Boiler kw sizing

[Loz]

Many thanks for quick reply, much appreciated, gives level of confidence in the 24kw model,

 

Cheers,

 

Lawrence

[Nickfromwales]

Go buy your boiler lol.

[Loz]

On 08/10/2021 at 18:05, Nickfromwales said:

Go buy your boiler lol.

Last question - Promise !  Looking at the Baxi 824 it has a 7L integral expansion tank - the megaflow 28 and platinum 32 have 10L, I have 13 radiators and bit less than 40m2 of underfloor heating and pipework running every where - is 7L big enough - not sure how to guesstimate ?!

 

Many thanks,

 

Lawrence

[Nickfromwales]

You can never have too much expansion. The MI’s for the boiler will suggest that a system volume calculation be done to ascertain the total volume / temp / expansion and more is added to meet the system requirements. I would say you’d be around 12-14L to allow a good chunk of headroom, so adding another 7-8L would be what I would be doing if I was fitting that lot. Others may want to scrimp and save and do a calculation to just give enough and no more, but a bit of redundancy is always a good provision afaic. Also means less frequent topping up on the filling loop too. 

[Marvin]

21 minutes ago, Nickfromwales said:

a bit of redundancy is always a good provision afaic. 

+1 and less stress on the whole system is a good idea. 

[Marvin]

I also usually install a PRV - pressure reducing valve - on an incoming water main for the same reason.

 

Mains water pressure fluctuates, and is usually higher than required in the home, putting unnecessary stress on every tap and valve connected if one is not fitted.

 

 

 

 

[James Newport]

My plumber just fitted a standalone expansion tank for this reason. 

[Loz]

Thanks for all replies, sounds like a need to nudge plumber into fitting an external expansion vessel - for my info is the external vessel in addition to internal one in volume or does the external one mean internal one is redundant ?

[James Newport]

It's additional.

It is literally a little tank that gets plumbed into a heating pipe and provides space for water to expand into. 

[Loz]

13 minutes ago, James Newport said:

It's additional.

It is literally a little tank that gets plumbed into a heating pipe and provides space for water to expand into. 

Thanks for clarifying

[Nickfromwales]

4 hours ago, Loz said:

Thanks for clarifying

It can also be mounted anywhere in the system as the original one is always specifically protecting the appliance  

[fezster]

Generally, it is better to pipe the EV on the return to the boiler. If on the flow, there is a chance of reverse circulation.

 

Also, if you require an external EV, I'd recommend a heat-only boiler - this has no integral pump or EV. This makes future maintenance far easier, as you do not need to open the boiler to change either of these and / or check the EV pressure. You can also size the pump and EV to your requirements.

Edited October 13, 2021 by fezster

[Adsibob]

On 05/10/2021 at 18:34, PeterW said:

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

I'm doing the calculation now. It asks me to insert the outside air temperature. What shall I put in? Below are the average winter high and low temperatures in my area:

 

December                   8° / 2°

January                       7° / 2°

February                     8° / 2°

 

[Adsibob]

Actually, can't even get the Stelrad STARS app to work; regardless of what temperature I put it, there is some bug on the website that doesn't let me save and click through to the next page.

[hendriQ]

On 06/10/2021 at 20:08, Nickfromwales said:

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. 

Speaking to my plumber today about @Nickfromwales’s idea of using a radiator running to infinity wherever the UFH is on, as a pseudo buffer. He said that with a boiler that modulates to 1/17th of its output, given we are having a low loss header, this is unnecessary. He is happy to do it, but he thinks I’m wasting money and space on unnecessary over-engineering. I’m going to have a gas system boiler, probably rated at 25kw (though he is trying to persuade me to buy a bigger 32kw boiler, “to be on the safe side”).

 Thoughts on whether the low loss header negates the need for any type of buffer in this case?

Edited November 18, 2021 by hendriQ

[Nickfromwales]

1/17th mod rate means that the boiler will very likely never short cycle. Having a LLH pretty much ticks every box so I agree with your plumber. He sounds a good guy. ?

[Nickfromwales]

You don’t need 32kW imho, but I would do W plan if you have an UVC. 

Edited November 18, 2021 by Nickfromwales

[markocosic]

How short is short for short cycling?

 

Can you control hysteresis (integrals even?) on the boiler such that flow temperature in CH mode is allowed to vary a lot (ufh not caring, rads not caring much either) to mitigate that short cycling in CH mode?

