Boiler kw sizing

[Loz]

Hi,

 

In middle of renovation, at the end will be a 4 bed detached house (1950's) with reasonable insulation (new extension will be to new regs).  There will be two electric showers and additional one from the hot water tank.  There are two zones of wet underfloor heating (zone 1 16m squared, zone 2 24m squared). Going for a system boiler with unvented indirect 210 litre cylinder.  We have 13 radiators.  Plumber wants to go for a Baxi megaflow which looks good but has been suggesting 24kw model, looking online that looks underpowered, they also do a 28 and 32, a friend in similar setup but with much larger tank has the 32.  What would people recommend in this scenario ?  Any help greatly appreciated

 

Many Thanks,

 

Lawrence

[Nickfromwales]

In heating mode you will never get anywhere near to using all of the 24kW unless you have the house freezing cold and want to heat it to 22oC ASAP. Nominal heat from a wall mounted boiler would be 12-14kW to heating ( max ) and then whatever you have for DHW will dictate how fast the cylinder can reheat ( aka "recovery" time ).

To be able to heat the cylinder and run the heating simultaneously it could be beneficial to go up to the 28kW as the minimum heat output ( maximum modulation ) will be pretty much the same, or you could stick with the 24kW and go W-plan ( domestic hot water priority ) which would allow either the heating OR the DHW only, and never the two together. The biggest issue you'll have here is running the UFH separately from the radiators. As these are two completely different beasts the plumber would normally specify a room stat for each UFH zone, with a 2-port zone valve shutting the UFH on / off respectively which then islands the UFH from the rest of the system. The cylinder and rads would also have 2 port zone valves that isolate them in line with heating and hot water demand.

If the UFH is running and nothing else, then the boiler will short cycle. To deal with that, you would need a 50L wall mounted / floor mounted buffer tank across the flow and return to the UFH manifold AFTER the UFH zone valve ( so it does not act as a bypass when the UFH is off and heat unnecessarily ).

This is multi-faceted as always!

Q: If you have 2 electric showers, and only one that uses DHW, then IMHO it is a waste of time and money to fit a system boiler and cylinder. Why don't you just fit a high flow combi such as a Vaillant 938? That will exceed you DHW needs for sure and then you don't need a lifetime of annual G3 service callouts for the UVC.

[Loz]

3 minutes ago, Nickfromwales said:

In heating mode you will never get anywhere near to using all of the 24kW unless you have the house freezing cold and want to heat it to 22oC ASAP. Nominal heat from a wall mounted boiler would be 12-14kW to heating ( max ) and then whatever you have for DHW will dictate how fast the cylinder can reheat ( aka "recovery" time ).

To be able to heat the cylinder and run the heating simultaneously it could be beneficial to go up to the 28kW as the minimum heat output ( maximum modulation ) will be pretty much the same, or you could stick with the 24kW and go W-plan ( domestic hot water priority ) which would allow either the heating OR the DHW only, and never the two together. The biggest issue you'll have here is running the UFH separately from the radiators. As these are two completely different beasts the plumber would normally specify a room stat for each UFH zone, with a 2-port zone valve shutting the UFH on / off respectively which then islands the UFH from the rest of the system. The cylinder and rads would also have 2 port zone valves that isolate them in line with heating and hot water demand.

If the UFH is running and nothing else, then the boiler will short cycle. To deal with that, you would need a 50L wall mounted / floor mounted buffer tank across the flow and return to the UFH manifold AFTER the UFH zone valve ( so it does not act as a bypass when the UFH is off and heat unnecessarily ).

This is multi-faceted as always!

Q: If you have 2 electric showers, and only one that uses DHW, then IMHO it is a waste of time and money to fit a system boiler and cylinder. Why don't you just fit a high flow combi such as a Vaillant 938? That will exceed you DHW needs for sure and then you don't need a lifetime of annual G3 service callouts for the UVC.

 

Wow, thanks for detailed reply, that really does help explain things, arms me with some questions for the plumber on whether to go up to the 28 to run the two simultaneously, we will always likely have the UFH and radiators on at same time so guess no need to isolate although has made me start to think on that as believe UFH takes longer to heat up so likely would want that to come on earlier - mmm not sure now.

