Thick screed wobbles….

[saveasteading]

17 hours ago, Oz07 said:

in the floor slab and have no screed 

Your logic for this would be of interest. 

Are you putting insulation onto sub-base or a screed? Reinforced slab? Where does the piping go?

 

My answer:

Because it is tricky to get it flat and smooth at a robust stage of the project. Also, most people seem to have overdesigned slabs IMHO.

for me, Sub-base, 100 - 150 slab, light or no mesh , pir, pipes, screed. That gives us a good hard working surface, then plumbing and screed in controlled circumstances.

[Oz07]

 

4 hours ago, saveasteading said:

Your logic for this would be of interest. 

Are you putting insulation onto sub-base or a screed? Reinforced slab? Where does the piping go?

 

My answer:

Because it is tricky to get it flat and smooth at a robust stage of the project. Also, most people seem to have overdesigned slabs IMHO.

for me, Sub-base, 100 - 150 slab, light or no mesh , pir, pipes, screed. That gives us a good hard working surface, then plumbing and screed in controlled circumstances.

I've always just used the structural slab as the only floor when building with a ground bearing slab. Not used UFH in those though.

I normally do hardcore, blinding, dpm, insulation, dpm 100mm slab and job done. Use the self compacting concrete to get a good flat finish. Bet it would be squeaky bum time putting some crack control cuts in the slab with UFH pipes in there! I can't see the logic in putting 2 floors in but obviously each project is unique and has reasons.

[scottishjohn]

On 28/09/2025 at 17:50, marshian said:

I’m basically running it at ASHP temps for CH

will that not cause the acid in the water vapour of the exhaust gases to rot the heat exchanger very quickly 

that temp for self condensing 55c, was to stop any build up of corrosive moisture in the heat exchanger 

 or that what i was told

[saveasteading]

14 hours ago, Oz07 said:

the structural slab as the only floor when building with a ground bearing slab.

Ok. No ufh makes it much easier. But getting a good finish is the issue.

[marshian]

1 minute ago, scottishjohn said:

will that not cause the acid in the water vapour of the exhaust gases to rot the heat exchanger very quickly 

that temp for self condensing 55c, was to stop any build up of corrosive moisture in the heat exchanger 

 or that what i was told

 

Hmmm - that sounds like you are mixing up with an issue that would be a result of running a non condensing boiler at condensing temps.

 

The aim of the condensing process is to recover latent heat from the exhaust gases. This process is not a fixed value of say 55 Deg C flow temp it's a scale of recovery

 

 

The lower the return temp the higher the % of heat recovery in the zone of condensing mode. I'm targeting flow temps in the mid 20's to low 30's and I've validated the boiler efficiency to be 97 % (that's quite a fun thing to do once you get your head around how) 

 

Anyway the boiler has a stainless steel heat exchanger with a radial burner in a boiler designed to run low flow temps - the condensing process keeps the products of combustion off the heat exchanger and sends them out via the condensate drain - it really doesn't matter to the boiler if you are condensing with a return temp of 50, 40, 30 or 20 deg - the only impact it has is on the volume of condensate produced.

 

It had it's first service a month or so back and the gas engineer commented how clean it all was inside the heat exchanger (he was as interested in the boiler condition as I was)

 

I'll post up my flow temps from this morning

 

[marshian]

10 minutes ago, marshian said:

 

I'll post up my flow temps from this morning

so OAT 7.0 Deg C - Weather Compensation curve says boiler should be targeting 27.4 deg C

 

3 mins after start boiler sitting happily with 7 deg delta (flow temp on left - return temp on the right) and maintaining a 28.7 deg flow temp at min modulation (10.4 %)

 

 

at 13 mins after start boiler reaches the overshoot temp (target temp plus 3 Deg C or return temp delta below 7 deg C - I haven’t worked out which it’s using it may even be using both on either or basis) either way the boiler shuts down and waits for restart temp (Flow = Return - at 7.0 deg OAT this will be around 20.8 to 21.2) this will be in about 50 mins time

 

 

It’s the 3 Deg hysterisis of the boiler that allows me to run low flow temps - it will keep a burn going for as long as it can as long as target flow temp doesn’t climb too high or delta between flow and return doesn’t drop below 7 deg.

[JohnMo]

42 minutes ago, scottishjohn said:

will that not cause the acid in the water vapour of the exhaust gases to rot the heat exchanger very quickly 

that temp for self condensing 55c, was to stop any build up of corrosive moisture in the heat exchanger 

 or that what i was told

That is correct for a non condensing boiler. It has been law that all replacement gas boilers are replaced with a condensing boiler since 2005 and oil boilers since 2007. So a non condensing boiler should no longer be able to be bought or installed. However nearly all new boilers are installed using the same mind set as a non condensing boiler. Even though the government at the spent millions on training.

