Oil boiler for UFH

[Rob55]

New build house with UFH - what condensing oil boiler would you go for?  It's in the garage so I presume a boiler house model of the appropriate size (70-90), but are there any other things to consider or brands to avoid?

[Temp]

Welcome Rob.

 

We have had a Grant Oil boiler feeding a Thermal Store and UFH for 12 years. It gets serviced when we remember which is probably every two years. Its been fine. Service engineer seems to like its Riello burner.

 

However some tips...

 

As far as I can tell no domestic oil boiler is fully modulating. The burner is either fully on or fully off. There is no variable mode like there is for gas boilers. This means that when your UFH only needs a little heat an oil boiler may short cycle (switch on and off frequently). This is a bit like driving a car driving in city traffic compared to the motorway. Adding a thermal store or buffer tank between the boiler and UFH allows the burner to run flat out without short cycling at times of low demand. However Thermal stores and buffer tanks leak heat and take up space so it's a bit swings and roundabouts.

 

Another thing we found was that our boiler was slightly too powerful.  So we fitted a smaller jet in the burner. This reduced the boiler power and improved efficiency a few % (according to the manual).

 

Our Grant boiler was made in about 2006 and has a steel heat exchanger which requires a minimum return temperature of 40C to prevent corrosion. To ensure this is always the case we have a mixer which returns some of the hot flow to the boiler return if the return is below 40C. We also keep the corrosion inhibitor topped up. Whichever boiler you choose check if it needs a return mixer. Perhaps not all do.

 

 

Edited October 3, 2019 by Temp

[ryder72]

I have a Grant Vortex boiler coupled to a 500l thermal store to eliminate the short cycling problem. I have reused this boiler from the previous house which was demolished. The boiler used to turn on and offer up to 4 times an house and used oil faster than a Boeing 747.

 

On a really cold day (ambient -5) with 2 people occupying/showering and the UFH heating a 250m2 house (floor area approx 170 m2) the boiler will not fire approx 3 -4 times in a 24 hour period and run for up to 45 minutes to take the store from 40 degrees to 90 degrees. The boiler is rated at a much higher capacity than it needs to be.

[Rob55]

So would the consensus be that a buffer tank is needed, I already asked both my architect and plumber this question and they thought it was overkill... but to me, a boiler cycling on and off isn’t a good thing?

 

Also, for the UFH, does anyone ever filter the water going into the system for the first time? I was thinking of buying a di vessel and getting a couple of clean IBC’s to filter the water into to prevent build up of crap inside the heating pipes.. 

[PeterW]

5 minutes ago, Rob55 said:

So would the consensus be that a buffer tank is needed, I already asked both my architect and plumber this question and they thought it was overkill... but to me, a boiler cycling on and off isn’t a good thing?

 

Also, for the UFH, does anyone ever filter the water going into the system for the first time? I was thinking of buying a di vessel and getting a couple of clean IBC’s to filter the water into to prevent build up of crap inside the heating pipes.. 

 

Boiler cycling on oil is not good - not sure how clued up your architect would be on this, but the plumber should know better..!

 

And what are you filtering...?? A full UFH system holds 80-150 litres of water at most depending on the loops - you fill it from mains pressure clean cold water, and add inhibitor. A decent mag filter is less than £75 and will keep all the crud and crap from building up anyway . 

[Onoff]

Why on a new build would you go for oil and not an ASHP?

[Rob55]

There’s no grants over here for ASHP and we have built an eco home which is both super insulated and airtight, so oil is the cheapest way of heating. Architect projects total bill for the year to be around £250! House is 2200sqft. Even if the oil bill was £500, which I fully expect that it will be, I don’t see ASHP as having a short payback.

 

As for water filtering I wasn’t sure on the capacity maybe a couple of IBC’s was just a throw away comment but my point was that a di vessel will leave the incoming water completely pure and remove hardness from the water. We had to do it on an industrial heating system with a couple of IBC’s and I always thought it seemed like a great idea for UFH as you don’t get a second chance if a pipe blocks up with calcium deposits after 10/20 years!

Edited October 5, 2019 by Rob55

[PeterW]

So the water is a closed system and the temperatures are so low you wouldn’t get precipitation anyway - an inhibitor also changes the pH so tbh it seems a lot of effort for nothing and I’ve never seen a UFH system with issues from deposits - it’s usually galvanic corrosion from different metals that causes the problems so a filter and inhibitor is your best bet. 

[Jeremy Harris]

29 minutes ago, Rob55 said:

There’s no grants over here for ASHP and we have built an eco home which is both super insulated and airtight, so oil is the cheapest way of heating. Architect projects total bill for the year to be around £250! House is 2200sqft. Even if the oil bill was £500, which I fully expect that it will be, I don’t see ASHP as having a short payback.