 

On the Combi question that was original plan, only problem we have is the boiler is in the garage and whilst close to kitchen is a long way from the upstairs bathrooms so was concerned on how long would take hot water to get to the basin taps.  The tank is situated between the three bathrooms giving quick hot water and also my partner really wants the luxury of pressured hot shower which will be the main one fed from the tank, I'll be honest I am a bit torn over the whole setup now.  We do have an airing cupboard for tank to go in (replacing ancient one connected to tanks in loft which had to be removed as part of renovation although could conceivably be recommissioned elsewhere in loft).

 

Many thanks,

 

Lawrence

[Nickfromwales]

4 minutes ago, Loz said:

UFH takes longer to heat up so likely would want that to come on earlier

Nail > head.

Also vice versa, it will want to shut off sooner too ( if you have self-learning energy saving controls or manually manipulate that to allow the residual heat to ebb away ). Those are referred to as set-back controls.

The new combi could go where the cylinder is now, and just go for a vertical flue. Done this hundreds of times. The kitchen tap may be the pain then though, but should be no worse than what you get now as the run would then be in 15mm not 22mm ( subject to you making that alteration to the existing plumbing of course  ) so less primary dead legs of water to discharge before getting hot water to the furthest outlets.

[Loz]

7 minutes ago, Nickfromwales said:

Nail > head.

Also vice versa, it will want to shut off sooner too ( if you have self-learning energy saving controls or manually manipulate that to allow the residual heat to ebb away ). Those are referred to as set-back controls.

The new combi could go where the cylinder is now, and just go for a vertical flue. Done this hundreds of times. The kitchen tap may be the pain then though, but should be no worse than what you get now as the run would then be in 15mm not 22mm ( subject to you making that alteration to the existing plumbing of course  ) so less primary dead legs of water to discharge before getting hot water to the furthest outlets.

 

Thanks for response, will definitely ask about the underfloor heating separately from radiators.  Getting gas to the current cylinder position would be tricky as would the flu as will double check but no easy access to external wall and upwards would be a problem but will take a look this afternoon.

[Nickfromwales]

?

[SteamyTea]

How low can the 24, 28 and 32 kW models modulate down to?

 

Don't think that running UFH is the same as radiators. The ability to modulate down to a few kW, and still be in condensing mode, is better than switching off and on again.

[Nickfromwales]

44 minutes ago, SteamyTea said:

How low can the 24, 28 and 32 kW models modulate down to?

 

Don't think that running UFH is the same as radiators. The ability to modulate down to a few kW, and still be in condensing mode, is better than switching off and on again.

The Worcester Bosch Lifestyle boilers can now modulate down to 3kW which is very impressive. Before, with the Greenstar range, they only got as low as 7kW. 
About 5kW for the current Baxi’s iirc. 

 

[SteamyTea]

10 minutes ago, Nickfromwales said:

The Worcester Bosch Lifestyle boilers can now modulate down to 3kW which is very impressive

That is pretty good. Does it still condense efficiently at that low rate?

 

[Nickfromwales]

1 hour ago, SteamyTea said:

That is pretty good. Does it still condense efficiently at that low rate?

 

Depends what temp the heat exchanger stays at I suppose. Honest answer = I’m not sure. I know the fan speed slows in line with modulation, so in theory the HeX temp should stay high enough in the right range.

Edited September 23, 2021 by Nickfromwales
Edit as per strikeout

[SteamyTea]

11 minutes ago, Nickfromwales said:

Honest answer = I’m not sure

The manufactures are probably tied by legislation, so should still be within the condensing temperature zone, or it would make a mockery of the whole thing.

No good having a boiler that only condenses when the temperatures are spot on, wind is not blowing up the exhaust and the sun in behind the yard arm.

[fezster]

Heating a HW tank and running CH at the same time is always a compromise. The flow temp required to fully heat a HW tank to 60-65 degrees is 70-75 degrees - this means that whilst the CH is ramping up to temperature, the HW tank may in fact be losing heat to the lower flow temps running through it's coil until the CH circuit has heated up sufficiently.

 

Much preferred is what Intergas refer to as x-plan. This is HW priority with modulating controls to boost the boiler to max temp (eg. 80 degrees) to heat the cylinder up as quickly as possible: https://www.intergasheating.co.uk/consumer/2019/11/05/tech-talk-3/

 

Most unvented cylinders have a < 30 mins recovery time with ~20KW coils, and some of the more modern high gain cylinders have much larger coils to heat up in half that time. The added advantage is that you can then run much lower flow temps on milder days by using weather compensating controls, whilst still heating your cylinder up adequately.