 

A condensing boiler has a heat exchanger in the flue gas stream, heating return water passes through the heat exchanger. The Flue gas is cooled. If the return water is below the flue gas dew point (53 degs) the flue gas releases water - moving from a gaseous state (steam) to water. This phase change releases energy (latent heat) which is utilised by the boiler to add efficiency.

 

A boiler with big plumes of white smoke from the flue is running above condensing temperature. The white smoke is the water vapour escaping, with energy it carries. Efficiency drop 10 to 15%, compared to running as described above by @marshian. His flue gas will be invisible or almost.

[marshian]

Just now, JohnMo said:

A boiler with big plumes of white smoke from the flue is running above condensing temperature. The white smoke is the water vapour escaping, with energy it carries. Efficiency drop 10 to 15%, compared to running as described above by @marshian. His flue gas will be invisible or almost.

 

Absolutely correct - most of the time it is impossible to tell from the flue gases that the boiler is running - When you can tell it's running is below 5 deg C OAT when a very thin plume is visible or when it's re-charging HW as the flow temp and modulation level is much higher (it starts on 50% modulation and ramps up to 100% before slowly ramping down to maintain a 7 deg delta between flow and return at whatever target flow temp I've set it to) - and after the first 10 mins of the HW cycle the return temp exceeds 55 deg C and the boiler is no longer in condensing mode and my boiler flue looks like all the other boiler flues around me - chucking out a white plume but that's mainly due to a small coil in the current HW tank meaning I can't heat HW to target temp without leaving the condensing mode.

 

It's on the list to sort - just haven't made up my mind to go with a new HW Cyl and ASHP coil or install a PHEX and circulation pump with the existing tank but we are a bit off topic now........

[marshian]

My flue on a CH cycle (just now) Boiler is running at 4kWh (min modulation 10.8%)

 

[scottishjohn]

out of interest i have done some searching and can  see NO boiler manufacturer that reccomends a boiler temp of less than 55c return temp 

 does you boiler have a temp control that allows you to set boiler temp to 30c  output ?

I am not wanting an argument .but i find it strange that al makers stick to the 55c return temp 

I am guessing this is a combi boiler ,so you do not have the leggonella problem  that would happen if you ran you DHW very low with no purge  on a regular bassis to much higher temp

I am just interested on how this will work long term and if you can have output temp of boiler so low  how will you stop it hunting  as it overshoots your set temp all the time 

the next question is if you can run  at so low a temp 

how will the running costs compare with a heat pump?

[JohnMo]

12 minutes ago, scottishjohn said:

does you boiler have a temp control that allows you to set boiler temp to 30c  output ?

Here is an copy of the weather compensation curves for our Atag boiler, can be run as low as 20 degs.

 

17 minutes ago, scottishjohn said:

see NO boiler manufacturer that reccomends a boiler temp of less than 55c return temp

This may be a boiler specifically designed for the UK market. Where S and Y plan are the norm. So it does CH and cylinder heating at a single flow temp. The cylinder is normally a small coil area, so rubbish heat transfer at low flow temperature. So to be able to run CH at low flow temps you need a boiler that runs two flow temps (like a combi) or a system boiler that is priority domestic hot water enabled. This can be a 4 pipe boiler or a system boiler set to X or W plan - not S or Y plan.

 

[marshian]

6 minutes ago, scottishjohn said:

out of interest i have done some searching and can  see NO boiler manufacturer that reccomends a boiler temp of less than 55c return temp

 

 

Sorry I've broken your quote down into bite size chunks to make it easier to reply too

 

You'll have to link me to those statements because it makes no sense - the whole point of a condensing boiler is to run them in condensing mode for that to happen the flow temp needs to be 55 deg or lower to ensure the return temp is below the dew point

 

6 minutes ago, scottishjohn said:

 does you boiler have a temp control that allows you to set boiler temp to 30c  output ?

 

 

Temp of the boiler can be set from 20 Deg C to 80 Deg C in non weather compensated mode

 

In weather compensated mode the Curve sets the flow temp and that will be between 20 deg and 80 deg  

 

6 minutes ago, scottishjohn said:

I am not wanting an argument .but i find it strange that al makers stick to the 55c return temp 

 

 

Discussion is good - it's not an argument and I have no problem explaining where I'm coming from 

 

As earlier link me up to where makers are saying stick to a 55 deg Return temp

 

6 minutes ago, scottishjohn said:

I am guessing this is a combi boiler ,so you do not have the leggonella problem  that would happen if you ran you DHW very low with no purge  on a regular bassis to much higher temp

 

Nope not a combi - I have a HW Tank heated by the boiler - however flow temp for CH and HW are not the same - I'm running DHWP so if there is a call (or Schedule) for HW to be re-charged the boiler flips from low temp CH running WC based on OAT to HW mode - where it runs an elevated temp and modulation rate to get the HW done asap.

