Storage combi or UVC choices for new build

[ruggers]

I'm looking for some advice on pro's & cons of each system. I know everyone has their preferences on models too but I'm just trying to narrow down my choices, I was looking at a system boiler or heat only with UVC with good modulation & then I recently discovered the Viessmann 222-F Storage combi. I'm just unsure if the Viessmann 200 or 222F can work with what I wanted, being some zoned areas controlled by room thermostats.

 

Info. New self build with MVHR, SAP levels are high so combi would help more than UVC. Wet UFH downstairs in screed, radiators upstairs on separate UFH manifold for end to end connections with actuators, 2 separate wiring centres. Wunda trade looked good. Also looked at Salus & heatmiser. Radiators sized for a 50 or 55C flow, UFH around 40/45C with room temp at 20C & -3C lowest. Occupants, 2 adults 2 kids. main bathroom bath with separate shower, downstairs bathroom shower within the bath. Could be a mixer shower or an electric one, mixer preferred.

 

Currently manage on 1 bathroom with a standard combi but need to prep for 2 showers simultaneously in future. Usually like 10-12 min showers, don't like high pressure ones. 222-F would suit our needs but can it work with zones & wiring centres? Weather comp, load sensing & priority hot water required if using UVC.

Divcons & low loss header has been recommended by an experienced heat engineer. Any other boiler suggestions welcome?

[Beckside]

We went low loss header and divicons with a Viessman W200 system boiler on weather comp, so no thermostats just one outside temp sensor, and a300l UVC. We have UFH on the ground floor and small rads and towel rails upstairs It was the ability to run two or three showers simultaneously that made the choice for us with regard to the tank. The divicons are not cheap but the system works really well. You would not need pumpsets on the ufh manifolds or any thermostats or actuators of you treat each floor as a zone.  We were going to use wunda and Heatmiser as thought we wanted/needed that level of control. In reality if your house is airtight enough to warrant MVHR then you could treat each floor as a zone. It works really well 

[JohnMo]

As mentioned if going weather compensation, don't waste money on flashy thermostats, the radiator just need TRVs, UFH as single zone.  Set up weather compensation so UFH runs all the time without overheating the house, flow temps will be closer to 25 than 45 degrees.  Radiators are you too late to increase size to future proof, set up with a max flow temp suitable for a heat pump.

 

Boiler sizing can be difficult with a combi, as you want a big one for DHW, but they tend to be huge in a well insulated house for heating.  So may benefit from a buffer to ensure long boiler run times.  We have a big combi (Atag A35ECX) with only a max heating demand of around 3kW.  Have a 160L buffer with a DHW coil in it for pre heating cold water to combi DHW.  The buffer is also heated by solar, so big reduction in gas consumption in the summer.  The water leaving the preheat coil, if it is above 45 goes straight to the taps, below 45 goes through the boiler heated to 55 degs.

 

We can run three showers from a combi.

 

If you are thinking combi, the only real way to get consistent performance all year round and the ability to run multiple outlets properly is with pre heat on the cold water.

 

Not sure where low loss headers come in on a domestic install, just use close coupled tees, £2.50 instead of £250.  Or if you have a buffer that gives you hydraulic desperation.

 

I would have a pump and thermostatic mixer on each manifold for UFH, as it easily allows you to run different flow temperatures and it also automatically give hydraulic separation from boiler pump and also acts as limit stop on flow temperature to protect the floor.

 

 

[ruggers]

Hi, thanks for your replies,  @BecksideAlthough more zones is probably adding more potential failures with the actuators, the reasons I wanted them was for control of both temp & times. Upstairs I have a mancave/office for working from home & I liked the idea of heating just the one room rather than the whole upstairs when I'm in there during the day or late at night if others are in bed and don't want the heating on, or not having the kids bedrooms coming on at 5 & 6am when I need heating on for work.

 

Same for the living room downstairs of a weekend evening when we aren't using the other ground floor rooms. I'd still have a set back temp on as I'm aware that hot goes to cold. I had it planned to leave towel rails & the hallway & landing (core of the house) on no zone so they warm up when other zones call for heat & to reduce any short cycling. 