 

Probably needs boiler OEM thermostat to support differing types of call for heat?

 

Else set flow temp and the anti cycle delay to prevent firing again unless you're temp head dropped enough. Eg flow temp 55 (for rads), anti cycle delay 30 mins, and if only ufh is on and the temp hasn't dropped enough after 30 mins it just waits another 30 and sees if it's low enough. Ufh won't much care about the incoming unmixed flow to varying. Rads will pull the flow temp down fast enough that the anti cycle timer has no effect?

[SteamyTea]

1 hour ago, Nickfromwales said:

Having a LLH pretty much ticks every box so

Explain why please.

I understand that they control flow rates, regardless of inputs and outputs.

I may have misunderstood it though.

[Nickfromwales]

9 hours ago, SteamyTea said:

Explain why please.

I understand that they control flow rates, regardless of inputs and outputs.

I may have misunderstood it though.

Combination heating arrangement here with a mix of rads and UFH, so the LLH provides both hydraulic separation, plus somewhere for the boiler pump to 'run off' to ( whilst it lights / pumps out an initial wedge of heat energy / waits to register the return temp to decide on the required amount of modulation ).

It'll act as a hydraulic buffer, but also an accidental energy buffer too.

[hendriQ]

12 hours ago, Nickfromwales said:

You don’t need 32kW imho, but I would do W plan if you have an UVC. 

So went through my heat loss calcs again and came to about 21.25kW for heat requirements of the house, though this is an estimate so could be out by 10% or so (though my margin of error is also an estimate, lol). Then I checked the requirements of our 300L UVC indirect cylinder and that is 15.5kW. So adding those two up gives 36.7kW. The boiler range we're going for comes in either 25kW or 32kW. Appreciate it's rare we'll have to heat the UVC from completely cold at the same time as running the heat at full blast, but I thought 25kW just seemed a bit too low and the boiler installer was pushing me to go bigger given the very impressive modulation range of the Vitodens 200W series. Am I making the right decision?

[Nickfromwales]

18 minutes ago, hendriQ said:

So went through my heat loss calcs again and came to about 21.25kW for heat requirements of the house, though this is an estimate so could be out by 10% or so (though my margin of error is also an estimate, lol). Then I checked the requirements of our 300L UVC indirect cylinder and that is 15.5kW. So adding those two up gives 36.7kW. The boiler range we're going for comes in either 25kW or 32kW. Appreciate it's rare we'll have to heat the UVC from completely cold at the same time as running the heat at full blast, but I thought 25kW just seemed a bit too low and the boiler installer was pushing me to go bigger given the very impressive modulation range of the Vitodens 200W series. Am I making the right decision?

With W plan, yes, with S-plan you'd be better off with the 32 I suppose, but there would be very little in it AFAIC.

The old Baxi 552's had 10-16kW max rating and these would be doing 10 rotten ( non-convector ) radiators for decades, in draughty 1g glazed houses with a small hot water cylinder also.

The 24 will suffice either way, just these days everyone gets in a flap and buys the bigger boiler to 'be on the safe side'. 

Edited November 18, 2021 by Nickfromwales

[fezster]

On 18/11/2021 at 21:39, hendriQ said:

So went through my heat loss calcs again and came to about 21.25kW for heat requirements of the house, though this is an estimate so could be out by 10% or so (though my margin of error is also an estimate, lol). Then I checked the requirements of our 300L UVC indirect cylinder and that is 15.5kW. So adding those two up gives 36.7kW. The boiler range we're going for comes in either 25kW or 32kW. Appreciate it's rare we'll have to heat the UVC from completely cold at the same time as running the heat at full blast, but I thought 25kW just seemed a bit too low and the boiler installer was pushing me to go bigger given the very impressive modulation range of the Vitodens 200W series. Am I making the right decision?

 

You'd be better off with HW priority. Having HW and CH running at the same time seldom works as you'd expect. The flow temps required by your heating will be lower than that required to fully heat up your cylinder to 60-65 degrees and having both running will mean water is unnecessarily circulating through your HW circuit as the CH slowly ramps up to temperature.

 

Instead, with HW priority, the HW tank is given full power to heat up as quickly as possible, with modern UV cylinders having very fast recovery times. The added advantage being that if your boiler supports it, you can run the CH at lower flow temps (including using weather compensation), whilst still allowing your HW cylinder to utilise maximum flow temps.