUDG, Improved Loft insulation etc)

 

My heat loss at -2 came out at 4.3 kWh - current boiler is 24kW with a very poor modulation ratio (min 10kW) the heat loss explained why it cycles so much as even at -2 it's trying to push 2.5 times the heat required to maintain house temperature.

 

We only have a few days a year at -2 most of the time it's 8-10 deg at which point it's trying to chuck 6 times the heat energy required to maintain the house temp.

 

I've experimented over the winter with flow temps and with the current boiler I can get down to 45 - 48 deg flow temps - however this is mainly due to 10kW min input and a boiler min flow temp of 39 deg C and flow rate of 10L/Min

 

I've also experimented with range rating the boiler to try to replicate what life would be like with a 11kW boiler and the warm up time is a little too long.

 

The heat engineer software has told me exactly what each room needs to maintain a 20 deg room temp and I've been thro room by room sizing the replacement rads to run lower flow temps in preparation to a boiler change this year. (Viessmann 16kW - 100W heat only boiler which will be set up with DHWP and weather compensation - Yes I could go for the 11kW version but the min rating of all the 100W boilers is 3.2 kW so nothing lost by going for a little larger, we heat the house according to a schedule as we are at work during the day and slightly larger improves warm up time)

 

So in summary

 

100% recommend a good heat loss survey

Size your boiler (and rads) according to the way you heat your house and how quickly you want your water recovery to be - if heating constant (low and slow) you can size the boiler to be smaller - if heating according to a schedule you are going to want the boiler to be slightly oversize to get quick recovery of room temps

[JohnMo]

37 minutes ago, marshian said:

heat loss at -2 came out at 4.3 kWh

Or should say 4.3kW. which is very good

 

39 minutes ago, marshian said:

Size your boiler (and rads) according to the way you heat your house

While doing that you really need to take a look at pipe sizes to make sure your required flow rates matches the pipe sizes. When you reduce flow temp the dT across the radiator also reduces. 40 deg flow and a decent boiler may be looking to control at 5dT. So your flow rates through the radiator may increase.

 

46 minutes ago, marshian said:

Viessmann 16kW - 100W heat only boiler

Or a Viessmann 200 boiler will go down to 1.8kW and not need to switch on and off in mild weather.

 

A system boiler will control the circulation pump and modulate it's speed for better control. So able to run two different speeds for hot water and heating.  Where a heat only boiler will more than likely be a fixed speed pump compromise.

 

53 minutes ago, marshian said:

if heating constant (low and slow) you can size the boiler to be smaller - if heating according to a schedule you are going to want the boiler to be slightly oversize to get quick recovery of room temps

Did find a setback energy calculator, must be on the old phone, but worth a play, as the saving from setback is extremely small. Due to the hotter flow temps required to catch back up - but every house is different.

[marshian]

1 hour ago, JohnMo said:

Or should say 4.3kW. which is very good

 

While doing that you really need to take a look at pipe sizes to make sure your required flow rates matches the pipe sizes. When you reduce flow temp the dT across the radiator also reduces. 40 deg flow and a decent boiler may be looking to control at 5dT. So your flow rates through the radiator may increase.

 

Or a Viessmann 200 boiler will go down to 1.8kW and not need to switch on and off in mild weather.

 

A system boiler will control the circulation pump and modulate it's speed for better control. So able to run two different speeds for hot water and heating.  Where a heat only boiler will more than likely be a fixed speed pump compromise.

 

Did find a setback energy calculator, must be on the old phone, but worth a play, as the saving from setback is extremely small. Due to the hotter flow temps required to catch back up - but every house is different.

We have a suspended ground floor with a sizeable void under it - I've insulated it and it made an absolutely massive difference to heat loss in the house - 4.3 was with an ACH of 0.5 - with default ACH it was around 6 from memory (this winter we were able to validate the 4.3 over a few days when it was around -2 for the whole period)

 

Flow and return are 22mm with short 15mm tails to the rads and I have TRV4 bodies on the rads to adjust flow rates

 

Viessmann 200 isn't suitable - won't fit the desired space - current boiler is 2 pipe flow and return at the top and I don't want to rip floors up to re-pipe for a 4 pipe set up - the difference between 1.8 and 3.2 isn't enough to convince me the upheaval required is justifiable

 

Minimal change to the system required to go to a 100W heat only although the quote I got from the only Viessmann installer in a 50 mile radius is more than expected I'm still going that route.

 

I tried setback with current boiler - it cost way more to run than scheduled heating times (almost certainly due to boiler min kW) I will try it again with Viessman when it's installed.

 

I should add I'm installing with existing vented HW system due to not trusting 30 plus year old pipework with mains pressure I've already found 2 poor joints in the 30 years we've lived here and all the upstairs CH pipes run in the void between the floors and are quite tricky to get at.

[JohnMo]

Another boiler to chuck into the boiler options is an Atag boiler.

[marshian]

4 hours ago, JohnMo said:

Another boiler to chuck into the boiler options is an Atag boiler.

Very difficult to find the full specs of those atag boilers especially modulation or turndown ratios

[JohnMo]

51 minutes ago, marshian said:

Very difficult to find the full specs of those atag boilers especially modulation or turndown ratios

Page 8 of the install instructions 

 

Turn down is not quite as good as the Viessmann. But having used an Atag, they are super quiet and work well. The way to keep any heat source happy is enough engaged water capacity, you need about 50L to support 6kW so it doesn't short cycle.