 

I'm not fixated on this way over just having an up and down zone only, but I do like the idea of some zoned rooms. Wunda stats weren't too bad at £30 each. Divicons & LLH are a new suggestion to me which I'll need to research more. As you say not cheap to buy but I've heard cheap running costs. 

 

Why did you choose divicons instead of a mixer & pump on your manifold whats the difference? Do you have a rapid recovery cylinder? How do you set your indoor temperature with only an outdoor WC sensor? 

 

@JohnMoCurrently haven't started, obtaining quotes again, been in the works a while with some issues, so still able to alter rad sizes or anything else. I'll be installing the UFH & rad system but leaving the plant room to someone qualified.

I think ASHP will be out of my budget at the moment but would like to size accordingly for using one in years to come hence why I was wanting to design for 50C Flow (DT20). This is the higher end of an ASHP scale, but if I go too low due to room sizes my rads will be huge & towel rails would need to be electric.

 

The Viessmann 200 system boiler & 222F 32Kw modulates to 1.9kw (1:17) it says. The 222F has a 100L built in storage tank that recovers quickly & can manage 2 showers easily so that was why I found the combi appealing, no UVC to service & storing less hot water should be cheaper, not sure about legionella. without divicons I was looking to have a mixer & pump on the UFH, and on the upstairs manifold, just use the boilers pump, no mixing valve required for this.

 

What were your reasons to choose the combi with buffer tank & not a heat only or system boiler?

 

[JohnMo]

First reason for a cylinder was to enable PV diverter and offset DHW and possibly heating/ adding to CH.  Future plan is to also add solar thermal. Thermal panel currently sat in garage waiting to be installed.

 

The buffer when in CH mode only gets heated to about 30 to 35, so heat loss is negligible, but it hot enough to pre heat water going to combi for DHW.

 

Reason for combi was hot water on demand, without having to store hot water and the associated heat loss.

 

To get everything to work sounds easy, but has taken a couple of revisions to get to it function efficiently.

[ruggers]

Thanks. Were you not tempted by an ASHP?  It seems that everyone using the Viessmann products opt for no zones or stats which I need to look into further, I'm not really understanding how that works, I know it's new tech with Weather comp, but 1 thermostat for a whole house without WC just sounds like how it was done 30 years ago. Are people also doing this in their houses with other brands of boilers?

 

The plus side to having a tank would mean that the house should be set up to swap from gas to something else in the future like ASHP without pulling ceiling in to alter pipes. I do like the sound of the storage combi for the reasons you mention being continuous DHW and the heat loss from cylinder. Although MVHR should recover some from that room.

 

I had a look at an ATAG system boiler but they don't mention any modulation or figures.

[JohnMo]

Atag manual has all the numbers, snapshot attached

[Chablais]

Hi Ruggers

Weather comp is not new tech, used in Europe for at least the last 25 years. I was in your mindset before we started our build, to the point we installed enough back boxes for a Heatmiser thermmostat in every room and bathroom. However, once you build to Passiv level insulation and airtightness, you will find that the house, as a whole keeps the heat in. Trying to seperate rooms as zones is very difficult to near impossible, in fact, over heating the first floor is probably a bigger concern, as is any room with South facing windows. If I did it all again I would fit air  conditioning units and use them to heat in winter anc cool in Summer, powered by PV. as much as possbile. I ended up with a UVC because we wanted the ability to run three showers at the same time. in reality it rarely happens, and a 222 would almost certainly have done the job, without the stadning losses of a cylinder. Yes, we are on PDHW with a solar cylinder, so has two coils, both connected to the boiler, so our re heat time is approx 12-15 mins. As others have said, the other reason we ended up with a cylinder was to have somewhere to divert excess solar PV to. We dont use any gas to heat water from about April - November, and we are in the Lake District. I did look at using a cylinder from ACV that is in effect a cyliner in a cylinder, and the 'jacket' water is used to heat the domestic water, so that would mean we couldhave used PV to heat both the heating water and shower  water, but I struggled to find any plumbers/heating engineers with nay experience of using them. It would seem we ware about 20-30 years behind Europe in our approach to heating houses. We fitted a Belgian MVHR system, and it is normal fot it to integrate with the heating and colling system and also the window bilnds etc. to control shading/heating/cooling of the building as a whole. Not that we have, but we should have!!