[SteamyTea]

4 hours ago, fezster said:

Heating a HW tank and running CH at the same time is always a compromise. The flow temp required to fully heat a HW tank to 60-65 degrees is 70-75 degrees

If done with a HP, it is almost certain that the axillary resistance element will be on at those temperatures.

Though some of the CO₂ HPs should be OK.

[Adsibob]

On 23/09/2021 at 11:55, Nickfromwales said:

If the UFH is running and nothing else, then the boiler will short cycle. To deal with that, you would need a 50L wall mounted / floor mounted buffer tank across the flow and return to the UFH manifold AFTER the UFH zone valve ( so it does not act as a bypass when the UFH is off and heat unnecessarily ).

I know your standard advice is always to install a buffer tank for the UFH to avoid short cycling, but I'm just trying to understand if that is always the case. For example, if I have a gas boiler that has very high modulation range of up to 1:19 (such as the latest Veissman Vitodens 35kw), wouldn't the boiler simply be able to modulate right down to the low temp requirements of the UFH?  1:19 is pretty unique on the residential boiler market and enables the 35kw version of the boiler to run as low as 1.8421kW. By comparison Vaillant and Baxi have modulation ranges of about 1:6 or maybe 1:10. 

[SteamyTea]

9 minutes ago, Adsibob said:

  1:19 is pretty unique on the residential boiler market and enables the 35kw version of the boiler to run as low as 1.8421kW

What happens if the amount of fluid in the pipes is below the recommended amount?

What happens if the heat load needed is only 0.4 kW?

Good engineering practice tries very hard to not work at the extreme capability if the equipment.

[Adsibob]

2 minutes ago, SteamyTea said:

What happens if the amount of fluid in the pipes is below the recommended amount?

What happens if the heat load needed is only 0.4 kW?

Good engineering practice tries very hard to not work at the extreme capability if the equipment.

I get what you mean, but is that likely to arise in practice? I will have about 500m length of 16mm pipe for my in-screed system at ground floor. At least 3/4 of that will be on for most days 4 months a year, probably for at least 9 hours a day if not longer. The house is fairly well insulated, but not to passive standards (it's a 1930s house which we've gutted and upgraded) so during winter's shoulder months I imagine we will still have the system on throughout most of the ground floor for at least 4 hours a day. I'm not yet sure how much we will have the UFH on in the first and second floors, but imagine it will come on on the first floor for a few hours in the evening and a few hours in the morning during winter, whereas on the second floor it will hardly ever be used. Although it's a multizone system, it's unlikely that we will ever have one small zone on only. So I'm just trying to understand how often it will be the case that the UFH will need less than 1.84kW?

[SteamyTea]

7 minutes ago, Adsibob said:

So I'm just trying to understand how often it will be the case that the UFH will need less than 1.84kW?

What does your room by room thermal calculations show?

[Adsibob]

24 minutes ago, SteamyTea said:

What does your room by room thermal calculations show?

Unfortunately I have done zero thermal modelling (other than some calculations to make sure our loft conversion gave us a decent decrement delay). Does anybody have a template excel spreadsheet I can use for this, or recommend an online calculator. I appreciate it's quite involved and usually done by professionals, but I don't have money for that and rather try and cobble together a basic version myself. 

[PeterW]

Register for the Stelrad STARS app and you can do room by room calcs. 

[Nickfromwales]

14 hours ago, Adsibob said:

Unfortunately I have done zero thermal modelling (other than some calculations to make sure our loft conversion gave us a decent decrement delay). Does anybody have a template excel spreadsheet I can use for this, or recommend an online calculator. I appreciate it's quite involved and usually done by professionals, but I don't have money for that and rather try and cobble together a basic version myself. 

Walk into most plumbers merchants with room sizes and window / doors / ceiling heights etc and tell them you need a quote for the whole house for new radiators. They'll size each room for you there and then in most instances, and be insistent on them doing it at the counter so you can make notes ( just tell them some rooms need ornate radiators and waste some time flicking through catalogues asking would that rad be ok for this room etc whilst you make notes  ).