 

I heat the tank to ~50 every day except one where I heat to 60 (Legionaire cycle) Mrs Alien likes a long, hot and deep bath once a week so as we only have  a 117 Litre HW tank I take advantage of HW requirements on that day to do a Legionaire cycle - If Mrs Alien didn't want a bath I don't bother heating the water to 60.

 

6 minutes ago, scottishjohn said:

I am just interested on how this will work long term and if you can have output temp of boiler so low  how will you stop it hunting  as it overshoots your set temp all the time 

 

Over shoot is an interesting subject - I can only assume by observation that the in the initial purge and burn on start up the flow temp exceeding the set point is ignored as the flow temp can typically hit 40 deg C in the start up phase but the boiler ramps down to min and the flow temp stabilises at very close to the target flow temp.

 

The only time the boiler "hunts" is on HW cycle for a min or two where it's trying to balance the modulation rate against the target flow temp (22mm primaries from tank coil to the boiler means at 0.6 m3/hr flow rate the rate of change is quite rapid and the boiler can get caught out by it - I really should close the gate valve a little to trim it.

 

6 minutes ago, scottishjohn said:

the next question is if you can run  at so low a temp 

how will the running costs compare with a heat pump?

 

I don't have a ASHP to compare but I can tell you costs

 

House is 4 bed Detached 113m2 Floor area across 2 floors, It's a stupid T shape so has a few rooms/areas that have 3 external walls - SUDG windows and doors - Blown CWI - 325mm of loft insulation and 75mm of PIR under the suspended ground floor

 

Calculated Heat loss is 4.5 kWh at -2.5 Deg C OAT (pre loft insulation improvements)

 

Annual Gas consumption is 9500 kWh at £0.05 per kWh

 

(Again this is historical consumption and does not take into account the recent improvements to Loft insulation - I am expecting to get back to my previous schedule based consumption of below 9000 kWh whilst heating 24/7)

 

That is broken down into 

HW 19%

CH 80% 

Cooking 1%

 

So just under £600 per year for all gas usage (including the standing charge - Take off £100 to the figure if you don't want it included) 
 

I'm sure an ASHP would be cheaper (especially if favourable tariffs were involved) but how much cheaper I don't think there would be much in it

 

However capital costs (under the BUS grant) would have been much higher compared to the boiler swap/system tweak from Y pan to X (or W plan). 

 

IHTH

[marshian]

48 minutes ago, JohnMo said:

This may be a boiler specifically designed for the UK market. Where S and Y plan are the norm. So it does CH and cylinder heating at a single flow temp. The cylinder is normally a small coil area, so rubbish heat transfer at low flow temperature. 

 

 

Absolutely ^ that - S or Y plan is middle ages approach - X or W plan enables boiler to run two independent temp profiles depending on what it's heating (CH or HW).

 

 

48 minutes ago, JohnMo said:

So to be able to run CH at low flow temps you need a boiler that runs two flow temps (like a combi) or a system boiler that is priority domestic hot water enabled. This can be a 4 pipe boiler or a system boiler set to X or W plan - not S or Y plan.

 

 

Or even a basic "Heat Only" boiler set up to run DHWP - doesn't need to be a system boiler - just needs to know if it's doing HW or CH and adjust the temp and modulation priority accordingly.

 

Out of the box my Viessmann "heat only" boiler is a dumb boiler - runs one flow temp for everything - for HW I need a min of 64 Deg C so I would have to run the CH circuit at that same temp except that's high temp heating and I have mostly K22 rads or verticals in the house and one K33 rad. They don't need 64 deg C flow temps they need 35 Max.

 

So the solution was to "Add" HWD (Hot Water Demand) box and an "outside temp sensor" and tell the boiler it's no longer a dumb boiler capable of only running one flow temp for everything but to run WC flow temps when doing CH and when it's asked to do HW to get it done in the shortest possible time with the flow temp selected for HW - in my case 64 deg for summer and 70 deg for Winter * see note

 

Note due the variables between summer and winter

 

1. Tank losses being higher to surrounding temps lower - 25 deg v 19 Deg

2. Incoming water temp being 10 deg in winter compared to 20 in summer

3. I want to heat the water in 30 mins max during the heating season (so I can use Octopus download to provide exact water heating energy consumption  - Outside the heating season I'm only heating water or cooking and water heating is done in the morning and cooking in the evening I can see my consumption clearly 😉)

Edited September 30 by marshian
Clarification

[marshian]