 

[ruggers]

Thanks @Chablais, I also liked the 222-F for the reasons you mention above. Two showers at the same time is the most we will ever need running, or a shower ran after someones ran a bath so quick refill times are required. I hear varying times for this so assuming I need a fast recovery cylinder. It's trying to figure out an option A & B, because heat engineers in business can't be giving away free advice all day, and not a lot of people locally seem to be interested in anything out of the norm, I'm also in Cumbria. I contacted Viessmann regarding their system & storage combi's and they said their warranty is 5 years but can be 10-12 years if fitted by a trained installer, the closest being 50 miles away. 

 

I could do with knowing which other boilers have good modulation & reliability to check out. I will be doing a good job of the air tightness but won't be able to achieve the insulation levels some of you have got.

What boiler did you end up fitting with the UVC? I suggested a solar twin coil cylinder to a local plumber with both coils connected to the boiler flow for quicker cylinder recovery but they dismissed it as a bad idea saying it wouldn't work.

[JohnMo]

I really would specify everything suitable for a heat pump, it will all give advantages to a gas boiler.

 

UVC, with a heat pump coil, will accept lower flow temps, which will in turn keep you inside condensing mode of the boiler.  You can heat to circa 50 instead of 60 plus.  The heat exchanger coil is huge compared a gas boiler, which will have a better approach temperature and quicker reheat time.

 

Radiators, use double panel rads to get twice the area, or half the size.

 

I would and do have electric only towel rails, use them any time of the year without have to fire up the heating.  Just 400W elements in them, have UFH in the bathroom and en-suites.

 

Operate in weather compensation flow temps are low, boiler are super efficient, running costs low whole house an even temperature, although with suitable balancing you can get cooler rooms if you want.  Read heat geek website for some good info. 

 

 

[Beckside]

There are not many boilers that modulatevas low as the Viessman. We fitted a W200, the big advantage is all the controls inc Wi-Fi are built into the boiler as std. it has a colour touchscreen and an app on a phone. Just the outside temp sensor, cylinder temp sensor and low loss header sensor to connect. Plus the Divicons if you go that route. If you do, you can buy indentical units to the Viessman Divicons for substantially less, they are very common in Europe so a lot cheaper. Join/ have a look at ‘heating system design’ a Facebook group, lots of info on people using Divicons etc.

I really struggled to find anyone that even knew what weather comp and Divicons are, let alone the knowledge to install them. I used a guy from Southport, defo not the cheapest but very good install, and a 12 year warranty on the boiler, subject to a yearly service. Which over 13 yrs would buy a new boiler. We installed the ufh ourselves @150mm centres. Highest temp I have seen going in was 29deg c when it was -2 outside. Boiler is always condensing unless heating the tank. We have very small rads upstairs but never been turned on other to commission them. Have dual towel rails in each bedroom, electric side controlled by a Heatmiser switch, they are only 300w but work well. 

[ruggers]

On 14/06/2022 at 18:32, JohnMo said:

I really would specify everything suitable for a heat pump, it will all give advantages to a gas boiler.

 

UVC, with a heat pump coil, will accept lower flow temps, which will in turn keep you inside condensing mode of the boiler.  You can heat to circa 50 instead of 60 plus.  The heat exchanger coil is huge compared a gas boiler, which will have a better approach temperature and quicker reheat time.

 

Radiators, use double panel rads to get twice the area, or half the size.

 

I would and do have electric only towel rails, use them any time of the year without have to fire up the heating.  Just 400W elements in them, have UFH in the bathroom and en-suites.