 

 

Are you sure about this?

 

Oil price at the moment is about 5.6p/kWh 

 

Electricity running an ASHP would be a  bit less if you don't have Economy 7, around 5p/kWh, and a lot less if you do have E7, less than 3p/kWh

 

An ASHP costs pretty much the same to install as an oil boiler, probably less, as there's no oil storage tank needed.  I paid £1700 for our ASHP, that is over-sized for our house, and it cost about another £300 for installation related stuff

 

Our house is all-electric, and a bit smaller than yours, at 130m² (around 1400 ft²) and our total electricity usage (including charging my car) is £48/month, so about £576/year.  Heating is only a small part of that total, maybe £150 at the most.

[Rob55]

30 minutes ago, JSHarris said:

 

Are you sure about this?

 

Oil price at the moment is about 5.6p/kWh 

 

Electricity running an ASHP would be a  bit less if you don't have Economy 7, around 5p/kWh, and a lot less if you do have E7, less than 3p/kWh

 

An ASHP costs pretty much the same to install as an oil boiler, probably less, as there's no oil storage tank needed.  I paid £1700 for our ASHP, that is over-sized for our house, and it cost about another £300 for installation related stuff

 

Our house is all-electric, and a bit smaller than yours, at 130m² (around 1400 ft²) and our total electricity usage (including charging my car) is £48/month, so about £576/year.  Heating is only a small part of that total, maybe £150 at the most.

 

Really, I must honestly admit that on "hearsay" I thought ASHP was more like £6-7k.  Will get looking into this asap, thanks for the advice

[Rob55]

6 hours ago, JSHarris said:

 

Are you sure about this?

 

Oil price at the moment is about 5.6p/kWh 

 

Electricity running an ASHP would be a  bit less if you don't have Economy 7, around 5p/kWh, and a lot less if you do have E7, less than 3p/kWh

 

An ASHP costs pretty much the same to install as an oil boiler, probably less, as there's no oil storage tank needed.  I paid £1700 for our ASHP, that is over-sized for our house, and it cost about another £300 for installation related stuff

 

Our house is all-electric, and a bit smaller than yours, at 130m² (around 1400 ft²) and our total electricity usage (including charging my car) is £48/month, so about £576/year.  Heating is only a small part of that total, maybe £150 at the most.

from some internet googling I think my original cost estimate of £7k isn’t far off the mark?

Quote:

 

The cost of an air source heat pump unit can range from £6,000 to £8,000 and will depend on the size of the property it needs to heat. On top of this is the cost of the installation and of additional works required to upgrade the distribution system. (Generally speaking installing a heat pump is not especially disruptive work, though you may want to carry out this work at the same time as other home renovations.)

The running costs of an installed system will also vary depending on how much heat it needs to produce, what type of distribution system you have and the CoP of the system (see above). The cost of the electricity used to run the heat pump for a typical two-bedroom home is around £900-£1,200 per year, and up to £1,800-£2,400 for a four-bed home. This is generally cheaper than the cost of heating with electricity, oil, LPG or coal but more expensive than gas.

Maintenance costs for air source heat pumps are low. They are reliable, work automatically and have a long life.

[Jeremy Harris]

Lots of us here have installed ASHPs for a great deal less than that.  Some have installed them for less than the ~£2k we paid.  There are lots of people trying to charge ludicrous prices.  Try looking at some real prices for ASHPs, like this randomly selected hit from a quick search: https://www.wolseley.co.uk/product/mitsubishi-ecodan-85kw-ftc4--ashp-pack/  For a new build you'd get the VAT back, making this full installation kit for a 8.5 kW ASHP less than £3,200.  At most it's a day's work to install; with no experience at all I installed our similar unit in half a day.

 

That's an expensive unit, too, if you shop around you'll get them cheaper, I'm sure.

[ProDave]

Here is a 5KW heat pump for £2195 including VAT. https://www.easyheatpumps.com/product/panasonic-h-generation-high-performance-55c-monobloc/?attribute_models=WH-MDC05H3E5&gclid=EAIaIQobChMI8anBycSF5QIVia3tCh1fkw-tEAQYASABEgI38PD_BwE

 

That was just the first one that came up in a quick search, that is before you start looking for cheap deals etc.  Don't let people tell you they are expensive.

[PeterW]

Seriously ..??

 

£2200 for a 5Kw Samsung, £2700 for the 9Kw if you need a bigger one. 

 

One control cable, armoured supply to a rotary isolator and two pipes ... 

[Rob55]

My UFH says it has a heat load of 16.5kw, plus say 3kw for domestic hot water, am I wrong to assume this needs a 20kw heat source?