[Nickfromwales]

15 hours ago, Adsibob said:

I get what you mean, but is that likely to arise in practice? I will have about 500m length of 16mm pipe for my in-screed system at ground floor. At least 3/4 of that will be on for most days 4 months a year, probably for at least 9 hours a day if not longer. The house is fairly well insulated, but not to passive standards (it's a 1930s house which we've gutted and upgraded) so during winter's shoulder months I imagine we will still have the system on throughout most of the ground floor for at least 4 hours a day. I'm not yet sure how much we will have the UFH on in the first and second floors, but imagine it will come on on the first floor for a few hours in the evening and a few hours in the morning during winter, whereas on the second floor it will hardly ever be used. Although it's a multizone system, it's unlikely that we will ever have one small zone on only. So I'm just trying to understand how often it will be the case that the UFH will need less than 1.84kW?

How long is a piece of string question as far as system design goes, but with that level of modulation I very much doubt you'd have any issues other than maintaining a minimum flow rate for the primary circulation pump. I would suggest that a single towel rad running in parallel with the heating load would more than suffice with that boiler.

[Adsibob]

4 hours ago, Nickfromwales said:

How long is a piece of string question as far as system design goes, but with that level of modulation I very much doubt you'd have any issues other than maintaining a minimum flow rate for the primary circulation pump. I would suggest that a single towel rad running in parallel with the heating load would more than suffice with that boiler.

This is very reassuring thank you. We will have three towel rails. I have only bought one of the towel rails so far, it is rated at 1607 btu (471W). 
Would you say that is about the right size to act as my pseudo buffer? I ask because the remaining two towel rails could be off a similar size, or slightly bigger, and I could run any one of the three as the pseudo buffer.

[Nickfromwales]

More the merrier tbh, so maybe put a ground floor rad on bypass ( no TRV ) and possibly fit the other two with TRV's as upstairs is likely to need some thermostatic control but downstairs should be fine to heat to 'infinity' without control and without any nuisance. 

[Loz]

Hi,

 

My original thread went off track a bit but I am back to my original problem,  I need to chose boiler size by Monday, the plumber is not going to do any heat loss calculations or do anything to work out accurately so I am on my own other than the expertise of this forum.  The plumber had said he thinks the baxi 824 (24kw) system boiler is fine, annoyingly they don't do a 28 in that range.  Could go for a 28kw baxi megaflow but an older boiler with less of a guarantee.  There is a Baxi platinum but that only comes in a 32kw model.  My preference is for the 824 24kw boiler if can cater for the house (there will be no further additions once renovation is complete).

 

Here is summary of the house:

 

House is 1955 detached house with cavity wall insulation, it is 4 bedroom two of which added to new building regs so well insulated.  House is not particularly cold in that there is only one room where radiator is on full in winter.

 

At the end we will have 13 radiators, many of them quite small, have done some calcs and KW of them all seems to add up to on 12kw - seems low as guidance seems to be 1.5kw per rad but a few of them are towel rails and small radiators.  Worse case guess would be 16kw.

 

We will also have 2 zones of under floor heating first zone 1 is 13m squared and second zone 2 is 24m squared using 16mm pipe.

 

We have 3 showers but 2 electric so only one will be running off tank, did consider combi but would be long way from where water needed and decided on the system approach with unvented tank in centre of house.

 

We will be going for a 210 litre indirect unvented tank from gledhill - https://www.gledhill.net/products/unvented-cylinders/stainlesslite-system-ready-indirect/, the manual lists the 210 tank having a primary coil rating of 20.5 kw - not sure if reading that correctly as cylinder calculations normally allow 3-5kw.

 

Any thoughts greatly appreciated.....

 

Lawrence
 

[Nickfromwales]

24kW will suffice. I've seen the ancient Ariston 23MFFI's ( micro--combi ) doing a church vestry and a bunch of much bigger rads after them, which is about as adverse as it gets. Don't be tempted by the 32 it's just not needed.

If DHW isn't a huge priority ( as you have only one mixer ) then it'll be fine when servicing both, but my recommendation would be to rid yourself of at least 1 of the remaining 2 electric showers and get the hot water flow up via the UVC. If you find you're struggling for DHW then, just change from S--plan to W-plan and you'll be fine.