 

Operate in weather compensation flow temps are low, boiler are super efficient, running costs low whole house an even temperature, although with suitable balancing you can get cooler rooms if you want.  Read heat geek website for some good info. 

 

 

I'll look into the cylinders soon, so many types & sizes, solar twins coils, fast recovery ones, Gledhill, megaflo and vitocell, this will determine my boiler size i guess.

 

If designing the flow temps for 50-55C with priority hot water stored lower than 60, how often does the legionella cycle need ran?

Are heat loss calcs just each room added up in watts and then totalled together after the relevant conversation factor has been applied based on your DT?  I thought my SAP would include the heat losses per room.

 

I was getting my wires crossed with load /room comp & WC. Looking further at WC, you select a flow temp, then a desired room temp, then your coldest day along with a suitable curve & it works with the outdoor sensor to modulate & maintain even temp. But is there an indoor temp sensor, how else can it know what the temp is inside your house and instruct the boiler to turn off once your desired temp has been met if it only has the water flow temp to measure from?

 

23 hours ago, Beckside said:

I really struggled to find anyone that even knew what weather comp and Divicons are, let alone the knowledge to install them. I used a guy from Southport, defo not the cheapest but very good install, and a 12 year warranty on the boiler, subject to a yearly service. Which over 13 yrs would buy a new boiler. We installed the ufh ourselves @150mm centres. Highest temp I have seen going in was 29deg c when it was -2 outside. Boiler is always condensing unless heating the tank. We have very small rads upstairs but never been turned on other to commission them. Have dual towel rails in each bedroom, electric side controlled by a Heatmiser switch, they are only 300w but work well. 

Yes most locally don't fit divicons & LLH. Do divicons work with any boiler or viessmann only? I had a look at the mixed ones, do they just fine tune the water temperature as an extra on top of the boiler modulation? 

I'll size the rads myself but I'll get a company like continel or wunda to do an UFH plan, I was going to go for 150mm centres, someone said I might need 100mm for future proofing for ashp? 

 

Out of interest, what size and model tank did you select to go with the W200.

[JohnMo]

legionella, not really sure that's needed in the UK as all supplied water is disinfected with chlorine.

 

OSO make some good cylinders with very low heat losses.

 

SAP calcs should give total house instead of room by room.  In the boffins corner on this site is a heat loss calculator.

 

Simplistically in weather compensation mode the boiler runs 24/7, it manages the system flows by looking at flow and return temps.  Once the delta between the two temps reduces, it shuts down the boiler, but continues circulation of CH water.  Once the temperature difference increases it fired the boiler.

 

My manual says

Explanation when used as a 100% weather dependent controller

The WiZe thermostat can be set to work as a 100% weather dependent controller. This means that

the room in which the thermostat is located is no longer imperative for temperature control. Together

with the information from the outside temperature, a central heating (CH) flow temperature is worked

out which is sufficient to heat the house. Thermostat valves on the radiators allow an individual

temperature setting for each room.

Required installation parts:

a. Outdoor sensor (boiler-specific) connected to a boiler (refer to boiler’s installation instructions).

Mounting on north – north/east facade of the house. Prevent outside influences such as snow,

ventilation air or chimney heat.

b. Thermostatic radiator valves on the radiators in the house to individually control the rooms.

c. If ALL radiators are fitted with thermostatic radiator valves, the installation MUST be fitted with an

automatic bypass valve. Refer to to boiler installation manual.

 

I take the thermostatic valves are there more as a limit stop on temp, so if you want the room at 21, you have balanced the system to give you 21, you set the thermostat at 22, so if the sun's shining in the window the heating is switched off in that room.

 

[ruggers]

Sorry for the delay, I follow topic but not getting notifications without checking.

I believe legionella has to be considered with new regs for builds for a number of years now. Either the water needs stored at a higher temp than 62C (ish) or set on some timer program that boosts it every 48 hrs/week/month, unsure of the interval. This is why I wanted to find the easiest solution if i must use an UVC. Having TMV at 5 sinks outlets and 2 baths then trying to hide them but also access them for replacement or servicing seems a right pain, I don't know how long they last.