54 minutes ago, marshian said:

1 hour ago, scottishjohn said:

I am just interested on how this will work long term and if you can have output temp of boiler so low  how will you stop it hunting  as it overshoots your set temp all the time 

Expand  

 

Over shoot is an interesting subject - I can only assume by observation that the in the initial purge and burn on start up the flow temp exceeding the set point is ignored as the flow temp can typically hit 40 deg C in the start up phase but the boiler ramps down to min and the flow temp stabilises at very close to the target flow temp.

just by way of illustration have a sequence of screenshots on fire up  

 

initial fire - boiler modulation is 58.2 % (58.2 % = ~12 kW)

 

flow temp is 40.9 Deg C

 

 

30 secs later

 

modulation has dropped to 30.9 % = ~8 kW

 

flow temp hasn’t dropped yet (due to initial heat up in HEX)

 

1 min later

 

modulation has dropped to 11.5 % (so ~5kW) 

 

flow temp is dropping rapidly to 30 Deg C

 

 

2 mins after start up 

 

Modulation is at min 10.8 % (so ~4 kW)

 

flow temp stabilised at 27.7 deg C 

 

 

It’s 14 deg C outside on North side of the house where the OAT is taken so the boiler will do a burn of around 8 mins before shutting down for around 60 mins 

 

Point to note - the boiler isn’t heating the house - it’s just replacing the heat that is being lost - all rooms are at or very close to target temp.

 

fires per day are around 15 right now

 

The burn and coast process is the same in shoulder season as it is in depths of winter (the only change is to the time the boiler runs for) 

 

at -5 outside last winter the boiler was running for 55 mins followed by a 5-10 min coast 

 

fires per day were 22 - 25 
 

probably very similar cycles to an ASHP would operate in this house

 

[JohnMo]

1 hour ago, marshian said:

probably very similar cycles to an ASHP would operate in this house

This why they state to get a heat pump close to the actual heat loss and with good modulation. Then the cycling reduces you get less standby losses etc.

 

3 hours ago, scottishjohn said:

the next question is if you can run  at so low a temp 

how will the running costs compare with a heat pump

I've run gas boiler, hybrid and just ASHP only thing changed is the heating source.

 

For us the cheapest is ASHP, by a considerate margin. We have PV, battery and use octopus cosy,  so we are on average paying 13p to import electricity, get 15p to export, which at this time of the year, off sets import costs by about 50%. Add into the mix solar generated. So overall cost this week for all electric for the house was 2p per kWh. So already way cheaper than gas without taking into account the standing charges for gas, and CoP gain by ASHP. Obviously in the depths of winter PV is rubbish. But even so a CoP of 3, is still cheaper than gas, plus you are not paying a second standing charge.

 

[marshian]

I think to bring this thread back round to topic and thickness of screed - to me it’s just making your “heat emitter” aka radiator as big as possible with the ability to soak up temp changes in the environment around us whilst treading lightly with it’s energy consumption.

 

As @JohnMo stated so well you need to optimise whatever heating process you use to cover your needs based on heat loss, occupancy, HW needs and cost - there isn’t a one size fits all - careful thought, some maths, and considered purchases as far as kit/rads, UFH, heat source and HW storage (volume and method of recharge) is required and we all have a duty of care to the planet and it environment to do as little damage as we can - maybe even leave it in a better shape that it was when we arrived on this earth with de-carbonisation.

 

I look around the estate I live on only 67 houses (mix of 3 and 4 beds) and see plumes from every flue first thing in the morning and early evening - so much energy wasted that could be harnessed/recovered with low temp heating.

 

multiply that out on every housing estate in the country and our emissions and energy consumption would drop.

 

oil and gas companies wouldn’t like it much but there is little they could do about it except cut their cloth accordingly……….

 

I’ll get of my soap box and get back to painting fences on what has turned out to be a very nice afternoon

[saveasteading]

30 minutes ago, marshian said:

off my soap box and    back to painting fences

Can you reach the top without your box to stand on? 

 

 

[marshian]

9 minutes ago, saveasteading said:

Can you reach the top without your box to stand on? 

 

 

Yes comfortable - being 5’ 13” in my socks helps with stuff lije that

[G and J]

Handy things, those platform socks. 
 

Odd innit.  One never knows what a thread will do when one asks a question.  I love this forum even though I currently spend v little time on it. 

[marshian]

1 hour ago, G and J said:

Handy things, those platform socks. 
 

Odd innit.  One never knows what a thread will do when one asks a question.  I love this forum even though I currently spend v little time on it. 

 

 

 

I have politely asked @Nickfromwales To do a quick clean up and move non relevant conversation that related to my boiler set up to my Viessmann Journey thread