Best resource to understand sizes of boiler/water tanks

[Meabh]

Hi - our new ICF build starts next week. We will have MVHR &PV (sorted with suppliers & installers). I have a couple of quotes for UFH downstairs/gas boiler combos that will be installed by local plumbers although they may want to supply their own products. We will have electric towel rails in bathrooms & electric UFH in an upstairs bathroom.

 

I know I need to look at total heat loss etc but am struggling to understand which bits of SAP to get the info from to check sizes of boilers etc. What is the best thread/website/book to read to understand

 - getting info from the SAP report

 - sizing boiler/water tanks

 - how to set the system up to link with PV (storing excess in the water tank but will consider battery once all costs in)

 

There are so many decisions to be made and I feel like I know a little bit about everything....but not enough to question. As we will use local plumbers I want to make sure they understand the house won't run like a traditional build in terms of energy etc.

 

Many thanks

 

[ProDave]

Why do you want a combi boiler?

 

With PV fitted it makes much more sense to have a system boiler and an unvented hot water tank, then you can easily divert surplus PV to water heating.

 

Choose the right UVC and you can change for a heat pump later with little pain.  In fact with an ICF build it should be pretty low energy so have you even considered a heat pump from the start?

[JohnMo]

I started our build with a combi boiler, and am in the process of changing to an ASHP.  I have had a completion certificate for about a month. Setup and run correctly they are cheaper to run than gas.

 

For the time being I will leave the gas boiler in place for DHW as the boiler is only been in place a short while.

 

If you are going gas boiler you need it set up to run priory hot water or X plan this will allow it to operate hot for cylinder heating and cool for UFH.  Set the system up exactly the same as a heat pump, with heat pump cylinder (3m2 + coil).

 

UFH, ensure you have plenty of insulation in the floor, 150mm PIR or more ideally. Set the the spacing at circa 200mm, limit number of zones where possible.

 

Read on here about UFH, ASHP, insulation and gas boilers.

[Meabh]

4 hours ago, ProDave said:

Why do you want a combi boiler?

 

With PV fitted it makes much more sense to have a system boiler and an unvented hot water tank, then you can easily divert surplus PV to water heating.

 

Choose the right UVC and you can change for a heat pump later with little pain.  In fact with an ICF build it should be pretty low energy so have you even considered a heat pump from the start?

We'd go for a condensing boiler and unvented water tank (think the line boiler/UFH combo has been misread as combi). It is all set up as heatpump ready but the cost of them doesn't make sense and we already have gas connection....plus others in the area who have installed heatpumps are having trouble with them (partly installer knowledge plus being in a coastal area where temps are quite low in the mornings and heat pump struggling in defrost mode)....but mostly it's the cost of them at the moment is not making financial sense. A heat pump was in the original plans.

[Meabh]

2 hours ago, JohnMo said:

I started our build with a combi boiler, and am in the process of changing to an ASHP.  I have had a completion certificate for about a month. Setup and run correctly they are cheaper to run than gas.

 

For the time being I will leave the gas boiler in place for DHW as the boiler is only been in place a short while.

 

If you are going gas boiler you need it set up to run priory hot water or X plan this will allow it to operate hot for cylinder heating and cool for UFH.  Set the system up exactly the same as a heat pump, with heat pump cylinder (3m2 + coil).

 

UFH, ensure you have plenty of insulation in the floor, 150mm PIR or more ideally. Set the the spacing at circa 200mm, limit number of zones where possible.

 

Read on here about UFH, ASHP, insulation and gas boilers.

THanks - We have a raft slab with 150mm so all good there. 

I'm not confident we'd get an ASHP set up properly and the initial cost vs gas boiler (already have gas connection) for a house that shouldn't need much heating isn't making financial sense at the moment.

Hadn't thought about the hot water cylinder  - although I think that's what was spec'd by Chelmer.

[JohnMo]

I paid £1000 for our gas boiler in a sale, and paid £1300 for the ASHP from eBay, the plumbing is basically the same. Two pipes connected to the heat source, supply split via a 3 way valve diverter, one to cylinder and one to UFH. And a return pipe.

 

Thick slabs are slow to react at low flow temps, our is 100mm, but with 300mm pipe spacing, if you rely on a normal thermostat for each room, your house temps are all over the place.  You need a 0.1 deg hysterisis thermostat. We are running just one thermostat, and for the past month or so we're batch charging the floor overnight with weather compensation and using the thermostat to stop the floor heating.  Generally the heating cycle was 7 to 9 hours heating on, the rest of the day off, house temp pretty stable.