[ProDave]

11 minutes ago, Rob55 said:

My UFH says it has a heat load of 16.5kw, plus say 3kw for domestic hot water, am I wrong to assume this needs a 20kw heat source?

What size house?  

 

It would take a very large house or a very high (higher than normal) temperature to get that much out of under floor heating.

 

I put no more than 3KW into ours, probably a lot less most of the time.

[Jeremy Harris]

10 minutes ago, Rob55 said:

My UFH says it has a heat load of 16.5kw, plus say 3kw for domestic hot water, am I wrong to assume this needs a 20kw heat source?

 

 

16.5 kW seems extremely high to me.  Our 130m² (~1400 ft²) house needs about 1.6 kW when it's -10°C outside, and most of the time it only needs around 300 to 400 W in winter.

 

Assuming that your house needs a great deal of heating, then the maximum heat output from UFH is usually around 50 to 60 W per m², so you can work back from that as an absolute worst case.  Our UFH rarely needs to deliver more than about 20 W/m², for example, and we have about 75m² of heated floor area.

[Jeremy Harris]

7 hours ago, Rob55 said:

There’s no grants over here for ASHP and we have built an eco home which is both super insulated and airtight, so oil is the cheapest way of heating. Architect projects total bill for the year to be around £250! House is 2200sqft. Even if the oil bill was £500, which I fully expect that it will be, I don’t see ASHP as having a short payback.

 

 

This is another clue that the 16.5 kW figure is way too high.  A £250 oil bill,  according to the architects calculations, with an 80% efficient boiler, would be around 3,571 kWh for the year.  Hot water will be a fair hunk of that, roughly 1,000 to 1,100 kWh per person per year, so that needs to be subtracted.  For two people, that would mean that heating energy use would be around the same as  ours, about 1,500 kWh/year.  That doesn't stack up with a 16.5 kW heating demand, as our typical heating demand is about 300 to 400 W, and in extremely cold weather this can increase to about 1.6 kW for a short period.  It sounds to me as if a decimal place may have been shifted to the right by accident.

[Ed Davies]

It's not 16.5 kWh per day, is it?

 

That'd be a reasonable annual average for a not tiny house. E.g, decent but not Passivhaus so 30 kWh/m²/a for 200 m² comes to 16.438 kWh/day.

[Rob55]

8 minutes ago, Ed Davies said:

It's not 16.5 kWh per day, is it?

 

That'd be a reasonable annual average for a not tiny house. E.g, decent but not Passivhaus so 30 kWh/m²/a for 200 m² comes to 16.438 kWh/day.

it could well be, the spec sheet just says “UFH Load 16.5kW”

 

If that is the case, any idea what size ASHP would cover it plus hot water? 141m2 downstairs and 64m2 upstairs.

[Jeremy Harris]

1 minute ago, Rob55 said:

it could well be, the spec sheet just says “UFH Load 16.5kW”

 

If that is the case, any idea what size ASHP would cover it plus hot water? 141m2 downstairs and 64m2 upstairs.

 

I think you need to bottom out what the real heating requirement is, and work from there.  At the moment there's a massive discrepancy between what your architect is telling you, which suggests a mean heating requirement of around 300 to 400 W, and your UFH figures, which suggest a massively higher heating requirement.  If both floors are heated (and this is worth thinking about, as with a passive house spec there is rarely a need for first floor heating) then 205m² at 50 W/m² is an absolute maximum heat input of 10.25 kW, and frankly that's the sort of heating requirement that a 40 year old house might need.  Something closer to passive house spec is unlikely to need more than about 20 W/m², so that would be a peak requirement of around 4.1 kW.  Our heat pump is rated at ~7 kW and cost around £2k installed.  It's massively too big for our needs, but was the cheapest unit we could find at the time.

[Ed Davies]

9 minutes ago, Rob55 said:

it could well be, the spec sheet just says “UFH Load 16.5kW”

 

I think we need more context to work out what that really means. Can you scan/copy the relevant chunk of the “spec sheet”?

[Ed Davies]

2 minutes ago, JSHarris said:

It's massively too big for our needs,

 

But it does little or none of your DHW. If you wanted it to recharge a hot water tank at all quickly it'd not be oversized.

[Jeremy Harris]

17 minutes ago, Ed Davies said:

 

But it does little or none of your DHW. If you wanted it to recharge a hot water tank at all quickly it'd not be oversized.

 

Depends.  The maximum power we can put into our hot water heat storage system is 2.8 kW, and that could be very easily met by the excess power available from the ASHP.  The only reason we don't use the ASHP to charge the hot water system is the problem with getting it to deliver water at a high enough temperature to charge the Sunamp.

[ProDave]

Our 5KW ASHP does DHW and UFH without problem.