 

So a couple of WC questions on what you've put.

-Does the heating pump run 24/7 in winter, or does it just over run for a period after the burner turns off?

-Do you still have a program to turn it off at night or in summer. The house inside is often below 20C in summer but I wouldn't want it calling for heat.

-Can you set more than one temperature for the day. Eg: My morning temp is set at a lower temp than at 5pm > in winter, then night is a set back.

-This ones confusing me...If you set your WC desired room temp at 20C, does that mean every room has to be 20C? I personally don't need my kitchen as hot as the living room for example. If using radiators, I guess you set the boiler for the warmest room & can then turn down other rooms using TRV's. Not sure how that works for UFH though.

-What is if you wood burner fitted, WC won't know whether it's on or off? 

[JohnMo]

WC, the circulation pump runs 24/7. Heat pump or boiler will kick in to manage the flow and return temp.

 

Ours has a summer set point, so you set the outside temp you want the system not to heat.

 

Ours allows three different set points, for temp, which really means the flow rate is reduced or increased.

 

You can get different temperatures in different rooms by balancing the system, reduce the flow rate in the rooms you want cooler.

 

Sun out or wood burner on, will affect the return temperature as not so much heat is being transferred to the room.  So the higher return temp will tell the boiler it can switch off, the circulation pump stays on.

 

I have made some mods to our heating system and started the heating up, the return temps from the floor at start up were 25 degC.  Had the CH running for an hour boiler didn't fire up as the return temps were too hot.  Had to increase the boiler set point much high to get the boiler to fire up.

[PeterW]

3 hours ago, ruggers said:

 

I believe legionella has to be considered with new regs for builds for a number of years now. Either the water needs stored at a higher temp than 62C (ish) or set on some timer program that boosts it every 48 hrs/week/month, unsure of the interval

Only in commercial installs, not domestic. 

 

3 hours ago, ruggers said:

Having TMV at 5 sinks outlets and 2 baths then trying to hide them but also access them for replacement or servicing seems a right pain, I don't know how long they last.

You don’t ..? You install one pretty much at the outlet from the UVC and blend down there. 

[ruggers]

7 hours ago, JohnMo said:

WC, the circulation pump runs 24/7. Heat pump or boiler will kick in to manage the flow and return temp.

-I didn't think the pump running constantly would be very efficient or allow it to last long, thought there might be an off period in the night like when your set back temp is on or does the heating come on in the night also?

7 hours ago, JohnMo said:

You can get different temperatures in different rooms by balancing the system, reduce the flow rate in the rooms you want cooler.

Is it best to set up WC room temperature for the room you want warmest & then reduce flow to the rooms you want cooler with UFH & turn down TRV's on radiators?

7 hours ago, JohnMo said:

 

Sun out or wood burner on, will affect the return temperature as not so much heat is being transferred to the room.  So the higher return temp will tell the boiler it can switch off, the circulation pump stays on.

So if you put the wood burner on, the return temps at the boiler will be detected and turn off or reduce the heating to the rest of the house? I think this is where room/load compensation might suit me more than WC. I can understand WC has great comfort levels, but a log burner or hot kitchen can affect the heating to the other rooms.

 

4 hours ago, PeterW said:

Only in commercial installs, not domestic. 

Thats good to know, whats the best temperature to store hot water so people don't scald their hands at the sinks?

4 hours ago, PeterW said:

You don’t ..? You install one pretty much at the outlet from the UVC and blend down there. 

I keep seeing mixed info on this and it stating commercial & domestic new build from 2010 require TMV's on baths to comply with part G of the regs. I always thought they had to be within a certain distance of the outlet otherwise the tank seems a good place. 

[JohnMo]

8 hours ago, ruggers said:

-I didn't think the pump running constantly would be very efficient or allow it to last long

 

If you run mechanical equipment in a steady state condition there is nothing to wear so the equipment lasts a very long time.  Each time you stop or start mechanical equipment there is wear of the moving parts; as lube oil and mechanical tolerances get to their design conditions, the same is also true of stopping mechanical equipment.  

