Heating System Critique

[tuftythesquirrel]

Heating System Critique

 

Hello,

 

Would it be possible to have some form of constructive feedback on my proposed heating system.

 

The house has been renovated to as close as we can get to Passive house in terms of insulation and airtightness, bearing in mind it was a retrofit of a 1950’s church hall.

 

In the first instance we will use a gas system boiler and possibly switch over to an ASHP at a later date. The intention is to see what is the lowest temperature the buffer and hot water cylinder can operate at, by empirical methods to prove it is suitable for ASHP temperatures. I’ve been observing the ASHP technology and it is still developing i.e. some new systems can now get a COP of over 5.

 

The overall heating design was based loosely on the Heat Geeks (https://www.youtube.com/@HeatGeek) mantra of getting as much plastic pipe in the floor as you can. Then the system can be run at a lower temperature (https://www.youtube.com/watch?v=9x1WeVDIAyA&t=45s). This is beneficial for both gas boilers (maximises condensing efficiency) and ASHP’s (maximises COP). It also means by running the system at a low temperature whilst running on gas, I will know whether it is suitable for an ASHP before I go and spend £000’s. I’ve deliberately used a thin floor screed to minimise the mass and improve response time.

 

Construction Details and U Valves

 

I have attached a few design drawings.

 

Roof - 0.12 W/m²K, 150mm of Kingspan Quadcore composite sheet.
Walls - 0.145 W/m²K, 200mm EPS external wall insulation system (EWI Store)
Floor – 0.09 W/m²K, 200mm PIR/Celotex insulation, UFH and 60mm Cemfloor screed (as the finished floor)

Floor area approx. 375m²

Airtightness < 2ACH

Building heat load at -2C/+21C is 10kW

Thermal bridge reduction where possible, thermolite blocks at wall floor level, compacfoam underneath door thresholds, steel roof trusses brought inside thermal envelope etc.

Gas system boiler 24kW.

Underfloor heating via 4 manifolds for downstairs only. Pipe centres at 150mm or less where I could fit them.

Separate manifold running at a higher temperature for towel rails and optional radiator upstairs.

Hot water cylinder is a 300L Gledhill unvented (PLUIN300), with 2 immersion heaters.

UFH uses a 300L Telford unvented buffer tank with 2 immersion heaters and feeds the UFH manifolds via an NRG 6 port zone manifold to optimise heat transfer.

Separate pressure regulators provided for the balanced water supplies, the boiler fill loop and non-balanced cold water outlets.

 

I’m putting in a whole house automation system based on the Idratek hardware  (http://www.idratek.com/) for the lighting and heating functions. It should allow customisable weather compensation, for the two tank temperatures.

 

I’ve added pressure and temperature gauges so I can evaluate the process. Some of these are also fed back to the automation system so I can log them and look back to see how it is operating.

 

I’ve selected both tanks with two immersion heaters so I can use them powered by solar PV and battery at a later date.

 

I have had plumbers quotes and specifically requested buffer tank for the heating but no one gave me the option as the said they always run the heating directly from the heat source. When I said I would prefer it to prevent boiler/ASHP cycling they said it wasn’t necessary. However, Kingspan have now introduced their Albion Aerocyl cylinder that provides exactly what I have but in a single tank, specifically for ASHP installations, so who is right? Am I correct in saying that although ASHP’s can modulate, this is limited to around 25% of maximum power. So if my UFH requires 1kW, a 16kW ASHP could only produce a minimum of 4kW and would be significantly overpowered and probably cycle.

 

Thanks in advance.

P01 - GF Plan Volume Computations - 19.10.2021.pdf P02 - FF Plan Volume Computations - 19.10.2021.pdf UFS Q36658-001d Ground Floor.pdf Visio-Piping Diagrams v0.54 HWC and BT.pdf

[JohnMo]

Just looking at your heat loss calculation at 10kW.  This looks very large for your u values.  Our house has very similar u values, but is 193m2. But at -5 we have a heat load of 3.2kW.  So yours should be in the ball park of around 6 to 7 ?

 

My biggest issue is a gas boiler too big, at low flow temps it will do a min of 6.7kW, against a min demand of 0.5 (ave 10 deg day) and max 3.2kW.  I have thick screed so chuck loads of heat at a time and use as a storage heater if I want.

 

With the amount of water you have in the floor, operate as a single zone and you will not need a buffer.

 

I would double check your heat demand, get as smaller gas boiler as possible.  Going big is a needless expense and is a pain in a low energy demand house.

