What size system boiler ?

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 new 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 800 series which believe is latest which looks good and is 24kw model (they don't do bigger in that range but do in the megaflow range). Is that big enough for this setup ? Friends with similar setup have bigger system boilers but to be fair they have bigger tanks and all the showers are from the tank rather my one. What would people recommend in this scenario ? Any help greatly appreciated


Many Thanks,

Lawrence

Heat loss calculations for the building are required. Otherwise it's just guessing.

Suggest you get rid of at least one of those electric showers, preferably both. Most electricity supplies to normal houses can't cope with two electric showers on their own, never mind with all the other electrical items in a typical home.

Rule of thumb 1kW per radiator plus 3kW for your UFH gives 16kW total, but a full heat loss calculation from Heat Engineer will give you a very accurate picture of what you actually need for the princely sum of £12.

The Baxis are fairly decent very basic boilers which have almost no configurable installation parameters. This means that they will get hot, but can't be set up to match the system they're fitted to, so real-world efficiency suffers. They also aren't able to accept controls which can vary the central heating water temperature to match the heat requirement - they're fixed at a high temperature which will heat the hot water cylinder but is almost always too hot to efficiently heat a room. They tend to be fitted by people who don't understand why this is important. If you're having a full renovation it would be worth using someone who understands system design and how to get the best efficency from a condensing boiler. They're not fully condensing out of the box, they need correct system design and setup to achieve their maximum efficency, otherwise they'll typically run at around 80% efficient instead of the 95% you're hoping for

My home three floors 5 bedrooms, two kitchens, two bathrooms and a shower room, two living rooms, and a dinning room, and fitted with an 18 kW oil boiler which is ample.

The big thing is speed to heat a room, I would set each room used to heat in sequence, but to do this the radiators need to be double the size of the boiler or with oil it will cycle or gas it will modulate.

A second vote for Heat Engineer, if you don't know the heat loss you'ere accepting inefficiency from the get go. Lets don't beat around the bush, it'll take a few hours of your time and some effort, you'll learn something but it'll be worth it.

Out of interest there's a Vokera webinar, Heat Optimisation and Efficiency, tomorrow evening at 7pm that may be of interest. Most of our webinars are for Gas safe engineers only but this one is open to all. You can guess some heat losses as part of a mini-quiz. Use this link to register if interested https://carrier.zoom.us/webinar/register/2215979244261/WN_T_kjmDt0TAyuMD4xmVw85g

Many thanks for all the replies, really useful, looks like a heat loss calculation is key, will mention it to plumber but he was keen to order boiler, think they normally just install the one they are used to. Have emailed baxi for advice as well but the heat loss thing I need to do some more reading on, doesn't seem to be much out there on system boilers where found tons of info/forums on combi's. Many thanks, Lawrence

Don't be pushed into a quick decision because they want to order it.
It's your house, you will be living in it and paying the bills... Not them!
By nature, builders generally find heating and plumbing works a necessary evil that has to be done but interferes with their desire to get done and get out, so it's often given very little thought!

As you will have a system boiler it would be important to operate it with priority domestic hot water reheat, separating the reheating of the cylinder from space heating. This is advantageous as DHW reheat is done at non-condensing temperatures (20% of energy used), space heating can be done at more efficient condensing temperatures (80% of energy used). This can be done several ways with different methods, weather compensation or OpenTherm control for heating and higher temps for DHW.

The boiler and controls need to be configurable for this and there are several arrangements and muggles makes a good point about Baxi regarding this.

The webinar I mentioned above and another on weather compensation https://carrier.zoom.us/webinar/register/1016010418540/WN_DvJ_SyFeSP6eo3dfRO4bsA are not sales pitches but almost generic (I do work for a manufacturer however). They might provide a fuller understanding of what is possible and there are Q&A breaks.

+1 for a boiler which can have separate CH and HW flow temps, rather than compromising to satisfy both. Also switch to PDHW so that the cylinder heats up as rapidly as possible, aka Intergas X-Plan: https://www.intergasheating.co.uk/consumer/2019/11/05/tech-talk-3/

If you can add weather compensation to the mix you will get even more efficiency on milder days, where lower flow temps are required.