[PeterW]

10 hours ago, ruggers said:

I keep seeing mixed info on this and it stating commercial & domestic new build from 2010 require TMV's on baths to comply with part G of the regs. I always thought they had to be within a certain distance of the outlet otherwise the tank seems a good place. 

Nope you have to limit the water to less than 45°C (IIRC and nowhere near my regs books) at a bath tap but there is no limit on where you do it. If you do go the HWRC route too, no point circulating water at tank temp constantly. 
 

HWRC I would put on either a timer or proximity for a bathroom but just run kitchen and utility in 10mm as the losses are minimal then. 
 

10 hours ago, ruggers said:

Thats good to know, whats the best temperature to store hot water so people don't scald their hands at the sinks?

I’d be storing at 48° from an ASHP boosted to 65°C via E7 if I wanted additional capacity or 85°C from solar via a dump controller. 

[ruggers]

@PeterWI think I'll need a HWRC, all outlets would be within 6m of an UVC so fine without one, but the kitchen sink is around 12m away. This will increase if I use a 222-F storage combi due to different location than the tank. 

If you add a HWRC after the blending valve located at the tank and set it to desired temp 45-47C, would this maintain it's temp enough over a larger HWRC?

 

1 hour ago, PeterW said:

HWRC I would put on either a timer or proximity for a bathroom but just run kitchen and utility in 10mm as the losses are minimal then

I’d be storing at 48° from an ASHP boosted to 65°C via E7 if I wanted additional capacity or 85°C from solar via a dump controller. 

Are you suggesting to not add all of the outlets to the HWRC? 12-16m in 10mm to the kitchen sink might still take a bit of time, i thought it would give poor flow at that diameter.

As an example, I currently have 16m from a combi in 15mm & it take 50 seconds for full temp, it feels like an age.

 

Any reason for the 48c boosted to 65C when using an ASHP but lower if from a gas boiler?

[ruggers]

I've just had a thought, so a question for anyone with WC. Which does it work best with, UFH or radiators?  WC is proactive & radiators react quickly to changes, but the delays in a screeded UFH system heating an cooling would fall behind what WC was instructing it to do. The mixed rad & UFH system would work completely different.

 

Does anyone know if you can use a Viessmann boiler with load comp instead of weather comp? Strangely their website mentions the addition of an indoor temp thermostat coupled with WC, not sure how that works at all.

[JohnMo]

10 to 15mm - 10mm is a 1/3 the volume, so instead of 50 seconds for 16m you will be closer to 12 to 15 seconds for you 12m.

[dpmiller]

we've got about the same length to the kitchen tap in 10mm here and I can assure you ther's plenty of flow.

[Beckside]

Viessman on WC can still have a room influence if required. As has been stated, if you are well insulated and airtight, your whole house will have a fairly even temp, first floor circa 1,1.5deg higher than GF. Yes UFH in a screed is slow to react, but that is a benefit when in weather comp. The whole theory is gradual change, not on/off. You do not set the flow temps, the boiler does. Ours has never supplied higher than 29deg C to the ufh. You can set a legionella heating cycle from the boiler/app. Yes you can have multiple temps and times, yes you can have nighttime setback. Yes you can set an outdoor temp at which the whole system shuts off. There are a lot of variables you can adjust, once set up you shouldn’t have to touch it again, that is the beauty of it. Why would you want a wood burner if you have a correctly sized/set up, UFH?

our cylinder is 300l, we have three beds and four showers and a bath. The tank is that size as we May holiday rent the place, and if six people come back after a day on the fells and all want a shower in quick succession then that is what I figured we needed. Our system has been in over a year and works exactly as expected. 

[PeterW]

6 hours ago, ruggers said:

12-16m in 10mm to the kitchen sink might still take a bit of time, i thought it would give poor flow at that diameter.

You’d be surprised the flow at 3 bar pressure from 10mm ..!! It will be fine - probably less than a litre to get to hot.