 

You have about 1km more pipe in the floor as me as I am on 300mm centres and 7 loops and with a floor u value of 0.09, a flow temp of 30 degrees for 7 hours is enough to keep the house warm until the next day.  Would expect a flow temp nearer 25 would be enough for you.  Not sure who did your UFH design, they are quoting 113W/m2, and 40 degree flow temps, that's getting to be about 40kW going in to the floor, bit bonkers.

[tuftythesquirrel]

Hi @JohnMo,

 

Thanks for the reply. It was really helpful to get feedback from someone actually living in a property, with real life experience and data.

 

Quote

Just looking at your heat loss calculation at 10kW.  This looks very large for your u values.  Our house has very similar u values, but is 193m2. But at -5 we have a heat load of 3.2kW.  So yours should be in the ball park of around 6 to 7 ?

 My biggest issue is a gas boiler too big, at low flow temps it will do a min of 6.7kW, against a min demand of 0.5 (ave 10 deg day) and max 3.2kW.  I have thick screed so chuck loads of heat at a time and use as a storage heater if I want.

 

I did my own (very) rough calculation based on the total areas for the whole building, rather than a room by room as is the normal convention, so I will re-check them. We are in a detached house, so heat loss in every direction and its nothing like the perfect form factor for a passive house. I didn’t have a quote from a professional heating engineer that didn’t specify a boiler power anything less than 30kW, one even went a high as 40kW. When I did start asking a few questions, all of them admitted they had only ever fitted combi-boilers and radiators into a bog standard spec. semi.

 

Quote

With the amount of water you have in the floor, operate as a single zone and you will not need a buffer. I would double check your heat demand, get as smaller gas boiler as possible.  Going big is a needless expense and is a pain in a low energy demand house.

 

Yes, but I wanted some storage for and solar PV or battery power. The entire UFH system holds about 180 litres, so with 2 x 300 Litres, at least I can buffer the energy in the tanks to use separately later. I totally agree with the principle of going as small as possible, but my concern was then with the hot water demand and recovery time. We have 4 bedrooms, but will operate as a B&B so the likelihood of people taking simultaneous showers in succession is high. I spoke to a couple of tank manufacturers and they all said their specifications were produced using water temperatures of 80C. So my 300 litre tank could produce 225 litres of water before the tanks temperature dropped to 45C. They said if the tank temperature was 65C, the tank would only produce 135 litres. Since I want to operate at a lower temperature (60C) to improve the boiler condensing effect, the point at which the boiler will required is therefore earlier. Just wonder how you have found this.

 

Quote

You have about 1km more pipe in the floor as me as I am on 300mm centres and 7 loops and with a floor u value of 0.09, a flow temp of 30 degrees for 7 hours is enough to keep the house warm until the next day.  Would expect a flow temp nearer 25 would be enough for you.  Not sure who did your UFH design, they are quoting 113W/m2, and 40 degree flow temps, that's getting to be about 40kW going in to the floor, bit bonkers.

 

It was the underfloorshop.co.uk. Excellent service and support and the most economic quote I had as well. Yes, I thought that about the heat output as well, but I was always going to have to do some experimentation regarding the operating flows and temperatures. However, I was really encouraged by your figures and hopefully my extra pipe in the floor can bring the temperatures down quite low as well.

 

The feedback was encouraging knowing that we have hopefully gone in the right direction. The number of times that “professionals” tried to talk us out of the level of insulation we wanted was quite disturbing.

 

Once again thanks for your input.

 

[JohnMo]

You may be better with a big thermal store, size the boiler to charge the store, do as a DHW tank in tank or big DHW coil, seen them at 6.3m2 and 8.25m2.  Charging coil about 2.5m2 in the same store, will work at low temps.

 

1000l?

 

https://thermal-store.co.uk/1000l.php#1000l-lmt

 

Then it would work down to about 50 degrees, or wack up the temp to 80 when you are full.  No need for a buffer at all, just size to suit the storage you want and the DHW needs.  UFH would come direct of the store, have a tapping for each manifold pump, would give you a simple system with full hydraulic desperation.  Tonnes of hot water flow also.

[tuftythesquirrel]

Hi @JohnMo,

 

I've sort of been through this on a long iterative process/adventure🤣

 

Our posi-joists are probably not up to supporting the weight concentrated into a single tank. I've been going round this loop for a while and still not quite sure a thermal store would do the job in terms of ultimate performance. They also seemed to work out considerably more expensive, but I was never quite sure why.

 

What solution did you use for hot water if you sized the boiler quite small to suit the heating demand?

[JohnMo]

This my system now. 

 

 

So basically a combi boiler that can take preheated water. It will do three showers at a time - just.

 

Doesn't use much gas in the summer, uses mostly PV solar divert to heat.